Ultrastructural modifications of the follicular epithelium accompanying the onset of vitellogenesis in the whirligig beetle,Gyrinus natator

Summary The ultrastructure of the follicle cells during previtellogenesis and early vitellogenesis have been studied. In previtellogenesis follicle cells are columnar with numerous bundles of microtubules located along the lateral plasma membranes. Oocyte-follicle cell gap junctions are not found in this stage. At the onset of vitellogenesis, the bundles of microtubules disappear and are replaced by an apically located ring of microtubules. The modification of microtubular cytoskeleton is not followed by the development of intercellular spaces between the follicle cells. Concurrently, numerous gap junctions are formed between specialized follicle cell processes and oocyte microvilli, which are arranged in characteristic cone-shaped aggregations. It is suggested that cytoskeletal changes and formation of heterologous gap junctions, occurring at the onset of vitellogenesis, are induced by juvenile hormone.