Figure 1.

 Three phases of follicle cell development during oogenesis. (A) ∼650 follicle cells form an epithelial layer that surrounds each maturing Drosophila egg chamber; in late stages most surround the oocyte, although a few follicle cells sheath the 15 nurse cells. Within follicle cells over the oocyte, two clusters of chorion genes on the X and third chromosomes amplify 15- and 60-fold, respectively. (B) Schematic representation of an ovariole is shown. Egg chambers originate in the germarium and migrate posteriorly down the ovariole as they develop. Follicle cells originate from two somatic stem cells in the germarium and complete approximately eight mitotic divisions over a 6-day period to achieve their final number during stage 6 (S6). Thereafter, follicle cells undergo three endocycles (see C); the exact stage at which endocycles are complete is uncertain (broken vertical line). Follicle cells then enter a postendocycle stage for the remainder of oogenesis. Amplification is known to occur during this phase (bottom solid line), but whether it initiates during the endocycles is not known (broken line). (C) FACS profile of follicle cell nuclear DNA content, showing 4C, 8C, and 16C classes produced by three endocycles. Egg chamber drawings modified from King (1970).