Structure of ovaries and oogenesis in entognathans (Apterygota)

Two categories of female gonads can be discerned among entognathans. In Protura, Collembola and Campodeina (Apterygota), the paired ovaries are sac-shaped, not divided into discrete ovarioles. In contrast, the ovaries of Japygina are composed of 7 metamerically arranged ovarioles. The ovaries (ovarioles) of proturans and japygids are panoistic, whereas those of collembolans and campodeids are polytrophic-meroistic. Germ-cell clusters of collembolans and campodeids are always chain-like (non-branched). The oocyte develops from the cell placed centrally within the chain. Other cells become presumptive nurse cells. Differentiated nurse cells synthesize rRNA that is later transferred to the developing oocytes. In the panoistic ovaries (proturans and japygids), oocyte nuclei (germinal vesicles) are large and contain huge, active nucleoli. Three types of reserve materials are deposited in entognathan oocytes: lipid droplets, yolk spheres, and characteristic dense granules. Vitellogenesis is of a mixed type. Egg envelopes are secreted by the follicular cells and/or by the oocyte. The possible evolution (anagenesis) of entognathan ovaries is discussed.