Electron Microscopy of Oocyte-Follicle Cell Relationships in the Rat Ovary

The ovary of the neonatal rat has been found to offer very favourable material for an examination of the structural relationship between an oocyte and its surrounding follicle cells, since a large proportion of the follicles are at a similar stage of development and can be examined within a single section under the electron microscope. This has made it possible to examine the changes in cell surface relationships during the development of the follicle, from a stage where the oocyte possesses an incomplete covering of flattened cells to that in which a complete layer of cuboidal cells is present. The material used in this study consisted of the ovaries of albino rats killed from a few hours to several days after birth. The organs were fixed in osmium tetroxide and in potassium perman-ganate buffered to pH 7.6 with veronal-acetate, and embedded in methacrylate and araldite respectively. Observations made on osmium-fixed ovaries from neonatal rats essentially confirm the findings of recent authors using osmium-fixed ovaries from the adult rabbit (1), mouse (2), and rat (3). The use of permanganate for fixation has, however , helped to resolve cell surface interrelationships to a finer degree. In an early stage of follicle formation, the cell membranes of the oocyte and follicle cell are smooth and closely applied to each other. The separation between the membrane is of the order of 125 to 140 A. The follicle cells are very flattened but their extremities, which may extend for a considerable distance around the periphery of the oocyte, tend to be filiform. Owing to curvature these may appear in thin sections as a small number of isolated circular or elongated profiles. At a slightly later stage, when the follicle cells have acquired a less flattened form, small "pock-ets" containing larger numbers of follicle cell processes appear at intervals around the oocyte (Fig. 1). It is believed that this increase in numbers is due to the formation of fine branches from the filiform ends of the cells, although clear evidence of branching has so far been seen only in follicles at a somewhat later stage of growth. Short club-like microvilli from the surface of the oocyte also extend into these "pockets" and inter-digitate with the follicle cell processes. The oocyte microvilli elongate and become cylindrical as the intercellular space widens. A further stage in development is marked by the lateral extension of the "pockets" to form …