Practice question

Epiboly is a major morphogenetic movement during gastrulation where sheet-like ectodermal cells spread, flatten and thin to enclose deeper mesodermal and endodermal layers and eventually yolk. It does not involve invagination but rather radial intercalation and expansion of superficial epithelium driven by remodeling of E-cadherin contacts, actin cytoskeleton and differential adhesion. In zebrafish, amphibians and mammalian embryos epiboly helps position ectoderm externally to form epidermis and neural ectoderm. Failure of epiboly leaves involuting marginal cells uncovered, disrupting body axis formation and gastrulation closure.