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Cleavage

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.

TermYolkCleavageResultModel OrganismsNotes
AlecithalLittle/NoneHoloblasticBlastocystMammal
CentrolecithalCenterSuperficialBlastodermInsect
IsolecithalEvenHolobasticBlastulaUrchin, Sand Dollar
MesolecithalUnevenHoloblasticBlastulaAmphibianSometimes known as telolecithal.
TelolecithalVery UnevenMeroblasticBlastodiscBird, fish, reptileSometimes known as extreme telolecithal.

alecithal yolk distribution

centrolecithal yolk distribution

isolecithal yolk distribution

megalecithal telolecithal yolk distribution

mesolecithal yolk distribution

telolecithal yolk distribution

FactorOverview
CDK
Cyclin
CytoskeletonIncludes microtubules and microfibers. Microtubules inhibited by colchicine and nocodazole (inhibit chromosome segregation); and microfilaments inhibited by cytochalasin (inhibits cytokinesis).
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