The role of microtubules in the differentiation of ovarian follicles during vitellogenesis inDrosophila

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Abstract
Microtubule distribution in vitellogenicDrosophila follicles has been analysed with a tubulin-specific monoclonal antibody. The functional role of microtubules in germ line cells and somatic follicle cells was tested by allowing vitellogenesis to proceed in vivo or in vitro in the presence of colchicine or other microtubule inhibitors. Resulting morphological abnormalities were studied in 1 μm sections of Epon-embedded follicles. In addition, the effect of the inhibitors on cytoplasmic streaming was analysed by time-lapse cinematography. The results can be summarized as follows: Cytoplasmic streaming in the oocyte of stage 10–12 follicles was inhibited by colchicine, while streaming of nurse cell cytoplasm into the oocyte continued. This differential effect on cytoplasmic streaming was also apparent in histological sections. Since in inhibitor-treated follicles the nurse cell cytoplasm streaming into the oocyte did not mix with the ooplasm, the oocyte became stratified with respect to cytoplasmic constituents (e.g. yolk spheres were restricted to lateral and posterior locations and nurse-cell-derived lipid droplets to anterior and central positions). The oocyte nucleus is held in place by a dense microtubule network. After treatment with the inhibitors, the oocyte nucleus became translocated towards the posterior egg pole. Actively migrating follicle cells (border cells and cells migrating centripetally between nurse cells and oocyte at stage 10B) stained brightly with anti-tubulin. In the pressence of colchicine, these groups of cells showed various abnormalities and no centripetal migration was observed. If inhibitors were applied after this cell migration had taken place, these follicle cells became disorganized and often produced a highly irregularly shaped vitelline membrane. At stage 12 the follicle cells at the anterior egg pole (where normally the chorionic appendages and other specific structures of the egg shell are formed) rounded up, became highly irregular and did not produce any recognizable structures. Exocytosis of yolk and components of the vitelline envelope from the columnar follicle cells surrounding the oocyte at stage 10 became abnormal in the presence of microtubule inhibitors. Preliminary experiments indicate that yolk accumulated in follicle cells while vitelline membrane products continued to be exocytosed. Apico-basal follicle cell polarity was apparently affected by the drug in so far as vitelline membrane material accumulated not only at the apical end (which is normal) but also laterally in the intercellular space between the follicle cells.