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 within 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 an increase in their motility.
Fig: The sperm entering the ovum using the acrosome head to break down the zona pellucida 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 secretory 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 various 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 . At the time of ovulation, the second meiotic division is in progress and a spindle has formed for separation of the second polar body .
At this stage the 'ovum' enters the infundibulum of the uterine tube and passes into the ampulla.
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) depolarisation 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.
Fig: Female PronucleusAt 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.
The male and female pronuclei 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.
Each of these 46 chromosomes splits into two . 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 .
This is called the two-cell stage of the embryo. Note that strictly speaking there is no one-cell stage of the embryo.
Important points to 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 and smaller untill they acquire the size of most cells of the body.
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 followed by a '4-cell' stage ,' 5 -cell' stage etc.
Fig: 8-cell embryo, at 3 days 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 is called the morula.
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 completely surrounded by an outer layer of cells, (Compaction). The cells of the outer layer will later give rise to a structure called the trophoblast . 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 the morula from the uterine cavity, and partially separates the cells of the inner cell mass from those of the trophoblast . 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 . 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 vegetal or abembryonic pole.
Fig: Blastocyst with an inner cell mass and trophoblast. Function of the zona pellucida - It prevent implantation of the blastocyst at an abnormal site.
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.
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 the embryonic disc (also called embryonic area,embryonic shield,or germ disc)The three layers that constitute this embryonic disc are.
(i) Endoderm (endo = inside)
(ii) Ectoderm (ecto =outside)
(iii) Mesoderm(meso = in the middle)
These are three germ layers. All 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. 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 .These cells 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 , filled byamniotic fluid, or liquor amnii. The roof of this cavity is formed byamniogenic cells(cells of Rauber) derived from the trophoblast, while its floor is formed by the ectoderm.
Fig: Histogenesis of the three germ layers
(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 .
(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, one large space is formed. This cavity is called the extra-embryonic coelom .With its formation, the extra- embryonic mesoderm is split into two layers. The part lining the inside of the trophoblast, and the outside of the amniotic cavity, is called the parietal or somatopleuric extra-embryonic mesoderm.
The part lining the outside of the yolk sac is called the visceral or splanchnopleuric extra-embryonic mesoderm , 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 developing 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 overlying trophoblast (on the outside) this is called thechorion . The other is the amnion which is constituted by the amniogenic cells forming the wall of the amniotic cavity (excluding the ectodermal 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 .
(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 .
(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 (antero posterier 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: Artificially colored - gestational sac, yolk sac and embryo (measuring 3 mm at 5 weeks)
(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 . The primitive streak is at first a rounded or oval swelling, Primitive streak later forms the Henson's node.
(l) The cells that proliferate in the region of the primitive streak pass sideways,pushing themselves between the ectoderm and endoderm (fig. 11). These cells form 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 midline, in the 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 in brain and rest of the neural tube differentiate in spinal cord so central nervous system is formed by neural tube.
During the first 3 months of pregnancy the basic structure of baby is formed.This involves cell division, cell migration, and the 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).