Recent findings on oogenesis of Drosophila melanogaster: I. Ultrastructural observations on the developing ooplasm

Abstract

ABSTRACT The ultrastructure of the developing ooplasm has been examined in 2–3-day-old flies of Drosophila melanogaster with a view to clarifying the origin of the yolk platelets. Previtellogenic ovarian chambers have been shown to exhibit a number of cytolysosomes; these are first formed in the nurse cells and are later brought to the ooplasm after having passed through the ring canals. Just prior to the beginning of vitellogenesis some of the cytolysosomes in the ooplasm acquire in places a yolk-like appearance. As vitellogenesis commences, in ‘early stage-8 chambers, the central ooplasm comes to exhibit a number of inclusions which combine in themselves features of both yolk platelets and cytolysosomes. The onset of vitellogenesis at stage 8 is marked by the appearance of several structural entities on the plasma membrane of the oocyte. At this site, there appear a number of depressions or pits in between the microvilli. Deeper in the cortical ooplasm there are also present a number of coated vesicles and tubules. While the former exhibit a typical threelayered structure similar to that of the pits, the tubules lack the outer spiked layer. Mature yolk platelets appear to be made up of three major components. These are a central homogeneous dense body, an outer superficial layer with a number of small spherules and a rounded body therein, and an external limiting membrane. Instances of fusion between vesicles of sizes progressively larger have been observed with frequency. This observation is discussed in relation to the formation of yolk platelets by means of pinocytotic uptake. A contribution made by the nurse cells to the formation of the yolk platelets is also taken into account.