Cleavage

The zygote undergoes a rapid series of mitotic divisions called cleavage.

Cleavage is a rapid succession of consists of cell mitotic divisions following DNA synthesis but without cell grown: no increase in size, only increase in cell number.. These divisions result in a collection of small adherent cells called blastomeres. They do not grow in size but divide the zygote progressively into smaller cells.

The type of cleavage depends on the type of the egg cell in animal development. The amount and distribution of yolk in each cell determines the rate of cleavage and the relative size of the blastomeres.

Cleavage may be complete (holoblastic) or incomplete depending on extent of cytoplasmic subdivision (total or partial subdivision respectively), and to be equal or unequal depending on the relative sizes of the early blastomeres.

A series of cleavage divisions result in formation of blastula. Type of blastula depends on the cleavage type (Tabl.3, Fig.12).

Tabl.3 The principal types of cleavage and blastula

Group	Type of the egg cell	Type of the cleavage	blastula	Blastula cavity (blastocoele)
Amphioxus, Mammals	Isolecithal, oligolecithal or microlecithal (little yolk)	Complete (or holoblastic): initial cleavage planes extend through egg); Equal (in amphioxus), or unequal (in mammals)	Sphere with single layer wall (celoblastula in Amphioxus; blastocyst in mammals)	Large, central sphere
Amphibians, fishes	Mesolecithal (moderate yolk)	Holoblastic unequal	Sphere with layered wall (amphiblastula)	Small sphere
Reptiles Birds	Telolecithal (heavy yolked)	Incomplete (or meroblastic): initial cleavages planes limited to animal pole, the region opposite the yolk)	Cell disk on surface of yolk, bilaminar - diskoblastula	Flat space between epiblast and hypoblast

Fig.12. Blastula Types: a) celoblastula; b) periblastula (in Insects); c) amphiblastula;

d) diskoblastula; e) blastocyst.

The egg cells (and hence the zygote as well as blastula) is always asymmetric, having an animal pole and a vegetal pole. The upper hemisphere is the animal pole; the lower hemisphere is the vegetal pole. The vegetal pole draws its name from its inactivity relative to the lively animal pole. In some animal groups the vegetal pole contains large yolky cells that divide very slowly.

Complete and equal cleavage of the oligolecithal egg of Amphioxus results in a simple hollow sphere of cells enclosing a cavity – blastocoele. This sphere is termed a celoblastula.

In result of complete unequal cleavage of the mesolecithal egg of frog it is formed a blastula with an eccentric blastocoele surrounded by small blastomeres at the animal pole and by larger blastomeres at the vegetative pole. This blastula is called amphiblastula.

During incomplete and unequal cleavage of the telolecithal egg of the chick, the yolk does not divide. Cytoplasmic division are limited to the animal pole, and bilaminar disk shaped mass of blastomeres, surrounding the yolk, is produced. A cleft between the laminae is homologous to the blastocoele of lower living forms’ blastula. A subgerminal cavity between blastomeres and main yolk mass is formed shortly thereafter. It is filled with nutritive liquefying yolk. This blastula is called diskoblastula.

Complete unequal cleavage of the secondary isolecithal egg of eutherian mammal results in a compact collection of cells called a morula (20-50 cells). Morula begins to absorb uterine fluid forming a central cavity. The blastocyst, as it now known, consists of a peripheral layer of blastomeres forming the trophoblast, with a mass of cells at one aspect, the polar trophoblast, bulging into the central lumen and known as the inner cell mass, or embryoblast from which the embryo proper arises.