The History of the Cytoplasmic Elements during Vitellogenesis in Drosophila melanogaster

Abstract

ABSTRACT At least two kinds of yolk are developed in the eggs of Drosophila melanogaster. The origin of the proteid yolk has been traced to precursors sent into the oocyte by the nurse cells, while the minute fatty yolk granules are transformed chondriomites also contributed by the nurses. The behaviour and morphology of Golgi material in the nurses indicate that it may be responsible for a third type of yolk. A yolk nucleus has been found in oocytes of an age when the germinal disk has been pushed to one side. So long as such a structure exists, all the proteid yolk precursors are concentrated and matured within its confine. Soon this structure disintegrates and disappears. But after its disappearance, yolk precursors continue to be sent by the nurses into the oocyte, and there they develop into mature granules apparently without the benefit of a yolk nucleus. On the basis of facts thus far obtained, it is, therefore, difficult to say just what role the so-called yolk nucleus plays in vitellogenesis in Drosophila melanogaster. Golgi material in the nurse cells has been observed to undergo two distinct proliferations. In young nurses, Golgi material occurs in a few discrete bits which, as the nurses grow in size, proliferate into a large number of intertwining threads so arranged as to suggest a net. Then follows a period in which only very few minute granules of Golgi material can be seen. This is definitely a period of scarcity. After this, a second proliferation takes place. This one is carried to such an extreme extent that the cytoplasm of the cells is just black with Golgi material. The Golgi threads, arranged more or less in a parallel fashion and very closely together, present the appearance of pieces of Golgi ‘tissue’. Next, the pieces of ‘tissue’ loosen up into separate threads from which individual Golgi granules arise. These granules enlarge to become vesicles. In the course of development of the nurse cells, the mitochondria also undergo a proliferation. Finally, the chondrioconts break up into granules which enlarge and eventually all transform into droplets of unsaturated fat. My observations point to the conclusion that the oocyte depends upon its nurses for the production of fatty yolk. Proteid yolk granules, however, are developed within the oocyte, though from precursors sent in also by the nurse cells. The follicular cells do not appear to contribute anything to vitellogenesis in Drosophila. An idiosome-like granule has been observed in oogonia, and also in oocytes up to a certain stage of development. For reasons stated elsewhere, it cannot be regarded as an idiosome nor as a centrosome. This structure has not been observed in nurse cells.