Cleavage (Segmentation)

Cleavage is an unique embryological process which transforms the single fertilised egg cell into a sphere of closely aggregated multitude of cells. Immediately after fertilisation the fertilised ovum (egg cell) undergoes a series of cell divisions. These divisions are called cleavage divisions, which are all mitotic divisions.

Cleavage in Mammalian Ovum

The development in mammals occurs inside the body of the female. The eggs are small with very little or practically no yolk. So they are called microlecithal and isolecithal.

Segmentation starts at about 14 - 15 hours after fertilisation as the egg moves down through the fallopian tube. Mammalian zygote undergoes holoblastic and equal cleavage. This means that the egg is completely divided.

Cleavage division in humans, result in the complete division of the zygote and the subsequent cells, the blastomers. This type of cleavage is known as holoblastic cleavage.

The first cleavage furrow is vertical passing through the imaginary axis that runs through the animal and vegetal pole. The 2 daughter cells which are thus formed are called as blastomeres. The second cleavage is also vertical but at right angles to the first one forming 4 blastomeres of equal size. Subsequent divisions occur one after the other in an orderly fashion. The divisions are rapid and the blastomeres become progressively smaller. During cleavage, the number of cells in the embryo increases but the size of the embryo remains unchanged.

Morula

After repeated cleavages, the embryo takes the form of a solid ball of cells and looks like a mulberry. This embryonic stage is called morula (a little mulberry). The embryo is now at the 16-celled stage.

Blastula

As the cells of the morula continue to divide, a cavity appears inside the morula. It forms the blastocoel. It separates a surface layer of blastomeres from an inner cell mass. The blastomeres on the surface of the embryo become flattened and form the trophoectoderm or trophoblast. This layer on further development produces extra embryonic membranes and the placenta which help in nourishing the embryo.

Implantation

The process of attachment of the blastocyst (blastula) on the endometrium of the uterus is called implantation. It generally occurs on the seventh day after fertilisation, thus establishing pregnancy.

After implantation, the endometrium undergoes profound changes and is called the decidua.

The decidua tissue can be distinguished into three layers:

a) decidua basalis - situated between the embryo and the myometrium

b) decidua capsularis - situated between the embryo and the human of the uterus

c) decidua parietalis - includes the rest of the decidua.

The inner cell mass is destined to form the embryo proper. The sphere of cells with the cavity located at the centre is called the blastodermic vesicle.

Two Stages in the Development of Blastodermic Vesicle or Blastocyst

The large sized blastocoel of a mammalian embryo at this phase of development signifies 2 things.

• The blastocoel is the site where embryos of ancestral forms lodged food material in the form of yolk.

• The blastocoel in the embryo increased in size due to accumulation of more and more fluid. This increases the outer layer of the blastodermic vesicle or blastocyst for drawing food for the yolkless embryo from the uterine circulation of the mother.