Endocrine and paracrine regulation of cumulus expansion.

Abstract

In a Graafian follicle, granulosa cells are classified into two principal cell subpopulations: cumulus cells, which are closely associated with the oocyte to form the cumulus cell-oocyte complex (COC), and mural granulosa cells, which are organised as a stratified epithelium at the periphery of the follicle. Following the preovulatory gonadotropin surge, cumulus cells lose contact with mural granulosa cells and start to synthesise and secrete a large amount of hyaluronan (HA), a glycosaminoglycan with high molecular weight and large hydrodynamic domains (Salustriet al., 1992). Proteins derived from serum (Chenet al., 1992, 1994) and synthesised by cumulus cells (Camaioniet al., 1993, 1996) organise the strands of HA into an intercellular elastic network that traps the cumulus cells and the oocyte in a unit which can not be mechanically dissociated – a process also referred to as cumulus expansion. At ovulation, the expanded COC is released through the ruptured follicle wall and transferred to the oviduct. The matrix in the expanded COC facilitates its extrusion from the follicle and its capture by oviductal fimbria, and provides, together with the cumulus cells, a suitable microenvironment for sperm penetration and fertilisation (for references see Salustriet al., 1993).