Identification of cells migrating from the thecal layer of ovarian follicles

In the mammalian ovary, oocytes are contained within ovarian follicles. These consist in an oocyte surrounded by supporting cells: an inner layer of granulosa cells and an outer layer of thecal cells separated by a basal lamina. At any one time, a developing cohort of follicles exists, from which only a small species-specific number are selected for continued development towards ovulation, with the remainder dying by follicular atresia. Here, we use in vitro methods to study interactions between two follicles in culture (follicle co-cultures). We show that, when two individual follicles are grown together in culture, cells and cellular processes migrate from the outer thecal layer of one follicle to the thecal layer of the other co-cultured follicle. These cells are identified as a mixed population containing primarily endothelial but also neuronal cells. Both are able to migrate through the ovarian interstitum, making contact with the basal lamina of other follicles and with similar cells from these other follicles. Networks of such cells might be involved in interfollicular communication and in the coordination of follicle selection for ovulation.