Cleavage is the mitotic segmentation of the zygote. Different types of animals have different modes of cleavage. It depends upon the conditions of the process of the embryogenesis and types of ova. The development of the human embryo takes place in the mother’s body, which provides its nourishment. That is why the human ovum contains a little amount of the yolk inclusions and the type of the human germ cleavage is complete and irregular.
At the beginning of the cleavage two cells are formed at first. Of these two cells one is larger than the other and this then divides so, that only three cells are present after second division. Then the other cell divides, thus producing four cells, further five and so on. So, all cells divide, but with different speed. Is why such division (type of cleavage) is called complete and irregular. The cells which appear during the cleavage are called blastomeres.
As cleavage is in progressing the zygote passes down the uterine tube and when the process has reached the stage of 12- 16 cells, the dividing germ is called a morula from its resemblance to a mulberry. It is a solid ball of cells. The first several days the membrane of fertilization remains around the dividing cells, whereby the blastomeres can not move aside from each other.
At that period the size of the whole structure remains unchanged, but the cleavage cells become smaller and smaller with each division. It happens in order to maintain the proper relationship between the amount of nuclear and cytoplasmic material. As it has been said, the correlation between the nucleus and cytoplasm of an ovum is one to ten, while each somatic cell has correlation one to three. That is why for the first days the common measurement of the dividing germ remains unchanged, that is the G1 of the cell cycle is absent at this period and cells growth is not observed.
Thus division continues and soon morula appears in the uterine cavity in the fluid, produced by the uterine glands. This fluid passes into the morula in between its outer cells to form a cavity. The outer cells which involve the uterine fluid form trophoblast, which will provide the protection and nutrition to the embryo. Inner cells are bigger, darker and form an assemblage at one pole is called embryoblast, which will form the embryo proper. When this has happened, the morula is transformed into blastula. The human blastula termed a blastocyst and the cavity in the blastocyst is called the segmentation cavity or blastocoele.
Eventually the membrane of fertilization disappears and the blastocyst becomes attached to the uterine membrane and the trophoblast produces the enzymes to lysis the inner uterine membrane. In the result the germ sinks into the substance of the uterine wall. This process called the implantation and happened on the 6–7 days after fertilization.
The cells of trophoblast proliferate and form a multinucleate mass of protoplasm outside – syncytial trophoblast and inside – cytotrophoblast and will be used for the nourishment of the embryo and for the subsequent establishment of the placenta (this will bediscussed futher).
So, on the seventh day of the germ development, it is in the uterus, where it becomes implanted to the uterus wall and after implantation will call embryo.
