epiboly

Epiboly is a morphogenetic movement in which cells spread as a cohesive sheet to enclose the yolk cell during early embryonic development. It is a characteristic process of many vertebrate and invertebrate embryos, most notably observed in amphibians, fish, and birds. During epiboly, the vegetal pole of the embryo expands anteriorly and laterally, driven by coordinated cell proliferation, shape changes, and intercalation, ultimately forming the embryonic ectoderm that will give rise to the epidermis and nervous system.

The mechanism of epiboly involves several cellular behaviors. In amphibian gastrulation, for example, deep cells undergo radial intercalation that thins the tissue, while superficial cells flatten and expand by apical constriction. In zebrafish, the enveloping layer (EVL) and the deep cell layer move together, aided by the actomyosin cortex that generates contractile forces at the leading edge. The process is regulated by signaling pathways such as Wnt/PCP, Nodal, and FGF, which modulate cytoskeletal dynamics and cell adhesion molecules like E‑cadherin. Disruption of epiboly can result in developmental defects, including failure to close the blastopore or abnormal body axis formation.

Epiboly is studied as a model for collective cell migration and tissue remodeling, providing insights into wound healing, cancer metastasis, and organogenesis. Comparative analyses across species have highlighted both conserved and divergent aspects of the movement, underscoring its fundamental role in shaping the embryo during the earliest stages of life.