Collective cell migration in a topologically closed epithelium
The follicle cell epithelium has its apical surface toward the germ cells and its basal surface
facing out. Consequently, the basement membrane ECM forms the egg chamber's outermost layer.
During egg chamber elongation, the follicle cells undergo a directed migration across the inner
surface of the basement membrane that causes the entire egg chamber to rotate within this stationary
matrix. Although migration has been intensively studied in single cells, the mechanisms by which
cells migrate collectively are less well understood. Moreover, follicle cell migration occurs in a
topologically closed epithelium with no leading edge, suggesting that novel mechanisms may polarize
this epithelium for forward movement.