Mechanochemical mechanisms driving zebrafish partial (meroblastic) cleavages

Cleavages - the initial divisions of the fertilized egg - represent a fundamental and evolutionarily conserved process marking the onset of metazoan development. Across species, two principal modes of cleavage are observed: complete (holoblastic) and incomplete (meroblastic). While holoblastic cleavages closely resemble conventional cytokinesis both in vitro and in vivo, the mechanisms driving meroblastic cleavages remain less well understood. Here, we demonstrate that meroblastic cleavages in zebrafish embryos follow a two-tiered process. First, the formation and contraction of a large, arc-shaped actomyosin belt initiate the indentation of the cleavage furrow at the animal pole of the egg. However, this belt contraction alone is insufficient to fully subdivide the animal portion of the egg. Completion of the process requires a second step: the invagination of the cleavage furrow ridge, driven by cadherin-mediated membrane adhesion. Critically, this furrow invagination step depends on the mechanical uncoupling of the ingressing actomyosin belt from the adjacent lateral actin cortex, which facilitates its inward movement. Thus, meroblastic cleavages are governed by mechanisms distinct from those of holoblastic cleavages, involving a sequential interplay between actomyosin belt contraction and cadherin-dependent membrane adhesion. 

Chair: Daan Brinks