Preovulatory changes in morphology of rabbit ovarian follicles.

To examine timed changes in the morphology of theca interna and granulosa cells of rabbit preovulatory follicles, mature female rabbits were injected with100 IU per kg of human chorionic gonadotropin (hCG) and follicles larger than1mn in diameter were excised for light and electron microscopic studies before and3, 6, 9and 12hr after hCG injection. Spindle-shaped theca interna cells had been transformed into rounded cells with pronounced hypertrophy within9hr after hCG injection, and degenerative changes in cellular appearance with reduction in cellular density became apparent12hr after hCG administration. Electron micrographs of thecal cells showed that the maximum proliferation of large mitochondria and the full development of smooth endoplasmic reticulum (sER) had occurred by6hr after hCG injection, while by12hr after hCG administration abundant lipid droplets had accumulated and sER had decreased. These morphological changes in the theca interna cells indicate an activated endocrine function in the earlier stage followed by diminution in steroidogenesis in the later stage of the preovulatory interval. Although no appreciable changes was noted in granulosa cell histology in the first 6 hr after hCG injection, luteinizing changes proceeded9hr or more after hCG administration, as evidenced by the enlargement of cytoplasmic volume. A series of cytoplasmic modifications of granulosa cells leading to the formation of the fine structures characteristic of steroid-producing cells developed gradually in the earlier stage and rapidly in the final stage prior to ovulation, indicating differentiation of granulosa cells into lutein cells. It is concluded that the major cellular site of steroid biosynthesis in ovulating rabbit follicles may shift from degenerating thecal cells to luteinizing granulosa cells in the course of the preovulatory process.