The Structural Basis of Oocyte-Granulosa Cell Communication

Abstract

The ontogeny of female germ cells in animals encompasses phases of prolonged storage, rapid growth and maturation, and mechanisms for releasing oocytes from the ovary (Wallace and Selman 1990). Ovarian follicles provide the multicellular unit for prolonged storage of oocytes both prior to and during the reproductive lifespan of females. The follicle, formed by the physical interaction of oocytes and granulosa cells, supports the rapid growth and maturation of oocytes (Canipari 1994). To accomplish this, the somatic granulosa cells surrounding the oocyte undergo periodic changes in structure and function that concurrently attend the needs of the developing oocyte and organismal reproductive cyclicity. The physical linkage between female germ cells and the ovarian soma is finally interrupted during the process of ovulation, even though in many animals the oocyte is released with somatic cells attached to the oolemma or investing extracellular matrices (Suzuki et al. 2000). Thus, from both a phylogenetic and ontogenetic perspective, oogenesis clearly involves a protracted period of contact between the oocyte and surrounding follicle cells. The morphological and physiological relevance of oocyte—granulosa cell contact in the mammalian ovary is considered below with reference to the form, composition and functions subserved by specialized structures known as transzonal projections (TZPs).