By Dr Ananya Mandal, MD
The process of embryogenesis begins with an egg or ovum being fertilized by a sperm cell to form a zygote. The zygote is a single diploid cell, but this zygote then multiplies through mitotic division to form a multicellular embryo.
This mitotic division does not result in any significant growth and the multicellular cluster that is formed is the same size as the zygote. This process is referred to as cleavage.
Initially, at least four cycles of cell division occur, giving rise to 16 cells, referred to collectively as the morula. The individual cells that occur as a result of the cleavage are termed blastomeres.
This cleavage may be total (holoblastic) if there is a rich supply of egg yolk (e.g. in reptiles and birds) or it may be partial (meroblastic) in animals such as humans where the egg yolk is less rich. In the case of holoblastic cleavage, the offspring go on to receive further nourishment from the placenta or from milk, while animals that undergo holoblastic cleavage obtain all of their nutrients from the egg.
The ovum or egg cell has two poles: an “animal pole” and a “vegetal” pole. Cleavage occurs in the vegetal pole, meaning the size and distribution of the cells is very uneven.
In animals where holoblastic cleavage occurs, the first cleavage is always along the vegetal-animal axis of the egg and the second cleavage is perpendicular to the first. Depending on the planes of cleavage, the blastomeres then follow various patterns in different organisms. For example, among holoblastic eggs, a radial pattern is seen in sea urchins; a bilateral pattern in amphibians; a spiral pattern in molluscs and a rotational pattern in nematodes. Among meroblastic eggs, the pattern is discoidal in fish, reptiles and birds and superficial in insects. Once the cleavage ends, the cluster of cells undergo midblastula transition, which marks the onset of zygotic transcription.
Reviewed by Sally Robertson, BSc
Last Updated: Aug 17, 2014