Apical membrane turnover is accelerated near cell-cell contacts in an embryonic epithelium

During embryogenesis, the killifish Fundulus heteroclitus forms a monolayered tight epithelium called the enveloping layer (EVL). These epithelial cells have been shown to rearrange during epiboly, as they spread to cover the large yolk cell. Membrane remodeling by exocytosis and endocytosis is important in establishing and maintaining the apical-basolateral polarity of many epithelial cells and is a necessary component of epithelial rearrangements, as cells constantly break contacts and reform tight junctions. To study these phenomena in Fundulus heteroclitus embryos, we labeled the apical membranes of EVL cells with fluorescent lectins and lipids and followed membrane dynamics. Apical membrane components were found to be highly immobilized, allowing us to observe localized sites of apical membrane turnover in situ, over the period of several days. We found that apical membrane turnover in the EVL cells of post-epiboly killifish embryos is accelerated at cell-cell contacts, in a peripheral band of apical membrane which closely borders circumferential tight junctions. Moreover, this turnover rate is increased during epiboly, when the cells are actively rearranging. To investigate whether this increased membrane turnover may be related to the mechanical forces experienced by the rearranging EVL cells, post- epiboly embryos, whose EVL cells no longer rearrange, were subjected to mechanical deformation. In these manipulated embryos, apical membrane turnover was accelerated at cell-cell contacts in EVL cells which experienced externally applied mechanical tension. These results suggest that local mechanical tension may modulate regional apical membrane turnover within EVL cells during the process of epiboly.