🔬 Embryology: Cleavage Patterns & Blastula Formation
Cleavage is a series of rapid mitotic divisions that partition the enormous volume of zygotic cytoplasm into thousands of smaller nucleated blastomeres without organismal growth ($G_1$ and $G_2$ cell cycle phases omitted).
1. Classification of Cleavage Types by Yolk Distribution
| Cleavage Class | Yolk Amount & Symmetry | Subtype & Geometry | Representative Taxa |
|---|---|---|---|
| Holoblastic (Complete) | Isolecithal (sparse, uniformly distributed yolk) | Radial (Sea urchins) Spiral (Molluscs, Annelids) Bilateral (Tunicates) Rotational (Mammals, Nematodes) |
Mammals, Echinoderms, Invertebrates |
| Holoblastic (Complete) | Mesolecithal (moderate vegetal yolk) | Displaced radial cleavage (unequal micromeres vs macromeres) | Amphibians (Xenopus) |
| Meroblastic (Incomplete) | Telolecithal (dense yolk filling egg; small animal pole blastodisc) | Discoidal cleavage (cleavage furrow restricted to blastodisc cap) | Birds (Chick), Reptiles, Fish (Zebrafish) |
| Meroblastic (Incomplete) | Centrolecithal (central yolk mass; peripheral cytoplasm) | Superficial cleavage (syncytial blastoderm followed by cellularization) | Insects (Drosophila) |