Holoblastic

Holoblastic is a term used in developmental biology to describe a mode of embryonic cleavage in which the entire fertilized egg cytoplasm divides during each mitotic event. In holoblastic cleavage, the cleavage furrows pass through the whole egg, producing blastomeres that partition the entire cytoplasm and eventually form a blastula.

This type of cleavage typically occurs in eggs with little or evenly distributed yolk, such as isolecithal or mesolecithal eggs. Because yolk content is not heavily biased toward one region, the entire cytoplasm participates in division. Holoblastic cleavage can take several patterns depending on the species, including radial (common in echinoderms and many amphibians), spiral (seen in many mollusks and annelids), and, in mammals, rotational.

In mammals, including humans, holoblastic cleavage is rotational, with the first two divisions creating four cells at right angles to one another. The divisions continue to produce a morula, then a blastocyst, as development proceeds.

By contrast, meroblastic cleavage occurs when yolk is abundant and affects division. In telolecithal eggs (dense yolk at one pole), such as those of birds and many reptiles, cleavage is partial and restricted to a region called the blastodisc, forming a discoidal meroblast. In insects with very centralized yolk, cleavage can be superficial or form a syncytial blastoderm rather than a true holoblastic division.

See also: cleavage, embryogenesis, isolecithal, mesolecithal, telolecithal.