Cleavage Patterns: Holoblastic and Meroblastic

Cleavage Patterns: Holoblastic and Meroblastic

Cleavage patterns are the way in which an embryo divides during its early stages of development. These patterns are shaped by the distribution of yolk within the egg and the factors in the egg cytoplasm that influence the angle of the mitotic spindle and the timing of its formation. Two primary types of cleavage patterns are holoblastic and meroblastic.

Holoblastic Cleavage

In holoblastic cleavage, the entire egg undergoes cellularization. This means that the yolk platelets are either absent or present as cytoplasmic inclusions that partition among cells. Cleavage furrows encompass the entire embryo, from the animal pole (corresponding to the region containing the meiotic spindle in the oocyte) to the vegetal pole.

Equal Holoblastic Cleavage

Equal holoblastic cleavage is a type of holoblastic cleavage where the cleavage furrow cuts the egg into two equal cells. This type of cleavage can be radially symmetrical, bilaterally symmetrical, spirally symmetrical, or irregular.

Unequal Holoblastic Cleavage

Unequal holoblastic cleavage is a type of holoblastic cleavage where the resultant blastomeres become unequal in size. This unequal division can be due to the distribution of yolk and other factors within the egg.

Meroblastic Cleavage

In meroblastic cleavage, cell division does not divide the embryo in its entirety. Instead, embryonic cells divide in the animal pole independently of the vegetally located yolk, with cells typically remaining syncytial to the yolk cell for a period of time that varies among species.

Superficial Cleavage

In superficial cleavage, only the rim of cytoplasm around the periphery of the cell undergoes division. This occurs in centrolecithal eggs, where the yolk is in the center of the egg.

Discoidal Cleavage

In discoidal cleavage, cell divisions occur only in a small disc of cytoplasm, giving rise to the discoidal pattern of cleavage. This pattern is observed in telolecithal eggs, where only one small area of the egg is free of yolk.

It is important to note that pure holoblastic or meroblastic cleavage patterns are idealized extremes, and many embryos cleave with an intermediate geometry, in which the entire embryo cleaves but the strong asymmetry of yolk leads to a bias of cleavage planes.