Rab5-mediated Yolk Cell Endocytosis modulates Zebrafish Epiboly Biomechanics and Tissue Movements

Morphogenetic processes demand the coordinated allocation of cells and tissues to their final destination in a spatio-temporal controlled way. Identifying how these morphogenetic movements are directed and implemented is essential for understanding morphogenesis. Topographical and scalar differences in adhesion and contractility within and between cells are essential, yet, the role that membrane remodeling may play remains less clear. To clarify how surface turnover and dynamics may modulate tissue arrangements we studied epiboly in the zebrafish. During epiboly the blastoderm expands as a result of an asymmetry of mechanical tension along the embryo surface. In this scenario, we found that the membrane removal by macropinocytosis of the external yolk cell syncytial layer (E-YSL) ahead of the blastoderm is key for epiboly progression In early zebrafish embryos, the activity of the GTPase Rab5ab was essential for endocytosis, and interference in its expression exclusively in the yolk cell resulted in the reduction of yolk cell actomyosin contractility, the disruption of cortical and internal yolk flows, a disequilibrium in force balance and as a result epiboly impairment. We conclude that regulated membrane remodeling is crucial for directing cell and tissue mechanics and coordinating morphogenetic movements during epiboly.