Organization and in vitro activity of microfilament bundles associated with the basement membrane of Drosophila follicles.

The microfilament pattern in the somatic follicle cells of Drosophila ovarian follicles has been studied by staining F-actin with rhodaminyl-phalloidin. Parallel microfilament bundles were observed at the basal side of the follicle cells at all analyzed stages, but the organization of the microfilaments was found to undergo characteristic changes during development. At previtellogenic and early vitellogenic stages the microfilaments formed very long and thin bundles which were oriented perpendicular to the long axis of the follicle. Actin-containing cell protrusions formed only at one side of each cell indicating a planar circular cell polarity (best seen at stages 7 and 8). At later stages densely packed parallel microfilaments were observed at the basal end of the follicle cells. This pattern was maintained until stage 14 when the microfilament bundles became thinner and more widely spaced and finally disintegrated. During late vitellogenic stages the cell shape differed basally and apically: while apically the cells formed regular and very precise patterns the basal cell borders were convoluted. When stage 10 follicles were isolated in simple saline solutions the diameter of the oocyte decreased during 30 min culture. This contraction, which was presumably due to the activities of the basal microfilament bundles, could be inhibited by cytochalasins as well as azide or dinitrophenol. The reaction was most likely induced by the in vitro culture conditions, since there is no evidence that the contraction also takes place in loco.