Activated after fertilization, cleavage is a period of rapid mitosis that divides cells into a roughly somatic size.
Despite appearing uniform, cells are committed to a particular cell fate (insects) or dorsal-ventral position (amphibians). Cleavage depends on maternal mRNAs instead of zygotic genes. Since cleavage depends solely on cytoplasmic factors, enucleated eggs still undergo cleavage up to the blastula stage. Enucleated eggs even undergo cyclical rounds of cortical contraction and cyclical rounds of DNA synthesis (if a DNA clone of the genome is injected).
An embryo's cleavage pattern depends on the amount and distribution of yolk. An egg has a yolky pole (vegetal pole) and a nucleic pole (animal pole). Mammalian eggs are small, alecithal and undergo complete asynchronous cleavage at a slow rate of 12-24 hours.
|Isolecithal||Even||Holobastic||Blastula||Urchin, Sand Dollar|
|Mesolecithal||Uneven||Holoblastic||Blastula||Amphibian||Sometimes known as telolecithal.|
|Telolecithal||Very Uneven||Meroblastic||Blastodisc||Bird, fish, reptile||Sometimes known as extreme telolecithal.|
|Cytoskeleton||Includes microtubules and microfibers. Microtubules inhibited by colchicine and nocodazole (inhibit chromosome segregation); and microfilaments inhibited by cytochalasin (inhibits cytokinesis).|