Human Reproduction, Class 12, Biology Chapter Notes PDF Download

Human Reproduction

 

INTRODUCTION :

Embryology is the branch of biology which involves the study of all those processes, which take place during the development of an adult from the egg.

 

FORMATION OF GAMETES :

Follicle stimulating hormone stimulates gametogenesis. Besides this hormone vitamin E is also essential for gametogenesis. Deficiency of vitamin E leads to sterility. Vitamin A is also required for the formation of healthy gametes.

Gemetogenesis is divided in three stages :

(i) Multiplication phase      

(ii) Growth phase                    

(iii) Maturation phase.

As there are two types of gametes, the spermatozoa and ova, gametogenesis can be studied under two broad headings : spermatogenesis and oogenesis. Spermatogenesis is the formation of spermatozoa, whereas oogenesis is the formation of ova. Both spermatozoa and ova originate from primordial germ cells or PGCs, which are extra-gonadal in origin. In humans, the PGCs originate during early embryonic development from the extra-embryonic mesoderm. Eventually, they migrate to the yolk sac endoderm, and ultimately, to the gonads of the developing embryo, where they undergo further development. You can recall that spermatogenesis occurs in the seminiferous tubules of the testes of oogenesis occurs in the follicles of ovary. Formation of gametes starts at puberty.

 

SPERMATOGENESIS

Spermoatogenesis : i.e. formation of sperms. In most of the animals spermatogenesis takes place in testes, (exception-earthworm). Mammalian testes contain seminiferous tubules and wall of seminiferous tubule is composed of germinal epithelium. It contains some special types of cells called primordial germ cells and these cells start spermatogenesis. On the basis of origin, primordial germ cells are extra embryonic mesodermal. Besides these cells, germinal epithelium contains some large sized cell called sertoli cells. Occurrence of sertoli cells is the unique feature of mammalian testis. Sertoli cells provide nutrition of developing sperm i.e. developing sperms are embedded in cytoplasm of sertoli cells and absorb nutrition. After maturation sperms comes out from sertoli cells and librate in seminiferous tubules.

Liberation of sperms from Sertoli cells is called spermiation.

Liberation of sperms from testes is called semination.

Liberation of sperms from body of male is called ejaculation.

Mammalian sperms are transferred to vagina of female by the process called insemination.Sertoli cells form 'blood testes barrier' and protect the sperm from immune system of the body. because

antibody may attach on haploid cells and destroy them. (Sperms are haploid and other cells of  body are diploid).

Sertoli cells function as an endocrine gland i.e. secrete three type of hormones :

(i)   Antimullerian hormone : function of this hormone is degradation of female gonads in male embryo. (In male seminal vesicle is the ruminant part of oviduct of female).

(ii)  Inhibin hormone : Function of this hormone is to control excess secretion of pituitary gland to prevent the over-production of sperms.

 

(iii) Androgen binding protein : Function of this hormone is to concentrate testosterone in seminiferous tubules because testosterone is must for spermatogenesis in seminiferous tubules.

 

 

STEPS OF SPERMATOGENESIS

Spermatozoa are formed in the wall of the seminiferous tubules of the testes. The various cell-stages in spermatogenesis are as follows (the number of chromosomes at each stage is given in brackets)

An adult male produces over 10¹² to 10¹³ sperm cells each day. These gradually move into the epididymis and the first portion of the vas deferens, where they undergo further maturation and are stored.

The spermatogonia (type A) or germ cells (44 X + Y) divide mitotically, to give rise to more spermatogonia of type A (spermatogenic lineage) and also spermatogonia of type B.

The spermatogonia (type B) (44 + X + Y) enlarge, to from primary spermatocytes (spermato cytogeneis)

The primary spermatocytes (44 + X  + Y) now divide so that each of them forms two secondary spermatocytes. This is the first meiotic division. it reduces the number of chromosomes to half.

Each secondary spermatocyte has 22 + X or 22 + Y chromosomes. It divides to form two spermatids. This is the second meiotic division and this time there is no reduction in chromosome number.

Each spermatid (22 + X or 22 + Y) gradually changes its shape to become a spermatozoon. This process of transformation of a circular spermatid to a spermatozoon is called spermiogenesis/spermateleosis

In spermiogenesis first, of all nucleus of spermatid shift at one side. Except chromatin material, all
the structures come out from nucleus as a result nucleus become small and light in weight. Then several golgi vesicles gathered just above the nucleus. Some of the Golgi vesicles develop granules called
proacrosomal granules, such granule containing vesicle is called proacroblast. Rest of the golgi
vesicles are called golgirest, the golgirest dissolve in cytoplasm. All the proacroblast fuse and form large vesicle called acroblast and it's granule is called acrosomal granule. Acroblast arranged just above the nucleus. Then cytoplasm of spermatid starts moving towards posterior side. As a result, plasma membrane shrinks and gets attached to acroblast and nucleus. Due to this head of sperm is formed. Now centriole starts forming axonema, then all the mitochondria of spermatid are arranged in spiral order around axonema. In this way middle piece of sperm is formed. Now axonema elongates towards posterior side and forms tail. Cytoplasm of spermatid continuously flows towards posterior side.

 

Structure of sperm :

The spermatozoon has a head, a middle piece and a tail.  The head is covered by cap the acrosomic cap, anterior nuclear cap, or galea capitis. Acrosome is a bag like structure filled with lytic enzymes called spermlysins. In the anterior part of middle piece neck is present. The neck is narrow : it contains a
proximal & distal centriole (or Basal body). An axial filament begins just behind this centriole, it passes through the middle piece and extends into the tail. At the point where the middle piece joins the tail, this axial filament passes through a ring-like structure called the annulus (or ring centriole or zensons ring). That part of the axial filament which lies in the middle piece, is surrounding by a spiral sheath made up of mitochondria. (Nebenkern sheath)

Nuclear part of  head of spermatozoa consist of chromatin (mostly DNA) that is extremely condensed. It contains a basic nature protein called protamin.

The basal body is made up of nine segmented rod like structures each of which is continuous distally with one coarse fibril of the axial filament.

The axial filament, that passes through the middle piece and most of the tail, is actually composed of several fibrils arranged. There is a pair of central fibrils, surrounded by nine pairs (doublets) arranged in a circle around the central pair (9+ 2). This arrangement of one central pair of fibrils surrounded by nine doublets is covered by nine solid protein (making the arrangement as 9 + 9+ 2).

In the proximal tail part it is covered by only two solid protein fibres (therefore arrangement is 2 + 9 + 2), while end part of tail has no protein covering (therefore arrangement is 9 +2). Immediately outside the fibrils there is a fibrous health.

Middle piece (also called as the energy chamber) is surrounded by spirally arranged mitochondria (Nebenkern sheath). Finally, the entire sperm is enclosed in a plasma membrane.

 

OOGENESIS

Like spermatogenesis oogenesis process also can be divided into three stages :

(A) Multiplication                  

(B) Growth phase                  

(C) Maturation phase

       

(A) Multiplication phase : In this stage primordial germ cells or ovum mother cells repeatedly divide by
 mitosis to form large number of diploid oogonia.

This process completes in embryo stage of female in most higher animals.

 

(B) Growth phase : Like spermatogenesis, in this process oogonia grow in size and form primary oocytes. The growth phase is the longest phase oogenesis (except humans). During growth phase size of egg increases many times.

During growth phase several changes occur in egg and all these changes are classified in 2 sub-stages

Previtellogenesis

Vitellogenesis

 

Previtellogenesis :

During previtellogenesis, changes occur in nucleus and cytoplasm of egg.

 

Changes in nucleus :

Amount of nucleoplasm increases in nucleus.

Number of nucleolus increase in nucleus.

Formation of lamp brush chromosome starts.

Activity of DNA increases in nucleus, as a result DNA become highly active and rapidly synthesizes different types of RNA. Increased activity of DNA is called as Gene redundancy/Gene amplification. Due to all these changes, size of nucleus increases and nucleus becomes vesicular. This vasicular nucleus is called germinal vesicle .

 

Changes in cytoplasm :

In cytoplasm, rate of protein synthesis increases. Cytoplasm rapidly synthesises different type of protein and enzyme. Due to more availability of  protein  and enzymes, synthesis of new  protoplasm takes place and size of egg increases.

Number of cell organelles increase in cytoplasm, specially endoplasmic reticulum, golgi-body and mitochondria Mitochondria become very large in number so mitochondrial clouds are found in cytoplasm of egg.

Later on all these 3 cell organelle (golgi body, endoplasmic reticulum, mitochondria) are arranged in the form of ring around the nucleus, it is called as Balbiani vitelline ring. In the stage golgi body of egg secretes a membrane around the egg which is it called as vitelline membrane. A space appears in between plasma membrane of egg and vitelline membrane called as perivitelline space, It is filled with a fluid called perivitelline fluid.

At the end of previtellongenesis endoplasmic reticulum disappear. Golgi bodies gets converted into corticle granule. Corticle granules are filled with mucopolysacharide. Large number of change occur in mitochondria also.

 

Vitellogenesis : During vitellogenesis egg stores food in the form of yolk.

Some part of yolk is synthesized in egg only, but major part of yolk is received from liver.  Yolk received from liver is less viscous and is therefore soluble but this type of food  cannot be stored for long periods (because it easily gets converted into simple form)

So mitochondria of egg with the help of kinase enzyme make the  yolk more viscous and insoluble.
2 types of yolk is found :

Granular yolk occurs in the form of fine granules.  

eg. Protostomia animals.

 

Yolk platelets occur in the form of plate disc like granule. 

eg. Deuterostomia animals (higher animals)

 

Chemical composition of yolk :

1.Most abundant compound in yolk is phospholipid

Most common phospholipids is lecithin.

 

2. Yolk contains different type of protein :

Simple protein : Albumin, Globin, Globulin

Phosphoprotein : Phosvitene, Ovovitelline

Lipoprotein : Lipovitelline

 

3. In yolk, least amount of substance found is carbohydrate.

 

(C) Maturation phase : Oogenesis (Fig. 2) takes place in the ovaries. In contrast to males the initial steps in egg production occur prior to birth. By the time the foetus is 25 weeks old, all the oogonia that she will ever produce, are already formed by mitosis. Hundreds of these diploid cells develop into primary oocytes, begin the first steps of the first meiotic division, proceed up to diakinesis, and them stop any further development. The oocytes grows much larger and completes the meiosis I, forming a large secondary oocyte and a small polar body that receives very little amount of cytoplasm but one full set of chromosomes.

In humans (and most vertebrates), the first polar body does not undergo meiosis II, whereas the secondary oocyte proceeds as far as the metaphase stage of meiosis II. However, it then stops advancing any further, it awaits the arrival of the spermatozoa for completion of second meiotic division. Entry of the sperm restarts the cell cycle breaking down MPF (M-phase promoting factor and turning on the APC (Anaphase promoting complex). Completion of meiosis II converts the secondary oocyte into a fertilized egg or zygote (and also a second polar body)   

Ova are derived from oogonia present in the cortex of ovary. Some important differences between oogenesis and spermatogenesis are  

(i)   Whereas one primary spermatocyte gives rise to four spermatozoa, one primary oocyte forms only one
ovum.

(ii)  When the primary spermatocyte divides, its cytoplasm is equally distributed between the two secondary
spermatocytes formed. However, when the primary occyte divides, almost all its cytoplasm goes to the
daughter cell which forms the secondary oocyte. The other daughter cell (first polar body), receives half the chromosomes of the primary oocyte, but almost no cytoplasm.

The first polar body is, therefore, formed merely to get rid of unwanted chromosomes. 

 

TYPES OF EGGS

(A) On the basis of amount of yolk

(i)   Alecithal or Microlecithal or Oligolecithal eggs – The amount of yolk is very small or absent in these types of eggs. (oligolecithal, or microlecithal or alecithal).

The term 'alecithal' (absence of yolk eg. Man) was given by Kent. And term microlecithal' (eg. Urchin) was given by Tori.

Examples :- Egg of Amphioxus, Eutheria, Metatheria and sea – urchin.

                       

(ii)  Mesolecthal Eggs : - In this type of egg, the amount of yolk is moderate i.e medium ,neither more nor less.

Example : - Eggs of Amphibia, Petromyzon and lung-fishes.

                               

(iii) Polylecithal or Macrolecithal or Megalecithal eggs : -

Eggs are with large amount of yolk. e.g. Bird's egg , Insect's egg.

 

(B) On the basis of distribution of yolk :

(i)   Isolecithal or homolecithal eggs : The yolk is evenly or homogenously distributed in these eggs. eg. : micro, oligo or alecithal eggs.

 

(ii)  Telolecithal eggs : The yolk is concentrated in one part of the egg.

eg. Mesolecithal eggs of amphibia. (Moderately telolecithal)

Discoidal eggs : A type of  telolecithal and megalecithal eggs. Where the yolk is in enormous quantity and concentrated in one part of the egg. Thus only a disc of cytoplasm called germinal disc remains in the egg which is located at the other pole of egg. (Heavily telolecithal)

eg. :  Eggs of reptiles, birds and prototherian mammals

 

(iii) Centrolecithal eggs : Megalecithal eggs where the enormous amount of yolk is located in the centre an cytoplasm is in the form of superficial layer around the yolk.

eg. : Insects egg.   

 

(C) Classification of Eggs on the basis of Shell : -

On the basis of shell, eggs are of 2 types : -

(i)   Cleidoic eggs : - Eggs surrounded by a hard shell are known as cleidoic eggs. These eggs are found in those animal  which  have a terrestrial mode of life or which lay eggs on land.

These eggs have more amount of yolk. These are adaptations to terrestrial mode of life. Shell prevents the egg from dessication.

e.g. : - eggs of  ''Reptiles'',  ''Birds'', ''Insects'' and ''Prototherians''.

 

(ii)  Non-Cleidoic eggs : -

Eggs which are not surrounded by a hard shell are called non-cleidoic eggs.

eg. : - all viviparous animals (Mammals) and all oviparous animals which lay eggs in water (Amphibians). Reptilia eggs are called leathery eggs.

 

STRUCTURE OF OOCYTE

The nucleus of egg is also called germinal vesicle.

Oocyte is surrounded by membranes termed as the egg-membranes.

Oocyte/Ovum along with the egg-membrane are termed as the egg.

Egg = Ovum/Oocyte + Egg membrane.

Majority eggs are oval but the eggs of insects are long and cylindrical. Smallest eggs are of 50m in polychaeta and the largest eggs are of an Ostrich.

 

Classification of egg - membranes :

On the basis of origin, egg-membranes are of 3 types : -  

(1)  Primary egg membrane :

This membrane is secreted by the oocyte itself.

eg. Vitelline membrane, Zona Pellucida (mammals)

 

(2)  Secondary egg membrane :

This is found outside the primary egg membrane and is secreted by the ovary.

 

(3)  Tertiary egg membrane :

The is present outside the primary egg membrane. It is either secreted by the oviduct.

 

Functions of Egg-membranes

(i)   To provide protection   

(ii) To check polyspermy   

(iii) To provide buoyancy to the amphibian eggs

 

Different types of eggs

(I)  INSECT EGG

Eggs of insects are megalecithal or polylecithal in them yolk is present in the centre, so the eggs are also centrolecithal.

Two egg membranes are present here, inner vitelline membrane (primary) and outer chorian (secondary).  

The sperm enters the egg through micropyle because on the head of insect sperm acrosome is absent.

The Cytoplasm here is found in two parts :

(a)  Central cytoplasm  : - It is present in a very small amount in the centre of the egg. Egg nucleus is located in it.

(b)  Peripheral Cytoplasm : - It is present in a very small amount along the periphery of the egg.

 

(II) FROG'S EGG

Eggs of frog are moderately Telolecithal & Mesolecithal

Two types of egg membranes are found in frogs egg :

(i)   Vitelline membrane : This is primary egg membrane which is secreted by the ovum around itself.

(ii)  Jelly coat :  This is tertiary egg membrane, secreted by oviduct. Jelly coat has air bubbles trapped in it due to which it floats on water. This group of frogs egg is called spawn, Jelly coat is bitter in taste so enemies do not eat it.

           

Secondary egg membrane is absent in frog's egg.

Internal part of the egg is divided in two parts :

(i)   Animal pole : This part has cytoplasm, egg nucleus in also located in this part. In the cytoplasm melanin granules are found which prevent the egg from harmfull radiations. They also help in protection of egg by camouflage.

A sperm always enters into the ovum at some point in animal hemisphere. This point is normally other than the animal pole itself.

As the sperm enters into the ovum, taking some pigment granules with it, a grey, crescent shaped region appears in the equatorial zone geometrically opposite the sperm entrance point. This region is called grey crescent. It is formed due to movement of some pigment granules away from it towards sperm entrance point.

(It marks the dorsal side of future embryo). This area of sperm entrance point marks the anterior side of future embryo. The side diagonally opposite to it in the vegetal hemisphere marks the future posterior side. Thus the sperm entrance establishes the anteroposterior and dorsoventral axis as well as bilateral symmetry of future embryo.

 

(ii)  Vegetal pole : yolk is concentrated in this part of egg.

 

(III) CHICK EGG

These eggs are megalecithal or polylecithal and discoidal eggs.

In these eggs, yolk is present in the centre of the egg in the form of a dense mass. The cytoplasm of the egg is in the form of a disc above the yolk, which is termed as the germinal-disc.

Yolk is of 2 types i.e. yellow yolk and white yolk.   

Yellow – yolk has more amount of phospholipids and is secreted during the night. White yolk has less amount of phospholipids and is secreted during the day time. Central white part of yolk is called latebra.

Both the types of yolk are arranged in alternate and concentric layers.Pander was the scientist, who discovered the 3 germinal layers i.e. Ectoderm Mesoderm and Endoderm in chick - egg.

Around the egg, porous shell of CaCO₃ is present which is secreted by the cells of the oviduct.

In between the vitelline membrane and the shell membrane albumin is filled which is also called the white of egg. It contains 13% proteins.

Thick albumin-fibres termed as ''Chalaza'' are present in the albumin part of egg.

 

(IV)     EGG OF MAMMALS

Mammalian eggs have very less amount of yolk, so the eggs are oligolecithal and isolecithal or microlecithal  and homolecithal.

The egg has 2 egg-membranes : -

(1)  Zona Pellucida : - This is a transparent membrane like covering and is a primary membrane secreted  by the ovum/oocyte itself.

(2)  Corona radiata : - This is a layer of follicular cells'' and these cells are attached to the surface of egg through ''hyaluronic acid'' This is a secondary membrane, which is secreted by the ovary. These eggs don't have tertiary membrane.

Mammalian eggs are approx 0.1 mm in size.

 

FERTILIZATION

The process in which union of male and female gametes (formed by gametogenesis) and fusion of pronuclei of sperm and ovum takes place thus diploid zygote is formed, is called fertilization. 

Fertilization has following processes : - The union of male and female gametes is called Syngamy. Where as intermixing of their cytoplasm is called plasmogamy. The fusion of pronuclei of sperm and ovum is called karyogamy. The intermingling of their chromosomes is called amphimixis.

Due to fertilization , a diploid zygote is formed, by the union of two different types of gametes.

 

SITE OF FERTILIZATION :

(A) INTERNAL FERTILIZATION - Fertilization in the body (i.e., genital organs of animal) is called internal fertilization. In this type of fertilization, sperms are discharged by male directly into the genital tract of female after coitus.

Whole process of fertilization takes place within the body of female. This is the most common
 adaptation in terrestrial animals.

Examples : - Aschelminthes, reptiles, birds and mammals

 

(B) EXTERNAL FERTILIZATION - External fertilization takes place outside the body of females i.e., in water. Example : - In most of the invertebrates, some protochordates, amphibian and most of the fishes.

 

TYPES OF FERTILIZATION : 

(a)  Self fertilization - This process takes place in the body of single animal i.e., fusion of male and female gametes produced by male and female organs of the same animal. This is called self-fertilization. This is possible only in bisexual or hermaphrodite animals.

Examples : - Animals of phylum porifera and most of the species Hydra.

 

(b)  Cross-Fertilization – Fertilization takes place between two (male & female) different animals of same 
 species.

This is called cross fertilization.  

This process is found in all unisexual animals. These animals are also called dioecious animalsCross-fertilization is also found in most of the bisexual or hermaphrodite animals because in these animals male genital organs develop first. This condition is called protandrous condition. In some of the species female organs develop first, this condition is called protogynous condition e.g. sponges.

 

MECHANISM - We can understand the process of fertilization in following steps.

1.   Movement of sperms towards ovum.

2.   Entrance of sperms in the ovum.  

3.   Activation of ovum.

Approach of sperm towards the egg – it is a chance factor, so sperms perform random (directionalless) movement. To increase the chances of approach of sperm towards egg there are mainly two adaption

(a)  Number of sperms is very high : - e.g. In man 20 to 120 million sperms are present per cubic mm of semen.

(b)  Egg is 1000 times larger than sperm

Some special proteins are found on the surface of egg and sperm to help in fertilization.

According of  Lillie, chemicals named as ''fertilizins'' are found on the surface of egg. Fertilins are glycoproteins or acid mucopolysaccharides. According to Ballinsky, an acidic protein named as Antifertilizin is present on the surface of sperms. ''Fertilin'' proteins are also present on sperm surface.Both the proteins are specific for a particular species. Antifertilizin present on sperm of a particular species will react with fertilizin of present on egg of the same species of animals.           

If we place some eggs of sea-urchin in sea-water, this sea water becomes viscous, this is called egg-water. When some sperms come in contact with this egg water, sperms adhere with each other. It is called agglutination.

Here the reaction of fertilizin (dissolved in water from egg) and antifertilizin of sperm is observed clearly.

Fertilizins behave like lock and antifertilizins behave like keys.

Fertilization is always intraspecific.

According to Washerman and Sailing (1989) a specific pair of protein molecules is found on the surface of mammalian sperm, which can recognize specific carbohydrates and proteins in ZP₃ region of zona pellucida. The bindin protein of sperm reacts with these molecules to initiate the changes in acrosome.

A specific sugar galactose remains attached with ZP₃ glycoprotein. The sperm fails to recognize the ovum of its own species, if this sugar is removed from zona pellucida.

In addition to these glycoproteins, there are some hormones also, which help in fertilization.

The hormones present at the surface of sperm are called androgamones. These are of two types. Androgamone first & androgamone second.

Androgamones I help in the energy conservation of sperms.

Androgamones II dissolve the gelatinous coverting present all over the egg.

Hormones present at the surface of egg are called gyanogamones these are of 2 types.

(a)  Gyanogamones I - this hormone neutralizes Androgamone I and activates sperm to move

(b)  Gyanogamones II - It makes sperm head sticky.

Enzyme of acrosome (Hyaluronidase and sperm lysins) dissolve the egg membrane. This is called acrosomal reaction. As a result sperm head make the contact with the plasma membrane of egg, now inner membrane of acrosome evaginates outside and form rigid tube is called acrosomal filament. Acrosomal filament provide stimulus to plasma membrane of egg and due to stimulus of sperm, egg is induce for fertilization. Mammalian sperms do not form this type of filament because mammalian sperms are highly active and provide stimulus to  plasma-membrane of egg without any filament. Mammalian sperms acquire activity at two places. First-epididymis and second-vagina. Vaginal secretion make the sperm highly active and sperm acquire capacity of fertilization is called capacitation.

 

Activation of egg : Due to stimulus of sperm an enzyme is induced in plasma-membrane of egg it
is called adenyl cyclase enzyme and function of this enzyme is to catalyze C-AMP in egg cytoplasm.
C-AMP is the second messenger. Cyclic AMP receive stimulus from plasma membrane of egg and transfers it in egg cytoplasm and induces all the response of egg for sperm.

All the response of egg for sperm are collectively called gyanogenesis.

Due to stimulus of sperm, permeability of plasma membrane of egg increases specially for k⁺ and Ca²⁺ ions. Function of Ca²⁺  ions is to inactivate the cytostatic factors in egg. As a result egg is now ready for cleavage (In egg cytoplasm special type of protein called cytostatic factor are present these factor prevent the cleavage in unfertilized egg)

Due to stimulus of sperm, H⁺ – Na⁺ pump activates and induces the plasma-membrane of egg. Function  of this pump is to continuously influx H⁺ ions and outflux Na⁺ ions. As a result concentration of H⁺ ion increases in egg cytoplasm and develops an acidic medium. In acidic medium, proteolytic enzyme become active and liberate the m-RNA from informosome. These m-RNA become active and rapidly synthesize different types of protein and enzymes. Due to more availability of protein and enzymes metabolic activity of egg increases.

 

Response of egg :

(1) Due to stimulus of sperm, meiosis-II is induced in human egg by excluding second polar body becoming mature ovum.

(2)  At the point of contact with sperm and plasma-membrane of egg a cone-like structure is formed called reception cone. After some time reception cone sinks in egg cytoplasm along with sperm (entry of sperm is a type of phagocytosis). With the entry of sperm all the cortical granules burst and secrete a membrane around the egg is called fertilization membrane (cortical reaction). It is secreted on inner surface of primary egg membrane and perivitelline space become more wide and amount of perivitelline fluid is also increase. Function of perivitelline fluid and fertilization membrane is to prevent the entry of sperm in egg. so normally only one sperm enter inside the egg (monospermy). Sometimes more than one sperm enter inside the egg (polypermy).

 

Two types of polyspermy are found in nature.      

(1)  Pathological polyspermy : In it the nuclei of all the sperms fuse with egg nucleus. In such type of condition  embryo development does not occur. (Due to polyploidy condition)

(2)  Physiological polyspermy : In physiological polysermy nucleus of only one sperm fuses with egg nucleus and rest of the sperm die in egg cytoplasm. Dead sperm are called merocytes. In physiological polyspermy normal embryo development occurs.

 

Polyspermy is absent in human beings. Polyspermy mostly occurs in megalecithal eggs.

Fate of sperm in egg - In majority of animals, only head and middle piece enter inside the egg and tail is left outside.

In mammals, whole sperm enters in the egg.

In some animals, only head of sperm enters in the egg tail and middle piece remain outside
e.g. Hydra, Neries etc. After entering inside the egg, sperm  rotates by 180°. All the structure  of sperm  dissolve in egg cytoplasm except sperm nucleus and proximal centriole. 

The centriole of egg itself degenerates at the time of second maturation division. So proximal centriole of sperm starts division, it divides into 2 daughter centrioles, which migrate towards opposite pole and start forming spindles.  

 

Fat of sperm nucleus : -

The nucleus of sperm absorbs water from egg cytoplasm and becomes enlarged. Now it is called male pronucleus.

After meiosis – II egg nucleus occur in the form of scattered vesicles then it is called as karyomeres and after some time ll the karyomeres assembled to form complete nucleus is called female pronucleus.

Male pronucleus and female pronucleus migrate through definite routes and come close to each other. These routes are called fertilization path. (It has following parts)

(1) Sperm penetration path - Male pronucleus for some distance, moves at the equator of egg. This is called sperm penetration path.

(2) Sperm copulation path - Male pronucleus starts migrating towards female pronucleus.

(3) Egg copulation path - Female pronucleus migrates towards male pronucleus. Both the pronuclei come close to each other.

(4)  Cleavage path - Both the pronuclei move together to their final position which is somewhere in animal pole. At this final position nuclear membrane of both the pronuclei degenerate and chromosomes of male and female pronuclei form pairs. The mixture of male and female chromosomes is called amphimixis.

Amphimixis was discovered by O.Hertwig in the eggs of sea – urchin. 

Newport was of all first observed the entry of sperm into the egg.

 

Significance of Fertilization

1.   Oocyte completes its second maturation division on coming in contact with the sperm.

2.   Amphimixis process leads to the formation of diploid zygote to restore the normal diploid number of the chromosomes.

3.   The centriole of sperm after entering into egg induces the egg to undergo cleavage.

4.   The paternal and maternal characters are transmitted to the off springs through  the process of fertilization.

5.   The peripheral changes occurring in the egg prevent the further entry of sperm into the ovum, thus checking polyspermy.

 

PARTHENOGENESIS OR VIRGINAL DEVELOPMENT

The development of embryo without fertilization is called parthenogenesis. The animals which are formed by unfertilized eggs are called parthenotes.

The discovery of parthenogenesis was done by Charles Bonet in the eggs of sea-urchins.

     

Parthenogenesis is of two types –

(a)  Natural           

(b)  Artificial

 

(a)  Natural parthenogenesis -

Some animals show parthenogenesis by nature e.g. Honey bees, wasps, ants, grass-hoppers, ticks, mites and sea-urchins. Natural parthenogenesis is of 2 types : -

(i)   Haploid parthenogenesis or Arrhenotoky  : - In this case eggs are formed by meiosis. Eggs are haploid, they have the power of fertilization sometimes male animals are developed by unfertilized eggs. In Honeybees, unfertilized eggs develop into males (drones), and fertilized eggs develop into queen and soldiers. Thus male honey bees are always haploid and queen with soldiers are always diploid.

(ii)  Diploid parthenogenesis or Thelytoky : - In this case, eggs are formed without meiosis division. Eggs are diploid they do not have the power of fertilization.

Diploid eggs give rise to female generation only. Male members are absent in these species.

Examples : - Lacerta sexicola armenica (lizard), Caresius aratus gibelio (Fish).

Diploid parthenogenesis may also be divided into two types –

 

(A) Ameiotic Thelytoky - In this type of parthenogenesis, during oogenesis first meiotic division does not take place but second meiotic division occurs as usual. In this situation the ovum still remains diploid. These ova, when reproduce parthenogenetically give rise to diploid off springs. For example, Trichoniscus, Daphnia, Daphnia pulex etc.

 

(B) Meiotic thelytoky - If the eggs are formed by normal oogenesis process, but by one or other reasons the eggs retain their diploid chromosomal number, then the parthenogenesis is called meiotic thelytoky. It may happen because of autofertilization. Some species of order – Lepidoptera exhibit this type of parthenogenesis.   

In some animals parthenogenesis alternates with normal sexual reproductive cycle. This is called cyclic parthenogenesis e.g., Honey bee.

In some animals, development of animals is always by parthenogenesis and  sexual reproduction is absent in these species. This is called complete parthenogenesis. Males are absent.

Example - Lacerta sexicola armenica (lizard)

(b)  Artificial parthenogenesis : -

This type of parthenogenesis is done by artificial methods. Artificial parthenogenesis is done by putting eggs in different atmospheres or by giving special stimulus to the eggs. Different artificial methods used for this purpose are as follows -

(1)  If we place eggs in brine or salt solution, KCl solution. Then eggs show parthenogenesis e.g. eggs of sea-urchins.

(2)  By short exposure of radiations on eggs or exposure of silk insect egg to sunlight.

(3)  If eggs are given shocks of temperature.

(4)   If eggs are pierced by needle dipped in the blood of same animal. The eggs of frog show parthenogenesis by  this method.
 

CLEAVAGE

The term 'Cleavage' was given by ''Von Baer''.

In fertilized egg or activated egg, the egg undergoes repeated cell divisions which occur rapidly producing a multicellular structure without changing its size. All these mitotic cells divisions collectively called cleavage  or segmentation. Due to the process of cleavage, a single celled zygote, through a successive mitotic cell divisions changes into a complex multicellular structure. Cells produced as a result of cleavage are termed as blastomeres. The total size of the embryo remains the same. Though the number of blastomeres as a result of mitotic cell divisions increases, the size of blastomeres gradually decreases are compared to parent cell. Interphase stage is very short in cleavage. In interphase only DNA duplication and histone protein synthesis takes place up to some extent. In the interphase of cleavage only 'S' phase is present, G₁ & G₂ phases are absent. Protein synthesis and RNA synthesis do not occur during this interphase. Nucleolus is absent in the nucleus of blastomeres. Size of blastomeres decreases during cleavage. When size of blastomere becomes equal to that of size of somatic ells, the divisions of cleavage are stopped. Only normal cell division take place. Cleavage can be observed till onset of gastrula stage. After gastrulation, cleavage is completely checked. Nucleous appears first in gastrula stage. The consumption of oxygen is increased during cleavage.

 

CLEAVAGE PLANE :       

The traveling path of cleavage furrow in fertilized egg is called cleavage plane. Different animal eggs show different cleavage planes : -

 

PATTERNS OF CLEAVAGE :

(a)  Radial Cleavage - In this pattern, cleavage furrows are straight and form right angle with each other. In
 this case I, II cleavages are meridianal, which are at right angle to each other. III cleavage is equatorial In this way 8-celled octate is formed. In radial cleavage, 4 blastomeres of upper tier and 4 blastomers of lower tier are on sampe plane i.e, Blastomeres are arrangement in radial symmetry in the beginning.

 

(b)  Biradial Cleavage - In this pattern, first two cleavages are meridianal and at right angle to each other all III cleavage is vertical. In 8-celled stage 4 blastomeres of central zone are bigger and 4 blastomeres of peripheral region are smaller.

Examples : - In the eggs of Ctenophora.

 

(c)  Bilateral Cleavage - In this patterhn, first two cleavages are meridianal in same plane and III Cleavage is transverse. i.e., embryo shows bilateral symmetry in 8-celled stage. The blastomeres of one side are smaller and blastomeres of other side are larger.

Examples : - In the eggs tunicata, cephalochordata, amphibian and amphioxus.

 

(d)  Spiral Cleavage - The cleavage furrow passes obliquely. In this pattern 4 blastomeres of  lower tier rotate  clockwise or anticlock wise. If this rotation is clockwise,  then it is called dextral spiral cleavage e.g. In Mollusca. If this rotation of lower tier blastomeres is anti-clockwise, then pattern is called sinistral spiral cleavage, e.g. In helminthes and annelida.

 

CLASSIFICATION OF CLEAVAGE :

On the basis of fate of blastomeres : -

A.  Determinate Cleavage - In this pattern of cleavage, the fate of blastomeres is fixed, determined i.e. each blastomere forms a particular portion of embryo. If (by certain reason) any blastomere is damaged or destroyed, then the part of embryo (which would have development from that blastomere) will be absent e.g., Nematoda, Annelida, Mollusca and Some chordates like amphibian & ascidians.

B.   Indeterminate Cleavage - In this type of cleavage, the fate of blastomeres is not definite. All the blastomeres form all the parts of embryo. If some blastomeres are lost, no loss is observed in this embryo. If in the early stages of cleavage, the embryo is cut into small pieces, then each piece of embryo will develop into a complex embryo, and all the embryos are identical. So identical twins are monozygous. This is the basis of embryo cloning.

 

CLASSIFICATION OF CLEAVAGE :

On the basis of amount of Yolk : -

A scientist named Balfour gave a law. According to him, rate of cleavage is inversely proportional to amount of yolk present in the egg. The yolk present in egg, disturbs the rate of cleavage. The rate of cleavage is slow in that part of egg, in which amount of yolk is more, and the rate of cleavage is faster in the portion of egg in which yolk is in lesser amount. Mostly cleavage is of 2 types : -

 

A.  Complete or holoblastic : - When cleavage  furrow passes through the egg completely. As a result of this the whole egg divides. Holoblastic cleavage is found in all the eggs except megalecithal eggs. The whole egg divides into blastomeres. No part of egg remains undivided. It is of 2 types : -

(a)  Equal holoblastic cleavage - In those eggs, in which amount of yolk is less and it is distributed evenly in the egg, cleavage occurs in whole egg, blastomeres (So formed) are of same size. All the parts of egg show same rate of cleavoage.Example : - It is found in microlecithal and isolecithal eggs.  

(b)  Unequal holoblastic cleavage - In those eggs in which amount of yolk is medium and it is distributed unevenly in the egg. The blastomeres are bigger and less in number where the concentration of  yolk is higherin the egg. The part of egg which contains small amount of yolk, blastomeres here formed are smaller and more in number. The bigger blastomeres are called megamere and smaller blastomere are called micromeres.

Examples : - Unequal holoblastic cleavage is present in mesolecithal and telolecithal eggs and human eggs.

 

B.   Meroblastic cleavage - This cleavage is found in megalecithal eggs, in which amount of yolk is large. Cleavage does not occur in that part of egg, where yolk is present cleavage occurs only Cytoplasmic part, yolk remains undivided. Meroblastic cleavage is of 2 types on the basis of distribution of yolk in egg.

(a)  Discoidal meroblastic cleavage  - Cleavage occurs only in blastodisc of egg. This is mainly found in megalecithal or polylecithal eggs, because in these eggs, cytoplasm is found in the form of a disc.  

Examples : - Reptilian eggs and birds eggs.

(b)  Superficial meroblastic cleavage – In insect egg, central cytoplasm shows free central division, due to which so many nuclei are formed. All these nuclei migrates towards peripheral cytoplasm.
Cleavage occurs only in peripheral region. As a result of this, a superficial layer of blastomeres is formed around the yolk. This type of cleavage is also called superficial meroblastic cleavage.

Example - In centrolecithal eggs

Significance of Cleavage -

1.   There is no change in shape and size developing embryo till blastula stage comes. Till then it remains
 just like undivided egg in shape.

2.   As a result of cleavage, unicellular zygote changes into multicelluar structure.

 

HUMAN EMBRYOLOGY

Fertilization-

Fertilization is the union of two opposite types of gametes, spermatozoa and ova. The semen is a mixture of spermatozoa and accessory fluids. Once deposited with in the vagina, the spermatozoa proceed on their journey into and through the uterus and on up into the oviducts. Although spermatozoa can swim several millimeters each second, their trip through the uterus and to the oviducts requires on increase in their motility.

On the first hand, ejaculation of semen in the vagina triggers motility of spermatozoa. This is aided further by muscular contraction of the walls of the uterus and the oviducts. An additional increase in sperm motility occurs due to activation of  the sperm by the viscous liquid secreted from the secreted cells of the epithelial lining of oviduct mucosa. This phenomenon of sperm activation in mammals is known capacitation. It takes about 5-6 hours for capacitation.

Before fusion of a spermatozoan with the egg, the spermatozoa are to penetrate a few barriers, the egg membranes, which cover the egg. The activated spermatozoa undergo acrosomal reaction and release varies chemicals, like hyaluronidase that acts on the ground substances of follicle cells, corona penetrating enzyme that dissolves corona radiata, and zona lysine which perforates the zona pellucida. All these chemicals are contained in the acrosome, located at the tip of the sperm head, and are collectively termed sperm lysins. An average human ejaculate of 3-4 ml of semen contains 80-100 million spermatozoa. Out of these, only one will succeed in entering the egg and fertilizing it. Fertilisation of egg with only one spermatozoan is known as monospermy.   

While the ovarian follicle is growing, the oogonium within it undergoes maturation. The oogonium enlarges to form a primary oocyte. The primary oocyte undergoes the first meiotic division to shed off the first polar body and becomes a secondary oocyte (fig. 5.2A). At the time of ovulation, the second meiotic division is in progress and a spindle has formed for separation of the second polar body (fig. 5.2B)

At this stage the 'ovum' enters the infundibulum of the uterine tube and passes into the ampulla (fig. 5)

Fertilization of the ovum occurs in the ampulla of the uterine tube. One spermatozoon pierces the zona pellucida and enters the ovum. In response to egg sperm binding two things happen

(1) depolarization of egg membrane

(2) cortical reaction i.e. formation of fertilization membrane to prevent polyspermy.

(After one spermatozoon has entered the ovum other spermatozoa cannot enter it). After the entry of the spermatozoon, the second  polar body is extruded. The chromosomes of the ovum now assume the shape of a nucleus called the female pronucleus. At the same time the head of the spermatozoon (which it will be remembered is formed from the nucleus) separates from the middle piece and tail, and transforms it self into the male pronucleus (Fig. 5.2 D).

The male and female pronculei meet, but they do not fuse to form one nucleus. Their nuclear membranes  disappear and their chromosomes become distinct. It will be recalled that each pronucleus has 23 chromosomes so that the fertilized ovum now has 46 chromosomes in all (fig 5.3 A).

Each of these 46 chromosomes splits into two (fig. 5.3B). Meanwhile, a spindle has formed, and one chromosome of each pair moves to each end of the spindle (as in mitosis). Leading to the formation of two daughter cells fig. (5.3C). This is called the two-cell stage of the embryo. Note that stricktly speaking there is no one-cell stage of the embryo (fig. 6A)

Important points note at this stage are that :

(i)   the two daughter cells are still surrounded by the zona pellucida :

(ii)  each daughter cell is much smaller than the ovum. As subsequent divisions occur the cells become smaller an smaller until they acquire the size of most cells of the body.

 

CLEAVAGE :

The two cells formed as described above undergo a series of divisions. One cell divides first so that we have a '3-cell' stage of the embryo (fig.6B) followed by a '4-cell' stage (fig. 6C), a 5-cell, stage etc. This process of subdivision of the ovum into smaller cells is called cleavage. These cells are called blastomeres.          

As cleavage proceeds the ovum comes to have 16 cells. It now looks like a mulberry and it called the morula (fig. 6D). It is still surrounded by the zona pellucida. If we cut a section across the morula we see that it consists of an inner cell mass that is iscompletely surrounded by an outer layer of cells, (Compaction). The cells of the outer layer will later give rise to a structure called the trophoblast (fig. 7A). The inner cell mass gives rise to the embryo proper, where as the cells of the trophoblast help to provide nutrition to the embryo.

Some fluid now passes into morula from the uterine cavity, and partially separates the cells of the inner cell mass from those of the trophoblast (fig. 7B). As the quantity of  fluid increases, the morula acquires the shape of a cyst. The cells of the trophoblast  become flattened and the inner cell mass comes to be attached to the inner side of the trophoblast on one side only (fig. 7C). The morula has now become a blastocyst. That side of the blastocyst to which the inner cell mass is attached is called the embryonic or animal pole. While the opposite side is the vegetable or abembryonic pole.

 

Function of the zona pellucida -

The trophoblast has the property of being able to stick to the uterine (or other) epithelium and its cells have the capacity to eat up other cells. As the embryo travels down the uterine tube, and the upper most part of the uterine cavity ; it is prevented from 'Sticking' to the epithelium by the zona pellucida.

 

FORMATION OF GERM LAYERS (GASTRULATION) :

As the blastocyst develops further, it gives rise not only to the tissues and organs of the embryo, but also to a number of structures that support the embryo and help it to acquire nutrition. At a very early stage in development, the embryo proper acquires the form of a three-layered disc. This is called embryonic disc (also called embryonic area, embryonic shield, or germ disc) The three layes that constitute this embryonic disc are.

(i)   Endoderm (endo = inside)

(ii)  Extoderm (ecto = outside)

(iii) Mesoderm (meso = in the middle)

These are the three germ layers. All the tissues of the body are derived from one or more of these layers. We have seen that the blastocyst is a spherical cyst lined by flattened trophoblastic cells, and that inside it there is a mass of cells, the inner cell mass, attached eccentrically to the trophoblast Further changes are as follows 

(a)  Some cells of the inner cell mass differentiate (i.e. they become different from others) into flattened cells, that come to line its free surface (fig. 8A). These are called hypoblast and constitute the endoderm, which is thus the first germ layer to be formed.       

 

(b)  The remaining cell of the inner cell mass become columnar (fig. 8D). These are called are called epiblast and form the second germ layer, the ectoderm. The embryo is now in the form of a disc having two layers.

 

(c)  A space appears between the ectoderm (below) and the trophoblast (above). This is the amniotic cavity (fig. 8C), filled by amniotic fluid, or liquor animal. The roof of his cavity is formed by amniogenic cells (cells of Rauber) derived from the trophoblast, while its floor is formed by the ectoderm.

 

(d)  Flattened cells arising from the endoderm, spread and line the inside of the blastocystic cavity. In this way, a cavity, lined on all sides by cells of endodermal origin, is formed. This cavity is called the primary yolk sac (fig. 8D)

 

(e)  The cells of the trophoblast give origin to a mass of cells called the extra-embryonic mesoderm (or primary mesoderm). These cells come to lie between the trophoblast and the flattened endodermal cells lining the yolk sac, thus separating them from each other. These cells also separate the wall of the amniotic cavity from the trophoblast (fig. 9A).

This mesoderm is called extra-embryonic because it lies out side the embryonic disc. It does not give rise to any tissues of the embryo itself.

(f)  Small cavities appear in the extra-embryonic mesoderm. Gradually these join together to from larger spaces and, ultimately, on large space is formed. This cavity is called the extra-embryonic coelom (fig. 9B), it will be seen that  the extra-embryonic coelom does not extend into that part of the extra-embryonic mesoderm which attaches the wall of the amniotic cavity to the trophoblast. The development embryo, along with the amniotic cavity and the yolk sac, is now suspended in the extra-embryonic coelom, and is attached to the wall of the blastocyst (i.e. trophoblast)  only by this unsplitted part of the extra-embryonic mesoderm. This mesoderm forms a structure called the connecting stalk.

 

(g)  Formation of chorion and amnion : At this stage, two very important membranes are formed. One is formed by the parietal extra-embryonic mesoderm (on the inside) and the overlaying trophoblast
(on the outside) this is called the chorian (fig. 9B). The other is the amnion which is constituted by the amniogenic cells forming the wall of the amniotic cavity (excluding the extodermal floor). These cells are derived from the trophoblast. We have already seen that the amnion is covered by the unsplit extra-embryonic mesoderm, and that the connecting stalk is attached to it.

 

(h)  With the appearance of the extra-embryonic mesoderm, and later of the extra-embryonic coelom, the yolk sac becomes much smaller than before and is now called the secondary yolk sac. This alteration in size is accompanied by a change in the nature of the lining cells. They are no longer flattended but become cubical (fig. 9B).

 

(i)   At this stage, the embryo proper is a circular disc composed of two layers of cells : the upper layer (towards amniotic cavity) is the ectoderm, the cells of which are columnar, while the lower layer (towards yolk sac) is the endoderm, made up of cubical cells (fig. 10)

 

(j) At one circular area a near the margin of the disc, the cubical cells of the endoderm become columnar.This area is called the prochordal plate. The appearance of the prochordal plate determines the central axis of the embryo (i.e. enables us to divide it into right and left halves), and also enables us to distinguish its head and tail ends (fig. 10)

 

(k)  Soon after the formation of the prochordal plate some of the ectodermal cells lying along the central axis, near the tail end of the disc, begin to proliferate, and form an elevation that bulges into the amniotic cavity. This elevation is called the primitive streak (fig. 11). The primitive streak is at first a rounded or oval swelling primitive streak later forms the Henson's node.

 

 (i)  The cells that proliferate in the region of the primitive streak pass sideways, pushing themselves between the ectoderm and endoderm (fig. 11). These cells from the intra-embryonic mesoderm (or secondary mesoderm) which is the third germ layer.

 

Notochord

The notochord is a midline structure, that develops from mesoderm in dorsal region.

 

Importance of the notochord

The notochord is present in all animals that belong to the phylum Chordata. In some of them e.g. Amphioxus, it persists into adult life and forms the central axis of the body. In others, including man, it appears in the embryo but only small remainants of it remain in the adult. Notochord elongates considerably, and lies in the midine, in the position to be position to be later occupied by the vertebral column. However, the notochord does not give rise to the vertebral column.

Neurulation : i.e formation of neural tube. After neurulation there occur three type of ectoderm.

(i)   Somatic ectoderm

(ii)  Neural tube ectoderm       

(iii) Neural crest ectoderm.

Anterior part of neural tube differentiate brain and rest of the neural tube differentiate in spinal cord so central nervous system is formed by neural tube.

 

TERATOGENY :

During the first 3 months of pregnancy the basic structure of baby is formed. This involves cell division, cell migration, and differentiation of cells into the many types found in the body. During this period, the developing baby called foetus is very sensitive to anything that interferes with developmental steps. Eg.

Virus infection of mother by rubella (German measles) virus or exposure to certain chemicals may cause malformation in the developing embryo. Such agents are called teratogens (Monster Forming agents).

 

GENERAL STAGES OF EMBRYONIC DEVELOPMENT

1. Morula - As a result of segmentation or cleavage activities, unicellular zygote changes into a solid ball like multicellular structure. In the later stage of cleavage, clusters of sticky, cohering, protruding (otuside) blastomeres are produced, which look like mulberry. This stage is termed as morula stage.

2.   Blastulation - Cleavage continues in solid ball like morula and new formed blastomeres start rearranging themselves. Cell-aggregation starts in blastomeres, due to the movement of these blastomeres a cavity appears in the embryo, it is called as blastocoel. This cavity is schizogenous cavity in origin i.e. it is formed by the separation of cells. Cell aggregation is also known as cohesion. Blastomeres arrange themselves in the form of a layer around the blastocoel, this layer is termed as blastoderm. The embryonic stage is now called blastula, and its formative activites is called blastulation.

 

Types of blastula

The shape of blastula depends on so many factors e.g. size of eggs, amount of yolk, distribution of yolk in the eggs, frequency of cleavage and number of cleavage divisions. According to these factors, we can classify blastula of different animals in different categories.

(a)  Coeloblastula                    

(b)  Stereoblastula                   

(c)  discoblastula                     

(d)  Blastocyst                         

(e)  Superficial blastula or Periblastula

 

(a) Coeloblastula - Blastocoel is wide and clear in this blastulation, it is completely surrounded by blastomeres on all the sides i.e, blastocoel cavity is situated totally inside the embryo. Blastomeres are very small in size as compared to blastocoel.

Example : - Eggs of Amphioxus, coelenterate, amphibia, sponges etc.

Coeloblastula of amphibians is called amphiblastula because in it blastocoel cavity is accentric in position and is more towards the animal pole (amount of yolk is more towards the vegetal pole).

Amphiblastula of sponges is a free swimming larva, its blastomeres are flagellated. This larva swims freely with the help of these flagella. It is a unique feature in sponges.

 

(b)  Stereoblastula - In this blastula, blastocoel is very narrow or obliterated. Blastomeres are large as compared to blastocoel. It is almost solid.

Example - eggs of  Neries, and members of phylum Mollusca.      

 

(c)  Discoblastula - It is found in those animals which have discoidal eggs. There is a cavity present in between blastomeres and yolk, it is called subgerminal cavity. It is a type of cavity, which is surrounded by yolk on one side and by blastomeres on the other side.

Examples - eggs of reptiles, birds and prototheria.

 

(d)  Superficial blastula or periblastula - In centrolecithal eggs, cleavage occurs only in peripheral region. The layer of  blastomeres surrounds the centrally situated yolk. Blastocoel is absent in this type of blastula. In place of blastocoel, subgerminal cavity is found.

Example - Eggs of insects.

 

(e) Blastocyst - blastula or Eutherian & Metahterian mammals is called blastocyst, because blastula is in the form of a cyst

Blastula - of mammals is called blastocyst. In blastocyst all the embryonal cells occur in the form of solid mass called embryonal knob. Embryonal  knob (inner cell mass) is covered by protective layer called trophoblast and it's cell just above the embryonal knob are trophoblast called  cells of Rauber (amniogenic cells). There occurs a cavity in between embryonal knob and trophoblast called albumin cavity. It is filled with nutritive fluid absorbed from the wall of uterus. So albumin cavity is also nutritive-cavity.

 

SPECIAL POINT

1.   The growth phase is the longest phase during male gametogenesis. But in human oogenesis, maturation phase is longest.

2.   The acrosome of sperm are produced by golgibodies.

3.   The smallest sperm is of crocodile and its size of 0.02 mm & largest sperm is of Discoglossus (2 mm)  

4.   74 days are required to complete the cycle of spermatogenesis in human being.

5.   In 1 ml of semen, 20 to 120 millions of sperms are present in human being.

6.   Deficiency in the number of sperms result in sterility which is known as oligospermia.

7.   Absence of sperms in semen is known as azoospermia .

8.   Formation of yolk in oogenesis takes place in the growth phase.

9.   Largest egg is of Ostrich (16 cm long with its shell).

10. Although normal number of sperm are present in semen but if these are completely non motile. The condition is known as necrospermia.

11. Smallest egg in birds is of humming bird.

12. Due to high mortality rate in lower animals, the production of egg is more.                                                   

13. Sequence of egg production is as follows.

Mammals < Aves < Reptiles < Amphibian < Pisces.

14. Cat and rabbit both are induced ovulator.

15. The life span of eggs in female reproductive organs in human being is 48 hrs.

16. The nucleus of egg is known as germinal vesicle.

17. At the age of 45-50 yrs. In female the ovulation process will stop which is known as menopause.

18. The spermiation (release of sperms from sertoli cells) in all sertoli cells occurs simultaneously.

19. Cortical granules are absent in rat.

20. Mosaic type of cleavage is found in the parasite Echinococcus granulosus.

     

Special features of some animals :

(a)  Sperms of some animals are not having flagella :

eg.    

(1)  Ascaris - sperm is amoeboid

(2)  Cray fish - star shaped, tail less sperm

(3)  In crab and lobuster the sperm are tail less and have three sharp process.

 

(b)  Biflagellated sperm :

eg.  In toad fish (Opsansus) head of many sperms unite together and form sperm boats

In Gastropods, the sperms are hexaflagellated.

Smallest sperm – Crocodile (0.02 mm)

Largest sperm – Discoglossus (2 mm) in chordates and Drosophila in entire animal kingdom.

 

Shape of head part of sperms :

(i)   Spherical – eg Teleostei

(ii)  Lance shaped – eg Amphibia and Reptiles

(iii) Spiral end – eg. Birds

(iv) Spoon shaped – eg. Mammals (in man)

(v)  Hook like – eg. Rat.

 

Germinal layers and their derivatives

The following description gives an account of the respective organs formed by the three germ layers. Most of the organs are the product of combination of more than one germ layers.

 

Organs derived from ectoderm

1.   Skin (epidermis) and their pigment cells.

2.   Mucosal membrane of lips, cheek gums, basal portion of mouth, some part of palate, nasal apertures.

3.   Lower part of anal canal.

4.   Glans penis.

5.   Labia majora and outer part of labia minora.

6.   Anterior epithelium of cornea, epithelium of conjunctiva, ciliary body and iris of eyes.

7.   Outer face of tympanic membrane, epithelium of labyrinth.

8.   Glands :

(i)   Exocrine –

(A) Sweat glands                     

(B)  sebaceous glands            

(C)  parotid glands

(D) mammary glands                          

(E)  lacrimal glands ;

 

(ii)  Endocrine –

(A) Hypophysis cerebri                    

(B)  adrenal medulla

 

9.   Hairs, nails, enamel of teeth

10. Lens of eyes.

11. Nervous system.

 

Derivatives of mesoderm

1.   Connective tissues, superficial and deep fascia, ligaments, tendons, dermis of skin. (from dermatome)

2.   Specialized connective tissues like adipose tissue, reticular tissues, bones, cartilages.

3.   Teeth.

4.   All muscles.

5.   Heart, all blood vessels and blood cells.

6.   Kidneys, ureters, urinary bladder, posterior urethra of female, upper glandular part of prostate.

7.   Ovaries, uterine tubes.

8.   Testes, epididymis, vas defeens and seminal vesicle, ejaculatory duct.

9.   Pleural cavities, peritoneal cavity and pericardial cavity.

10. Joints.

11. Cornea, sclera, choroid ciliary body and iris related material.

12. Microglia, duramater etc.

 

Derivatives of endoderm

1.   Epithelial part of mouth, some part of palate, tongue, tonsils, pharynx, oesophagus, stomach, small
 and large intestine, upper part of anal canal.

2.   Pharyngo-tympanic tube, middle ear, inner face of tympanic membrane.

3.   Respiratotry tract.

4.   Gall bladder, pancreatic duct.

5.   Major protion of urinary bladder, complete urethra of female except posterior part, complete urethra
 of male except anterior and posterior part.

6.   Whole inner part of vagina including inner face of labia minora.

7.   Glands :

(i)   Exocrine –

(A) Liver                  

(B)  Pancreas

 

(ii)  Endocrine –

(A) Thyroid              

(B)   parathyroid            

(C)  thymus                 

(D)  islets of Langerhans

In addition to the above, the glands of gastrointestinal tract, major part of prostate etc. are also formed by endoderm.

 

Implantation

The attachment of developing embryo to the appropriate body layer or surface to obtain nutrition is called implantation. This phenomenon is a common event in most mammals (except prototheria) in which embryo (blastocyst stage) after reaching in uterus attaches itself with the wall of the uterus. In other animals like fishes, reptiles, birds, prototherian mammals etc., this nutritive connection is established with the yolk present in egg. In higher mammals including men, the blastocyst on its contact with endometrium of uterus gets completely buried in the wall of the uterus.

Initially the oocyte after its  release from ovary, comes into fallopian tube where the process of fertilization is completed, Just after fertilization, embryonic development starts and a blastocyst is formed after cleavage and morulation. In human being, the blastocyst gets attached with the uterine endomdetrium in about four days after entering in uterus. At the same time, the cells of endometrium of implantation area separate out and adhere with embryonic cells with the help of certain enzymes secreted by the cells of trophoblast. In human, the site of implantation is generally mid-dorsal or mid-ventral part of uterus. Implantation of blastocyst takes about 7-8 days after fertilization in human and by 12^th day it is completely buried in the wall of the uterus. The place of entry through which the embryo enters into the wall, is completely closed by a fibrous and cellular plug, known as closing coagulum.

 

Types of Implantation

On the basis of the position of attachment in the uterus, implantation is of three types -

1.   Central or Superficial implantation - In this type the blastocyst attaches  superficially with the wall of uterus,  and remains suspended in the lumen of the uterus. This type of implantation occurs in lower chordates, e.g. cow, pig, dog etc.

 

2.   Interstitial implantation – The blastocyst is buries deeply inside the wall of uterus and covered by endometrial tissues lying under epithelium. This type of implantation occurs in human being.

 

3.   Eccentric implantation – It occur in rat, squirrel etc. In this type of implantation , the blastocyst settles in the flods of epithelium of uterus. After some time it is completely surrounded by these folds.

 

9 months      –          placenta attains maximum size, nails on fingers appear. In the next 10 days the foetus is ready to born as a little bady.

The above mentioned timing are approximate time periods. Some times, due to some reasons, certain babies are born before stipulated time. The babies born in 7^th month may also survive as normal babies.

 

EXTRA EMBRYONIC MEMBRANES AND PLACENTA

Extra embryonic membranes

In chordates like reptiles, birds and prototherian mammals, shaped blastula is a disc structure called as blastodis. The cellular layer formed of blastomeres remains as blastoderm. The central part of blastoderm gives rise to embryo proper, while the peripheral portion does not take past  in the formation of embryo. This peripheral part is known as extra embryonic region. This region takes part in the formation of certain membranes called extra embryonic membranes. These extra embryonic membranes provide facilities for nutrition, respiration and excretion to the embryo. Extra embryonic membranes are of four types –

1.   Amnion                      

2.      Chorian                     

3.     Yolk sac             

4.     Allantois

 

On the basis of presence of absence of amnion, two groups of vertebrates are categorised.

1.   Amniota - This group is characterized with the presence of amnion in the embryos of its members. For example members of class Reptilia, Aves and Mammalia.

2.   Anamniota - Animals of this group are devoid of amnion in their embryos. For example class cyclostomata, pisces and amphibia.

 

Extra embryonic membranes in human

The process of gastrulation in embryo results into the formation of endoderm or hypoblast, ectoderm or epiblast, amniotic cavity, yolk sac, extra embryonic parietal and visceral mesoderm, connecting stalk etc.  Extra embryonic membranes are also formed during this process. Each extra embryonic membrane is derived from two layers.

 

1.   Amnion - It is formed by the layer of amniogenic cells present around the amniotic cavity and the extra embryonic mesoderm. Extra embryonic mesoderm layer surrounds the amnion. The connecting stalk is also attached with it. With a gradual increase in size the amnion covers the embryo from all sides. After about eight weeks of fertilization, amnion is completely incorporated into connecting stalk, which finally forms the umbilical cord. Embryo, in this stage, is called as foetus remains hanging in amniotic fluid.

2.   Chorion - It is formed by the extra embryonic parietal layer of mesoderm and the cell of trophoblast. After implantation of blastocyst, the trophoblast gives out several figner like processes, the chorionic villi which get embedded into uterine endometrium Mesoderm also contributes in the formation of these villi. After a period of four these villi disappear from all parts except the connecting stalk where they grow rapidly and participate in the formation of placenta.

3.   Yolk sac - Yolk sac is formed by the cells of extra embryonic visceral mesoderm and endoderm. Initially the sie of yolk sac is larger as compared to that of the embryo. About eight  weeks after fertilization, the yolk is reduced in size and changes into a tubular structure. Ultimately a placenta is developed with the incorporation of yolk sac and mesodermal connecting stalk with the amnion and chorion.

4.   Allantois - It is a solid and cylindrical mass formed by embryonic mesoderm. A small cavity lined by endodermal cells develops in it. The mesoderm of allantois forms many small blood vessels in this region. These vessels connect the embryo with placenta and ensure nutritional and respiratory supply to embryo. In human, allantois does not function to store the excretory wastes as it does in reptiles and birds.

 

 

PLACENTA

The eggs of viviparous animals are unable to develop into their embryos outside the uterus independently. This is because of the very little or negligible amount of yolk present in these eggs, which can not fulfill the nutritional and other physiological demands of a developing embryo. Here the embryo depends upon maternal tissues for shelter, nutrition, respiration etc. These animals therefore, have developed adaptation, respiratory and other physiological requirements from mother's body.

Placenta is found in all viviparous (exept sub-class-prototheria; oviparous) animals.

 

Structure of Placenta

Placenta is not a simple membrane. It is made up of the tissues from two different sources –

1. Maternal tissue  - These include uterine epithelium, connective tissues and blood capillaries.

2. Embryonic tissue - These include extra embryonic membranes (mainly chorion). Yolk sac and allantois may also take part in placenta formation.

 

Embryonic connective tissues and blood capillaries are also constituents of it. On the basis of extra embryonic membranes, the placenta is of three types.

1.   Yolk sac placenta - It is formed by yolk sac and uterine epithelium. For example, Elasmobrancs (Sharks), Mustelus etc.

 

2.   Choria-vitelline placenta - It is formed by chorion and yolk sac combinely. Hence it is called as choriovitelline placenta. For example, Didelphis, Macropus and other metatherian mammals.

 

3.   Chorio-allantoic placenta - This type of placenta is formed by embryonic chorion and allantoic membranes. It is also referred to as a true placenta. It is found in eutherian mammals.

 

Chorio- allantoic placenta in mammals.

1.   In this type of placenta, allantoic mesodern and the mesoderm of umbilical cord jointly form the blood vessels of umbilical cord. The endodermal part of the allantois remains as a very small cavity.

2.   To obtain nutrition from maternal blood several finger like processes or villi are formed by chorion which penetrate deeply into the crypts of uterus. Initially the villi are scattered over the whole surface of chorion but later they become restricted in the deciduas besalis region. The chorionic villi on the remaining surface disappear shortly. The part of chorion, which helps in placenta formation is known as chorionic frondosum.

 

Classification of Placenta

On the basis of different characters, the placenta are classified in following manner –

                       

1.   On the basis intimacy

After implantation, the wall of uterus is called as deciduas, instead of endometrium. The part of

deciduas, where placenta is formed is called deciduas basalis whereas, the part separating the embryo from lumen of uterus is called deciduas capsularis. The remaining part of lumen of uterus is called deciduas parietalis. Decidua also comes out from uterus at the time of parturition. On the basis of intimacy between embryo and uterine wall the placenta is classified into three classes –

 

(i)   Non-deciduate or Semi placenta - In this type of placenta, there is no close and rigid association between embryo and the wall of uterus. Hence, at the time of parturition, there is no bleeding as the chorionic villi are easily pulled out from the crypts of uterus. For example, cow, buffalo, horse, pig.

 

(ii)  Contra-deciduate placenta - There is a close association between embryonic and maternal tissues. However at parturition, the damaged maternal and embryonic tissues along with the part of placenta remain inside the uterus which are absorbed is situ by leucocytes. For example – Parameles, Talpa etc.

 

(iii) Deciduate placenta - This type of placenta is found in human, dog, hare etc. It is characterized with a very close association between chorionic villi and uterine wall. At the time of birth, the mucosal covering of the uterus is also damaged and discarded outside. This results in an extensive bleeding at child birth. This placenta is known as a true placenta.

 

2.   On the basis of implantation

Three types of placenta are found on the basis of implantation.

(i)   Superficial - When the placenta is situated in the lumen of uterus. For example, Parameles, pig, cow,
      cat etc.

(ii)  Eccentric - The placenta is situated in the fold or pocket of the cavity of uterus. For example - rat,
      squirrel etc.

(iii) Interstitial – This type of placenta is found in man, guinea pig, apes etc. The chorionic sac (placenta) penetrates deep inside the wall of uterus. Hence, the association between embryo and maternal part becomes very close.

 

3.   On the basis of distribution of villi

On this basis, the placenta are of four types.

(i)   Diffused placenta – The villi are scattered on the whole surface of placenta. For example pig, horse, lemur etc.

(ii)  Cotyledonary placenta – The villi are distributed in small isolated groups on the chorionic surface. These groups of villi are called as cotyledons. For example, cow, buffalo, sheep, deer etc.

(iii) Zonary placenta - This type of placenta have the villi distributed in a belt shaped zone which is large sized and circular.

Zonary placenta is of two types –

(a) Complete zonary  placenta - The belt of villi is complete and ring shaped in it. For example - dog, cat, lion etc.

(b) Incomplete Zonary placenta - The belt of villi is incomplete in it. For example - raccoon.

(iv) Discoidal placenta - In this type of placenta, whole of the chorionic surface is covered by villi in initial stage, but the villi disappear later from later from most area except the region of implantation, that is only a disc like region is left with villi. Discoidal placenta is also of two types –

(a)  Mono discoidal placenta - The villi are present only on dorsal surface in single circular disc like area. For example – human, hare etc.

(b)  Bi discoidal placenta - If the villi are distributed in two disc like areas, the placenta is called as bidiscoidal, eg. Monkeys.

 

4.   On the basis of histology

The blood of maternal and embryonic do not mix together through placenta. The blood circulations of the two sides are kept separated by one or more layers described below –

The transportation of various materials takes place by diffusion through these six layers the intimacy between maternal and embryonic tissues in different mammals is determined by the presence or absence of these layers in placenta. Therefore, on the basis of presence or absence of the above layers, the placenta is of five types.

1.   Epitheliochorial - It is the most primitive type of placenta in which all the six layers. mentioned
earlier, remain intact. For example pig, horse etc.

 

2.   Syndesmochorial - In this type of placenta the uterine epithelium is eroded by chorionic villi, so only two maternal layers remain functional. Therefore, along with three foetal layers, total five layers are preent in this placenta. For example - sheep, goat, cow etc.

 

3.   Endotheliochorial - Here uterine connective  tissue layer is also damaged along with uterine epithelium layer. Therefore only four layers (3 foetal and one maternal) are found in this placenta eg. dog, cat, etc.

 

 4.   Haemochorial – All the three maternal layers are penetrated in this placenta. The chorionic epithelium comes in direct contact with uterine blood sinusoids. For example – man, monkey, bat etc.  

 

5.   Haemoendotheliochorial/Haemoendothelial : It is the most typical placental in which the  trophoblastic epithelium of embryo is also eroded along with all three maternal layers. The foetal capillaries are in direct contact with maternal blood. For example rat, guinea pig, rabbit etc.

 

      The placenta of human mainly secretes two steroid hormones like estradiol and progesterone, and two protein hormones like human chorional gonadotropin HCG and human placental somatomammotropin HCS large amount of –HCG, hormone is secreted, during early pregnancy, from the placenta. Because of this reason its quantity increases in the urine of pregnant lady. On the basis of this fact, pregnancy test is performed. The above hormones are also held responsible for keeping the corpus luteum active, protection of embryo, prevention of abortion and growth of mammary glands.

 

Functions of placenta

1.   Exchange of important materials between foetal and maternal blood.

2.   The essential materials are exchanged by diffusion, pinocytosis or active transport.

3.   The small molecules like O₂, CO₂, H₂O etc. and other inorganic substances like chlorides, phosphates, sodium,  potassium, magnesium etc. are also diffused through placenta.

4.   Large molecules like lipids, polysaccharides, carbohydrates proteins etc. are obtained by pinocytosis process.

5.   The nutritional substances are supplied to embryo from the mother through placenta.

6.   Placenta also serves as a respiratory medium for exchange of O₂ and CO₂ between embryo and mother.

7.   The nitrogenous and metabolic wastes from foetus are released into the blood of mother by diffusion through placenta.

8.   The antibodies for measles, chickenpox, polio etc. present in the blood of mother reach the embryo through placenta.

9.   Pathogenic viruses may also enter in embryo through placenta.

10. If a female takes some harmful chemicals, liquor, drugs etc. during pregnancy, these may cross the placenta and on reaching into foetus may cause deformity during organogenesis. (eg. Thallidomide)

11. Placenta itself secretes some hormones like progesterone, estrogen, lactogen, HCG, HCS etc.

12. Progesterone, maintains and supports the foetus during the whole pregnancy period. At the time of parturition, relaxin is secreted by placenta which lubricates and widens the birth canal to facilitate child birth.      

 

REPRODUCTIVE HEALTH

Methods of contraception 

An ideal contraceptive should be user-friendly, easily available, effective and reversible with no or least side-effects. It also should in no way interfere with the sexual drive, desire and/or the sexual act of the user. A wide range of contraceptive methods are presently available which could be broadly grouped into the following categories, namely Natural/Traditional, Barrier, IUDs, Oral contraceptives, Injectables, Implants and Surgical methods.

(1) Natural methods :

work on the principle of avoiding chances of ovum and sperms meeting. Periodic abstinence is one such method in which the couples avoid or abstain from coitus from day 10 to 17 of the menstrual cycle when ovulation could be expected. As chances of fertilisation are very high during this period, it is called the fertile period. Therefore, by abstaining from coitus during this period, conception could be prevented.

Withdrawal or coitus interruptus is another method in which the male partner withdraws his penis from the vagina just before ejaculation so as to avoid insemination. 

Lactational amenorrhea (absence of menstruation) method is based on the fact that ovulation and therefore the cycle do not occur during the period of intense lactation following parturition. Therefore, as long as the mother breast-feeds the child fully, chances of conception are almost nil. However, this method has been reported to be effective only upto a maximum period of six months following parturition. As no medicines or devices are used in these methods, side effects are almost nil. Chances of failure, though, of this method are also high. 

 

(2) Barrier Methods :- 

In barrier methods, ovum and sperms are prevented from physically meeting with the help of barriers. Such methods are available for both males and females. Condoms are barriers made of thin rubber/latex sheath that are used to cover the penis in the male or vagina and cervix in the female, just before coitus so that the ejaculated semen would not enter into the female reproductive tract. This can prevent conception. ‘Nirodh’ is a popular brand of condom for the male. Use of condoms has increased in recent years due to its additional benefit of protecting the user from contracting STDs and AIDS. Both the male and the female condoms are disposable, can be self-inserted and thereby gives privacy to the user.  

Diaphragms, cervical caps and vaults are also barriers made of rubber that are inserted into the female reproductive tract to cover the cervix during coitus. They prevent conception by blocking the entry of sperms through the cervix. They are reusable.

Spermicidal creams, jellies and foams are usually used alongwith these barriers to increase their contraceptive efficiency.

 

(3)  Intra Uterine Devices (IUDs) :

These devices are inserted by doctors or expert nurses in the uterus through vagina. These Intra Uterine Devices are presently available as the non-medicated IUDs (e.g., Lippes loop), copper releasing IUDs (CuT, Cu7, Multiload 375) and the hormone releasing IUDs (Progestasert, LNG-20).

IUDs increase phagocytosis of sperms within the uterus and the Cu ions released suppress sperm motility and the fertilising capacity of sperms.  

The hormone releasing IUDs, in addition, make the uterus unsuitable for implantation and the cervix hostile to the sperms.

IUDs are ideal contraceptives for the females who want to delay pregnancy and/or space children. It is one of most widely accepted methods of contraception in India.

 

(4) Oral contraceptives :

Oral administration of small doses of either progestogens or progestogen–estrogen combinations is another contraceptive method used by the females. They are used in the form of tablets and hence are popularly called the pills. Pills have to be taken daily for a period of 21 days starting preferably within the first five days of menstrual cycle. After a gap of 7 days (during which menstruation occurs) it has to be repeated in the same pattern till the female desires to prevent conception.

They inhibit (primarily) ovulation and implantation as well as they alter the quality of cervical mucus to prevent/retard entry of sperms. Pills are very effective with lesser side effects and are well accepted by the females.

Saheli – the new oral contraceptive for the females contains a non-steroidal preparation. It is a ‘once a week’ pill with very few side effects and high contraceptive value. It was developed by scientists at Central Drug Research Institute (CDRI) in Lucknow.

Progestogens alone or in combination with estrogen can also be used by females as injections or implants under the skin  Their mode of action is similar to that of pills and their effective periods are much longer.

Administration of progestogens or progestogen-estrogen combinations or IUDs within 72 hours of coitus have been found to be very effective as emergency contraceptives as they could be used to avoid possible pregnancy due to rape or casual unprotected intercourse. This is termed as emergency contraception

 

(5)  Surgical methods :

Also called sterilisation, are generally advised for the male/female partner as a terminal method to prevent any more pregnancies. Surgical intervention blocks gamete transport and thereby prevent conception. Sterilisation procedure in the male is called ‘vasectomy’ and that in the female, ‘tubectomy’.

In vasectomy, a small part of the vas deferens is removed or tied up through a small incision on the scrotum whereas in tubectomy, a small part of the fallopian tube is removed or tied up through a small incision in the abdomen or through vagina. These techniques are highly effective but their reversibility is very poor. 

 

Medical Termination of Pregnancy (MTP)

Intentional or voluntary termination of pregnancy before full term is called medical termination of pregnancy (MTP) or induced abortion. Nearly 45 to 50 million MTPs are performed in a year all over the world which accounts to 1/5th of the total number of conceived pregnancies in a year. Obviously, MTP has a significant role in decreasing the population though it is not meant for that purpose. Whether to accept / legalise MTP or not is being debated upon in many countries due to emotional, ethical, religious and social issues involved in it. Government of India legalised MTP in 1971 with some strict conditions to avoid its misuse. Such restrictions are all the more important to check indiscriminate and illegal female foeticides which are reported to be high in India.

MTP is performed to get rid of unwanted pregnancies either due to casual unprotected intercourse or failure of the contraceptive used during coitus or rapes. MTPs are also essential in certain cases where continuation of the pregnancy could be harmful or even fatal either to the mother or to the foetus or both. MTPs are considered relatively safe during the first trimester, i.e., upto 12 weeks of pregnancy. Second trimester abortions are much more riskier. One disturbing trend observed is that a majority of the MTPs are performed illegally by unqualified quacks which are not only unsafe but could be fatal too.

Another dangerous trend is the misuse of amniocentesis to determine the sex of the unborn child. Frequently, if the foetus is found to be female, it is followed by MTP- this is totally against what is legal. Such practices should be avoided because these are dangerous both for the young mother and the foetus. Effective counselling on the need to avoid unprotected coitus and the risk factors involved in illegal abortions as well as providing more health care facilities could reverse the mentioned unhealthy trend.

 

Sexually Transmitted Diseases (STDs)

Diseases or infections which are transmitted through sexual intercourse are collectively called sexually transmitted diseases (STD) or venereal diseases (VD) or reproductive tract infections (RTI). Gonorrhoea, syphilis, genital herpes, chlamydiasis, genital warts, trichomoniasis, hepatitis-B and of course, the most discussed infection in the recent years, HIV leading to AIDS are some of the common STDs. Among these, HIV infection is most dangerous

Some of these infections like hepatitis–B and HIV can also be transmitted by sharing of injection needles, surgical instruments, etc., with infected persons, transfusion of blood, or from an infected mother to the foetus too.

Except for hepatitis-B, genital herpes and HIV infections, other diseases are completely curable if detected early and treated properly.

Early symptoms of most of these are minor and include itching, fluid discharge, slight pain, swellings, etc., in the genital region. Infected females may often be asymptomatic and hence, may remain undetected for long.

Absence or less significant symptoms in the early stages of infection and the social stigma attached to the STDs, deter the infected persons from going for timely detection and proper treatment. This could lead to complications later, which include pelvic inflammatory diseases (PID), abortions, still births, ectopic pregnancies, infertility or even cancer of the reproductive tract.

STDs are a major threat to a healthy society. Therefore, prevention or early detection and cure of these diseases are given prime consideration under the reproductive health-care programmes. Though all persons are vulnerable to these infections, their incidences are reported to be very high among persons in the age group of 15-24 years.

 

Preventive Measures :  

(i) Avoid sex with unknown partners/multiple partners.

(ii) Always use condoms during coitus.

(iii) In case of doubt, go to a qualified doctor for early detection and get complete treatment if diagnosed with disease.

 

Infertility

A large number of couples all over the world including India are infertile, i.e., they are unable to produce children inspite of unprotected sexual co-habitation. The reasons for this could be many–physical, congenital, diseases, drugs, immunological or even psychological. In India, often the female is blamed for the couple being childless, but more often than not, the problem lies in the male partner. Specialised health care units (infertility clinics, etc.) could help in diagnosis and corrective treatment of some of these disorders and enable these couples to have children. However, where such corrections are not possible, the couples could be assisted to have children through certain special techniques commonly known as assisted reproductive technologies (ART).

Assisted reproductive technologies (ART)

In vitro fertilisation (IVF–fertilisation outside the body in almost similar conditions as that in the body) followed by embryo transfer (ET) is one of such methods. In this method, popularly known as test tube baby programme, ova from the wife/donor (female) and sperms from the husband/donor (male) are collected and are induced to form zygote under simulated conditions in the laboratory. The zygote or early embryos (with upto 8 blastomeres) could then be transferred into the fallopian tube (ZIFT–zygote intra fallopian transfer) and embryos with more than 8 blastomeres, into the uterus (IUT – intra uterine transfer), to complete its further development.

Embryos formed by in-vivo fertilisation (fusion of gametes within the female) also could be used for such transfer to assist those females who cannot conceive. Transfer of an ovum collected from a donor into the fallopian tube (GIFT – gamete intra fallopian transfer) of another female who cannot produce one, but can provide suitable environment for fertilisation and further development is another method attempted.

Intra cytoplasmic sperm injection (ICSI) is another specialised procedure to form an embryo in the laboratory in which a sperm is directly injected into the ovum.

Infertility cases either due to inability of the male partner to inseminate the female or due to very low sperm counts in the ejaculates, could be corrected by artificial insemination (AI) technique. In this technique, the semen collected either from the husband or a healthy donor is artificially introduced either into the vagina or into the uterus (IUI – intra-uterine insemination) of the female.

 

General Information

In lower animals, as a result of asexual reproduction the structure formed is termed as blastema and it gives rise to the complete animal. This is termed as ''blastogenesis'' and  the animals so formed are termed as blastozoids.

In majority of animals, as a result of sexual reproduction the structure formed is termed as the zygote and it gives rise to complete animal. This is termed as ''embryogenesis'' and the animals, so formed are termed as oozoids. They have the most advanced characters.

                                                                                                         

HISTORY :

(1)  Aristotle is known as ''Father of Embryology'' he first studied the development in chick and other embryos. He gave its description in his book ''Historia Animalia''.

 

(2)  Leeuvenhock (1671) - He observed and described human sperm for the first time.According to Hartsoeker and Leeuvenhock there is a small model of developing animals present in the head of the sperm of that animal. This small model is  called homunculus. Both these scientists are called spermists, and this theory is called '' Theory of spermist ''.

 

(3)  Swammer Dame, Haller, Bonette & Malpighi : - According to these scientists, small model of animal is always present in the egg. These scientists are called Ovists, and their theory is known as 'Ovists theory'

 

(4)  Schleiden & Schwann : - Both the scientists established the cellular structure of egg and sperm.

 

(5)  Pander : - He described the presence of three germinal layers in chick embryo.

 

(6)  Fredrich Wolff : - He first presented the ''theory of epigenesis''.

 

(7)  Muller : - He gave the recapitulation theory.

 

(8)  Haeckel : - He gave the details of Recapitulation theory and named it as the bio-genetic law.

- Bio-genetic Law : - According to this each organism during its embryonal development, passes through all stages, through which its species has evolved or embryo repeats its ancestry. i.e. Ontogeny recapitulates its Phylogeny.

 

(9)  ''Carl Ernest Von Baer'' : - He is known as the ''father of modern embryology.'' He gave the Baer's Law which in turn  proves the recapitulation theory.

According to this law, during embryonal development,  the development of general structures takes place earlier and specific structures develop at last or later on.

 

(10) A. Weismann : - He gave the theory of germplasm or the theory of continuity of germplasm. According to him, there are 2 types of protoplasm in the body of animals : -

(i)   Somatoplasm     

(ii)  Germplasm

Somatoplasm dies but the germplasm is never destroyed, rather it is transferred to the progenies.    

 

(11) Wilhelim : - He studied embryonal development in frog and gave the mosaic theory.

- He said that there are some presumptive areas in the eggs of frog. These areas form specific structures during embryonal development. This is termed as ''Promorphology .'' and these type of eggs are termed as the mosaic eggs.

 

(12) Hand Driesch : - He studied embryonal development is sea-urchin and gave the regulative theory.

- In the eggs of Sea-Urchin presumptive areas are not found i.e. promorphology is not found. So, each part of the egg is capable of forming the complete embryo. These type of eggs are termed as regulative eggs.

 

(13) Boveri & Child : -

They gave the gradient theory to explain the mosaic development in eggs.

According to them, a metabolic gradient is present inside the eggs.

Different parts of the egg have different metabolic rates.

The rate of metabolism is faster at the animal-pole of the egg and is slower at the vegital pole of the egg.                     

 

(14) Spemann :- He gave the ''Theory of organizers''.

- According to it embryo has some special type of tissues, which induce development of some specific  structures. 

These are termed as the organizers.

-  These organizers secrete some special chemicals called evocators which induce the formation of

specific structures.            

- Spemann got the Nobel prize for his theory of organizers.

 

(15) R.V. Graff : -

- He studied a follicle in human ovary and termed it as ''Graafian follicle''