Structure-Function Relationships in the Wall of the Ovarian Follicle

Abstract

This paper reviews current knowledge of the light and electron microscopic structure of the three layers of the mammalian follicular wall-follicular epithelium (membrana granulosa), theca interna and theca externa-and discusses correlations between structure and function. The ultrastructure of follicular epithelial cells in growing follicles emphasizes their protein synthetic and secretory functions; features suggestive of a major steroidogenic function appear only at later stages. Regional differences in follicular epithelial cell function are probably important, although structurally these cells are relatively homogeneous. Structural diversity is more marked in the thecal layers : differentiation in the theca interna is towards fibrocytic and steroidogenic cell types, while that in the theca externa is towards fibrocytic and myoid types. Adherens and gap junctions are present between cells in all layers; however, tight (occludens) junctions have not been convincingly demonstrated between the cells in any of the three layers. Blood and lymph vessels are confined to the thecal layers. However, follicles possess no structural barrier comparable to that associated with the 'blood-testis barrier', and show a correspondingly greater permeability to large molecules than seminiferous tubules. Interactions between the layers of the follicular wall have not yet been intensively investigated, but are likely to play an important role in follicular function. To date, the best-documented interaction between layers is that described in the 'two-cell hypothesis' of oestrogen production. Some potentially useful directions for future research are proposed.