Cleavage (embryology)

Cleavage is a series of rapid mitotic divisions that subdivides an egg into smaller cells, called blastomeres, without increasing the embryo’s overall size.

Blastomere: A blastomere is one of the cells produced by cleavage of an egg. Cleavage partitions the egg into blastomeres while conserving its total volume.
Oocyte: An oocyte is an immature egg cell that can mature into a female gamete. Cleavage begins after fertilization of the oocyte, whose size constrains early divisions.
Amphibian development: Amphibian development is the sequence of embryonic changes from fertilized egg through hatching or birth. Amphibian eggs show unequal, holoblastic cleavage shaped by yolk concentrated at one pole.
William Harvey: William Harvey was an English physician who studied circulation and animal reproduction in the seventeenth century. His embryological observations helped establish systematic study of development from eggs.
Cell cycle: The cell cycle is the sequence of growth and division through which a cell duplicates its contents and divides. Early cleavage cycles often shorten by omitting or reducing growth phases.
Fertilization: Fertilization is the union of sperm and egg that initiates development of a zygote. The fertilized egg enters cleavage as its first embryonic developmental phase.
Xenopus: Xenopus is a genus of aquatic frogs widely used in developmental biology. Its large eggs made the mechanics and organization of early cleavage experimentally accessible.
Karl Ernst von Baer: Karl Ernst von Baer was a nineteenth-century biologist who founded comparative embryology and discovered the mammalian ovum. His comparative work placed early embryonic stages within a broader account of animal development.
Holoblastic cleavage: Holoblastic cleavage divides the entire egg during each cleavage furrow. This pattern occurs when yolk does not prevent furrows from crossing the whole egg.
Mitosis: Mitosis is nuclear division that produces two daughter nuclei with the same chromosome complement. Cleavage relies on repeated mitoses to increase cell number without enlarging the embryo.

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