| Abstract: | The egg chamber of Drosophila melanogaster consists of 16 interconnected cells surrounded by a monolayer of follicle cells. Each 16 cell cluster (from which the oocyte and 15 nurse cells differentiate) arises within the germarial region of an ovariole. To study the ultrastructure of the early stages in the formation and differentiation of egg chambers, a three dimensional reconstruction was made from serial thin sections through a germarium from a 24-hour old, virgin female. The germarium was found to be subdivided into three regions: (1) The mitotically active area where clusters of 16 cells originate from a series of cystocyte divisions, (2) the region where these cells interact with mesodermal cells, and (3) the region where the germarial cyst is transformed into the first egg chamber in the vitellarium. Since cystocytes were found to decrease in size with each division, the possibility exists that cell size may determine when the divisions cease. Models are presented which mimic with varying degrees of success the developmental changes the germarial cells undergo with time. Hypothesis are developed to explain why stem line oogonia are restricted to the anterior portion of the germarium, why mesodermal cells first interact with cystocytes in region 2, and why the oocyte is oriented posteriorly. The nuclear differentiations of the component cells of the chamber are described and correlated with observed differences in radiosensitivity. Symbionts were observed in the germaria of several strains of Drosophila, and the bearing of these findings upon nutritional studies is discussed. |
