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In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. [1] The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same size as the original zygote.
Human embryology is the study of this development during the first eight weeks after fertilization. The normal period of gestation (pregnancy) is about nine months or 36 weeks. The germinal stage refers to the time from fertilization through the development of the early embryo until implantation is completed in the uterus .
Because cleavage is impeded in the vegetal pole, there is an uneven distribution and size of cells, being more numerous and smaller at the animal pole of the zygote. [8] [9] In holoblastic eggs, the first cleavage always occurs along the vegetal-animal axis of the egg, and the second cleavage is perpendicular to the first.
Cleavage is the very beginning steps of a developing embryo. Cleavage refers to the many mitotic divisions that occur after the egg is fertilized by the sperm . The ways in which the cells divide is specific to certain types of animals and may have many forms.
The division of blastomeres from the zygote allows a single fertile cell to continue to cleave and differentiate until a blastocyst forms. The differentiation of the blastomere allows for the development of two distinct cell populations: the inner cell mass, which becomes the precursor to the embryo, and the trophectoderm, which becomes the precursor to the placenta.
The difference between a mammalian embryo and an embryo of a lower chordate animal is evident starting from blastula stage. Due to that fact, the developing mammalian embryo at this stage is called a blastocyst, not a blastula, which is more generic term. There are also several other differences from embryogenesis in lower chordates.
As cleavage continues, the cavity expands to become the developed blastocoel. The blastocoel is a crucial component of amphibian embryo development. It permits cell migration during gastrulation and prevents the cells beneath the blastocoel from interacting prematurely with the cells above the blastocoel.
The cleavage divisions of CS2 embryos do not occur synchronously. And the fate of the blastomeres is not yet determined. The two-cell embryo is spherical and surrounded by the transparent zona pellucida. Each of the blastomeres that form is also spherical. On approximately day 3, at the eight-cell stage, compaction usually begins. [2]