Ultrastructure de l'epithelium folliculaire et des enveloppes de l'ovocyte chez Lepismachilis targionii (grassi) (Thysanura : Machilidae)

Abstract

During oogenesis in Lepismachilis targionii, the follicular epithelium undergoes important transformations, synchronous with those of the oocyte. Three successive phases can be distinguished: (1) During previtellogenesis and primary vitellogenesis, a monolayered epithelium is formed around the oocyte; the cells then become more numerous through repeated mitoses: (2) From the beginning of secondary vitellogenesis, the follicular cells acquire new characteristics. Phenomena of the endomitosis type lead to the formation of polyploid, highly lobate nuclei, while the cytoplasm becomes richer in organelles, indicating intense metabolic activity. The secretion reaches the perioocytic space, where the so-called previtelline envelope is deposited. The oocyte draws yolk precursors from this secretion, which probably contains nutrients supplied by the haemolymph through the intercellular spaces. At the end of vitellogenesis this secretion becomes very dense and forms the vitelline envelope: (3) Once vitellogenesis is over, the follicular cells become flat and present new cytoplasmic changes, with the formation of numerous vesicles of various kinds. Junctions of the adhaerens type, which were absent during secondary vitellogenesis, become well developed. Finally the five different layers of the chorion are deposited.