Development of interstitial cells and ovigerous cords in the human fetal ovary: an ultrastructural study.

Differentiation and subsequent regression of interstitial cells in the human fetal ovary was studied by light and electron microscopy in specimens obtained from twelve fetuses between 12 and 40 weeks of gestation. Interstitial cells possessing the ultrastructural features associated with steroid production first differentiate from fibroblast-like cells by 15 weeks. This occurs within the medullary fibrous tissue penetrating the cortex. The number of such cells is maximal at 18 weeks around the ovigerous cords in the inner half of the cortex. In addition, gap junction formation is seen in the pregranulosa cells forming ovigerous cords. From 21 to 31 weeks primordial follicles develop from ovigerous cords, which themselves develop from undifferentiated cortex, in a sequence that is most advanced in the medullary portion of the gonad and least advanced at its surface. During this time the interstitial cells decrease in number. They are not observed around the follicles, but are occasionally seen in the outer layer of the cortex which is still at the stage of ovigerous cords. At 40 weeks, growing follicles associated with several layers of enveloping theca cells are observed in the innermost region of the cortex. Interstitial cells are rare in all parts of the ovary. The development of interstitial cells independent of follicles in the human fetal ovary implies that such cells are distinct from theca cells, and that they correspond to the primary interstitial cells of other mammalian species. In addition anatomical and temporal relationships are observed between the interstitial cells, the ovigerous cords, developing primordial follicles and the formation of gap junctions in pregranulosa cells.