Figure 1.

Polar cells are produced in excess and restrict to a pair of cells during oogenesis. (a) Schematic drawing of an ovariole showing the anteriorly positioned germarium from which follicles (or egg chambers) are formed, subsequently maturing progressively as they move toward the posterior (stages 2 through 9 follicles shown of 14 total stages). Each follicle is composed of 15 nurse cells (nc) and one posteriorly positioned oocyte (o) all of germline origin, surrounded by a monolayered epithelium of somatic follicular cells (fc). Specialized somatic cells are indicated by different colors, polar cells in green, interfolicular cells in red and border cells in blue. Groups of supernumerary polar cells restrict to only a pair of cells between stages 2 and 5. As of stage 7, anterior polar cells induce differentiation of adjacent border cells and during stage 9, the border/polar cell group delaminates from the follicular epithelium and migrates between the nurse cells to reach the oocyte. (b–e) Confocal images of upd-Gal4; UAS-mCD8:GFP follicles in which polar cells are identified by membrane mCD8:GFP whose expression is driven only in these cells (green) and immunodetection of endogenous Fas3 (red). (b′–e′) Panels are magnified views of boxed areas in the corresponding (b–e) panels. Polar cells are marked with asterisks. The anterior of all follicles is to the left and the apical side of polar cells in (c′–e′) also to the left. (b and c) At early stage 2, the membrane mCD8:GFP marker allows specific identification of polar cells, while the Fas3 marker is expressed in both polar cells and surrounding follicular cells. At this stage, polar cells are flattened with their long axis perpendicular to the antero-posterior axis of the follicle. Some follicle extremities contain more than two polar cells (four in b) or two polar cells (c). (d) At stage 3, Fas3 and mCD8:GFP both show specific polar cell accumulation. At this stage, polar cells in clusters of three cells acquire a more rounded shape, are equivalent in size and organized around an axis of radial symmetry, all maintaining apical contact with the germline (arrowhead), as well as basal contacts with the basement membrane. (e) At stage 6, follicles present two equivalent polar cells at each extremity, each contacting the germline apically (arrowhead)