The cleavage is one of the initial steps of fetal develop. It consists of the first mitotic divisions, which occur without the increase in cell volume. Therefore, the first mitotic divisions are atypical, as there is no cytoplasmic recovery and, therefore, there is no increase in the total embryo volume.
→ Cleavage and the development of the human embryo
After fertilization, the zygote moves towards the uterus. At that moment, the cleavages begin, which form blastomeres (first cells resulting from division). Blastomeres don't all divide at the same time, so there are often odd numbers of cells in the embryo.
The 16-cell embryo is called a morula., as it resembles a blackberry and reaches this stage approximately three days after fertilization. The blastomeres during this period are well compacted. When the embryo reaches 32 cells, the blastomeres begin to secrete fluid into the embryo. This fluid is concentrated inside the blastocele, a cavity. At that moment, we call the embryo a blastocyst and it is it that will immerse in the endometrium.
→ Cleavage Patterns
As stated, the mitotic divisions that occur with the embryo early in development are called cleavage. These divisions feature different patterns, which differ mainly according to the calf distribution. We can say that the less yolk an egg (zygote) has, the easier it is for division and vice versa.
The cleavage can be of two types: holoblastic or total and meroblastic or partial.
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Holoblastic or total cleavage: the zygote splits completely.
Equal:The zygote divides and forms blastomeres of the same size. It is seen in alecitic eggs (with reduced amount of yolk) and oligolocytes (small amount of yolk, which is evenly distributed).
Unequal:The zygote divides faster at the animal pole than at the vegetative pole, which leads to a greater number of cells in this region. It is seen in mesolocyte eggs (large amount of yolk, which is unevenly distributed). In the vegetative pole, there is a large amount of calf; in the animal pole, there is less calf.
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Meroblastic or partial cleavage: the cleavage occurs in only one region of the zygote.
Discoidal:Splitting occurs in an area of disc that does not have a yolk. Occurs in telolocyte eggs (large amount of yolk, which occupies almost the entire cell).
Superficial:The division takes place at the periphery of the zygote. Occurs in centrolecite eggs (large amount of yolk in the central region of the egg).
By Ma. Vanessa dos Santos
Source: Brazil School - https://brasilescola.uol.com.br/o-que-e/biologia/o-que-e-clivagem.htm
