Influence of granulosa cells and of different somatic cell types on mammalian oocyte development in vitro

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Abstract
In mammals the ability of an oocyte to become fertilised is the result of a complex process occurring within the ovarian follicle which depends on the stagespecific expression of oocyte genes and the presence of granulosa cells (for a review see Buccioneet al., 1990a). The coordinated development of germinal and somatic components of the follicle is regulated by two principal systems of interaction, based on the presence of gap junctions and on the production of paracrine factors. Gap junctions link granulosa cells to each other and to the oocyte (Anderson & Albertini, 1976), and represent a major route for the transfer of small molecules involved in oocyte metabolism (for a review see Mangiaet al., 1992) and regulation of the arrest and resumption of meiosis (for a review see Eppig, 1993). The production of paracrine factors by granulosa cells has been suggested by the findings that these cells express the production of theSteellocus, the Steel factor (SLF) or kit ligand (KL; Motroet al., 1991; Manovaet al., 1993), and that this factor promotes oocyte growthin vitrowhen used at high concentrations (Packeret al., 1994). Since KL is too large to be transmitted through gap junctions, it must necessarily be released in the extracellular environment before binding to the c-kitreceptor present on oocyte membrane (Manovaet al., 1990; Horieet al., 1991).