Cleavage is the repeated mitotic division of zygote to form a solid ball of cells called morula which later changes into a hollow ball of cells called blastula.
Cleavage of human zygote occurs within the fallopian tube. It is holoblastic, i.e., it divides the zygote completely into daughter cells or blastomeres.
The first cleavage takes place about 30 hours after fertilization. It divides zygote longitudinally into two blastomeres (one slightly larger than the other). The second cleavage occurs within forty hours after fertilization. It is at right angles to the plane of the first resulting in four blastomeres. The third cleavage takes place about 72 hours after fertilization. During these early cleavages, the young embryo moves slowly down the fallopian tube towards the uterus (Fig. 3(B).10).
At the end of the fourth day, the embryo reaches the uterus. It looks like a mulberry and is known as morula. This solid ball like morula has thirty two cells. In human zygote the cleavage is radial (blastomeres are arranged in radial plane around the polar axis) and indeterminate type (fate of each blastomere is not predetermined).
During the early cleavage in mammals capacitation occurs. It occurs at 8-cell stage when the loosely attached blastomeres are held tightly due to production of proteins called cohesions on their surface.
Significance of Cleavage:
(i) It converts a unicellular zygote into a multicellular embryo.
(ii) It maintains the cell size and nucleo-cytoplasmic ratio of the species.
(iii) Cleavage produces large member of cells or blastomeres required for the building of offspring’s body.
(iv) During cleavage quick mitotic division of blastomeres occurs following which there is no growth of blastomeres.
(v) Cleavage brings about the distribution of cytoplasm among the blastomeres.