Relationships of the Ovular Surface with Follicle Cells and Origin of the Zona Pellucida in Rabbit Oocytes

In mammals, immature oocytes are surrounded by a layer of flattened cells. At this early stage of development, these "follicle cells" and the plasma membrane of the oocyte are in close proximity. As the oocyte matures, the follicle cells multiply and become cuboidal or radially elongated, and a jelly-like substance containing polysaccharides (1) appears between the oocyte cortex and the follicle cells. When fully developed, this jelly will constitute the zona pellucida (ZP). Although separated by the zona pellucida, the bodies of the follicle cells and the mature oocyte maintain a close relationship throughout the life of the latter. Two cellular extensions effect this relationship: the multibranched prolongations of the follicle cells, and slender microvilli arising from the oocyte surface. Clearly, the zona pellucida contains both cellular extensions and amorphous substance. The present observations on rabbit follicles are intended to elucidate two problems that arise in considering the nature of these two components-the relationship between follicle and oocyte surface, and the origin of the amorphous substance. The material examined in this work was obtained from recently matured female rabbits and fixed in OsO4 dissolved in a solution of polyvinylpyrrolidone ("periston" Bayer), according to a method outlined in a previous communication (10). Cellular Extensions and the Relationship between Oocyte and Follicle Cells.--Many authors have interpreted the follicle cell prolongations as anastomotic bridges connecting the cytoplasms of follicle cell and oocyte (6, 8); others consider them to be extensions of the follicle cells that end at the oocyte surface, thereby forming a contact relationship only (2, 5, 7). Histologists seem to favor the second opinion, but Kemp's recent observations in the electron microscope (4) have caused new interest in the first concept. In a recent EM study of rat oocytes at various stages of growth, S0telo and Porter state that they could find no evidence of continuity between oocyte and follicle cell cytoplasms in their electron micrographs (9).