Germband extension is a tissue remodeling event that occurs during the gastrulation stage of the Drosophila embryonic development. During this process the germband, which is the region of the embryo that gives rise to various segments of the body, doubles in length along the anterior-posterior axis while simultaneously narrowing down along the dorsal-ventral axis through a series of cellular rearrangements.
Cell intercalation is the most prominent type of cellular rearrangement driving Drosophila germband extension. Intercalation is accompanied by the formation of rosette-like patterns on the apical sides of cells along the anterior-posterior axis, which is driven by actomyosin contractions and apical junction reorganization events. However, the dynamics occurring on the basal region of cells were largely unknown.
In this study, Sun et al have used live imaging of embryos to reveal the formation of rosette patterns on the basolateral sides of cells during germband extension, and specifically, cell intercalation. In most cells, basolateral rosettes are seen to form earlier than the apical rosettes. The formation of basolateral rosettes are initiated by the dorsal most and ventral most cells of the cell cluster, which put forth actin-based, wedge-shaped protrusions at their cell fronts. The protrusions facilitate the movement of the two cells towards each other along the dorsal-ventral axis, causing the neighboring cells to realign into a rosette pattern on the basolateral side.