Local regulatory factors controlling folliculogenesis in pigs.

Although many excellent reviews have summarized follicular development from the perspective of gonadotrophin-induced follicular differentiation and steroid secretion (Richards, 1980), the process of selection and dominance of ovulatory follicles during each ovarian cycle remains poorly understood. The present review will focus primarily on the role of intragonadal substances in the regulation of follicular maturation. Since previous summaries have considered follicular maturation with respect to prostaglandins (Aksel et at, 1977), insulin, somatomedins (Adashi et at, 1985), relaxin (Bagnell et al., 1987), lipoproteins (Gwynne & Strauss, 1982) and gonadotrophinreleasing hormone (Gn RH) (Knecht et al., 1983), details of these topics will not be repeated here. Rather, we will review the growing body of data from experiments using predominantly follicular tissue and cells from the sow which address the intraovarian regulation of folliculogenesis. Gonadotrophins play a role in initiation and maintenance of follicular growth, selection of dominant follicles and their maturation to preovulatory status. Evidence for this is derived from changes in blood concentrations of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), concomitant alterations in the number and type of gonadotrophin receptors in granulosa and theca cells, and from the secretion of steroidal and non-steroidal products by these cells in response to gonadotrophin stimulation. However, follicular growth cannot be accounted for entirely by the changing concentrations of gonadotrophins. Rather, the developing responsiveness of follicles to stimulation by FSH and LH results from changes in the production of and alterations in follicular sensitivity to intraovarian paracrine and/or autocrine factors (Tonetta & diZerega, 1986). In this context, gonadotrophins are necessary, but not sufficient by themselves, to account for the ovarian cycle. In recent years, several intragonadal regulators have been identified and characterized which can alter follicular maturation. In addition to the well-known paracrine factors discussed below, additional paracrine regulators have been identified but their function is still uncertain (see Tonetta & diZerega, 1989).