Developmental studies of vitellogenesis in a dipteran insect, Chironomus thummi

Vitellogenesis of developing oocytes of a Dipteran insect Chironomus thummi has been investigated. The onset of yolk deposition is marked by the differentiation of the oolemma including the formation of microvilli and endocytosis. These changes are accompanied by the appearance of small electron dense granules, similar in density to the yolk platelets, arising through the sequential accumulation of material into the matrices of the multivesicular bodies (MVBs). These latter structures are produced in the previtellogenic oocytes of the pharate pupae and early pharate adults. Often the limiting membrane of the MVBs bears bristle coats resembling those of the coated vesicles of pinocytotic origin, suggesting that it is through the fusion with the pinocytotic vesicles that the accumulation of dense material in the MVBs results. That the Mvbs transform into structures resembling yolk granules is supported by statistical analysis which indicates that the decrease in the number of electron-dense MVBs coincides with the increase in the occurrence of small dense yolk granules. In the late pharate adult stage the yolk granules are considerably larger than those of earlier stages. It is during this period that at least one type of electron-dense granule occurs at the oocyte follicle cell border, and that these apparently contribute to the formation of the vitelline envelope. The results of the present study indicate that preformed oocytic elements, the MVBs, play a strategic role in the formation and arrangement of the yolk granules in Chironomus. Since these structures account for the bulk of the ooplasm, it appears that the MVBs are at least partly responsible for the correct ordering of the cytoplasmic constituents of the oocytes, which is critical for the proper development and differentiation of the embryo.