Origins of follicular cells and ontogeny of steroidogenesis in ovine fetal ovaries.

Using fetal sheep as the experimental model, we have elucidated some of the key events that culminate in the formation of primordial follicles. A special effort was made to determine the source of the somatic cells that ultimately become granulosa cells of primordial follicles. Between gestational days 38-100: (1) light and electron microscopy was used to characterize changes in ovarian histoarchitecture; (2) incorporation of BrdU was used to identify populations of proliferating cells within fetal ovaries before, during and after, follicular formation; and (3) in situ hybridisation was used to determine the cell-specific and temporal patterns of expression of mRNAs encoding for selected steroidogenic enzymes. At day 38 somatic (pregranulosa) cells were in contact with oogonia and easily distinguished from endothelial and mesenchymal cells. Between days 38 and 45, pregranulosa cell-oogonia complexes progressively coalesced to form 'tube-like' structures referred to as ovigerous cords. These cords consisted of pregranulosa cells and oogonia arranged such that pregranulosa cells formed the outer wall of the cords. Ovigerous cords were avascular, enveloped in a prominent basal lamina, open-ended where they interfaced with the ovarian surface epithelium, and formed a separate compartment whereby oogonia/oocytes were segregated from the surrounding stroma and vasculature until the time of follicular formation. The structural integrity of ovigerous cords was maintained through day 75, at which time primordial follicles (type 1 and type 1a) first emerged from the cords at the interface of the cortex and medulla. On the basis of the sequential structural changes that occurred during the differentiation and development of fetal ovaries and location of proliferating cells identified by the incorporation of BrdU, we conclude that the majority of the granulosa cells in primordial follicles are derived from mesothelial cells originating from the ovarian surface epithelium. In addition, from the cell-specific distribution and temporal pattern of expression of mRNAs for key steroidogenic enzymes we hypothesize that steroid hormones may play a pivotal paracrine/autocrine role in the formation and/or function of ovigerous cords as well as the development of the ovarian vascular network.