General Properties of Embryonic Development | Additional Study Material for NEET PDF Download

GENERAL PROPERTIES OF EMBRYONIC DEVELOPMENT
1. Morula :- As a result of segmentation or cleavage activities, unicellular zygote changes into a solid ball like multi-cellular structure. In the later stage of cleavage, clusters of sticky, cohering, protruding(outside) 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 blastocoel. This cavity is a 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 a blastula, and its formative activities are 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 / solidblastula
(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, Coelenterata, amphibia, sponges etc.

Coeloblastula of amphibians is called amphiblastula because in it blastocoel cavity is accentric in position and 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 flagillated. 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.

Example - Eggs of Neries, 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 sub-germinal 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. 

Example - Eggs of insects.

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

Blastula - 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 called cells of Rauber (amniogenic cells). There occurs a cavity in between embryonal knob and trophoblast called albumin cavity (Blastocoel). It is filled with nutritive fluid absorbed from the wall of uterus. So albumin cavity is also nutritive- cavity.

3. Gastrulation

Gastrula: In gastrula stage rate of cleavage division is slow and ultimately stops at the end of gastrula. Gastrula stage is the most important stage in embryonic development because two main events take place during gastrula stage.
(a) Differentiation of blastomere: As a result of differentiation of blastomere; three germinal layers i.e. ectoderm, mesoderm and endoderm are formed. Formation of three germinal layers is the significance of gastrula stage. All the preparation of differentiation of blastomere are completed in late blastula stage.

(b) Morphogenetic Movements: During gastrula stage blastomere perform amoeboid movement and reach to their definite place in embryo because after the gastrulation organogenesis has to start in embryo. Morphogenetic movement requires enormous energy. So respiratory activity of egg increases. Embryo consumes maximum O₂ during gastrula stage.

Method of Gastrulation :
(a) Epiboly: Movement of ectoderm forming blastomere
(b) Emboly: Movement of mesoderm and endoderm forming blastomere.

(a) Epiboly: In epiboly, ectoderm forming blastomere undergo division to form new blastomere. New blastomeres perform amoeboid movement and cover the embryo from outside. Epiboly is clearly visible in animal with amphiblastula. In amphiblastula ectoderm is derived from outermost layer of dividing micromeres.

Fig: Process of Epiboly

These micromeres perform amoeboid movement and cover the megamere from outside. Megameres without any movement establish inside the embryo because they are heavily filled with yolk. At one definite place in embryo micromeres do not perform any movement to cover the megameres this place occurs in the form of depression is called gastrulation slit or future blastopore. It is formed in grey crescent area.

In some animals there occur special type of epiboly is called Apiauxis. In this process ectoderm forming micromere are received from two sources.

(1) By the division of micromere

(2) By budding of megamere. (Eg-Ctenophora. Annelida, Mollusca and Fishes)

(b) Emboly: Mesoderm and endoderm forming blastomere perform movement and establish inside the embryo. There are three methods of emboly.

(i) Invagination: Emboly mainly takes place by invagination in animal with simple coeloblastula. In coeloblastula a part of blastoderm invaginate in embryo, as a result, blastocoel cavity degenerates and new cavity forms outside to inside called archenteron. The opening of the archenteron is called the blastopore. Archenteron forms an alimentary canal. Blastopore forms mouth in protostome animals and anus in deuterostome animals. As a result of invagination blastomere which fills the blastocoel form mesoderm and endoderm. Blastomere of outer layer form ectoderm i.e. all the three germinal layer are formed by emboly in coeloblastula. Epiboly is absent in coeloblastula.

(ii) Involution : Rolling movement of blastomere. Emboly mainly takes place by involution in animal with amphiblastula. 

• In amphiblastula micromeres move from different directions towards future blastopore,this movement is called convergence. 
• After convergence, second step is involution (i.e. blastomere perform rolling movement and enter in blastocoel with the help of future blastopore) now all the blastomeres move in different direction in blastocoel, this is called divergence. 
• As a result of divergence blastocoel cavity degenerates and new cavity called archenteron is formed. Opening of archenteron is called blastopore. 
• All the involuted blastomeres are collectively called 'Chorda mesoderm'. At the end of gastrula, outgrowth occurs in some megamere i.e. megameres evaginate outside and close the blastopore temporarily (yolk-pluge stage) yolkpluge stage indicates completion of gastrulation.

Fig: The process of Involution

(3) Poly-invagination or ingression: Emboly mainly takes place by poly-invagination in discoblastula, periblastula.

Fig: The process of poly-invaginationGastrulation in discoblastula:
In discoblastula, gastrulation takes place by two methods:

(i) Delamination : In delamination all the blastomere of blastoderm undergoes division. As a result new blastomere form and new blastomere fall on the floor of sub-germinal cavity. So embryo become double layered. Upper layer of blastomere is called epiblast and lower-layer of blastomere is called hypoblast. Hypoblast differentiates in endoderm.

(ii) Poly-invaginations: All the blastomere of epiblast undergo division to form new blastomere and new blastomere fall in subgerminal cavity form different direction and fill the subgerminal cavity. Blastomere which fill the subgerminal cavity are collectively called chorda-mesoderm. (chorda mesoderm forms mesoderm) Blastomeres which are left outside i.e. blastomere of epiblast form ectoderm. In such blastulas no new cavity is formed during gastrula stage i.e. archenteron formation does not occur during gastrula stage.

SPECIAL POINTS
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 is 0.02 mm & largest sperm is of Discoglossus (2mm)

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. This condition is known as necrospermia.

11. Smallest egg in birds is of humming bird.

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

13. The 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 lobster the sperm are tail less and have three sharp processes.

(b) Biflagellated sperm : 
Eg. In toad fish (Opsansus) head of many sperms unite together and form sperm boats. In Gastropods, the sperms are hexa-flagellated.

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 deferens 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. Respiratory tract.

4. Gall bladder, pancreatic duct.

5. Major portion 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.