Cleavage (embryo)
In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. Depending mostly on the amount of yolk in the egg, the cleavage can be holoblastic (total) or meroblastic (partial). The different cells derived from cleavage (up to the blastula stage) are called blastomeres.
See also: embryogenesis.
| Contents |
Holoblastic
In the absence of a large concentration of yolk, four major cleavage types can be observed in isolecithal cells - radial holoblastic, spiral holoblastic, bilateral holoblastic, and rotational holoblastic cleavage. These holoblastic cleavage planes pass all the way through isolecithal zygotes during the process of cytokinesis. Coeloblastula is the next stage of development for eggs that undergo these radial cleavaging.In holoblastic eggs the first cleavage always ocurrs along the vegetal-animal axis of the egg, the second cleavage is perpendicular to the first. From here the spatial arrangement of blastomeres can follow various patterns, due to different planes of cleavage, in various organisms.
Radial
Radial cleavage is characteristic of some animal groups, for instance vertebrates and echinoderms, in which the spindle axes are parallel or at right angles to the polar axis of the oocyte. In most other groups, embryos undergo spiral cleavage, in which
Bilateral
Spiral
In spiral cleavage, the cleavage planes are oriented obliquely to the polar axis of the oocyte. At the third cleavage the halves are oblique to the polar axis and typically produce an upper quartet of smaller cells that come to be set between the furrows of the lower quartet. All groups showing spiral cleavage are protostomia, such as annelids and mollusks.
Rotational
Mammals display rotational cleavage, and an isolecithal distribution of yolk. Because the cells have only a small amount of yolk, they require immediate implantation onto the uterine wall in order to receive nutrients.
Meroblastic
Discoidal
Superficial
| Holoblastic | Meroblastic |
|---|---|
|
See also: embryogenesis.