Ovarian folliculogenesis is a dynamic process characterized by a marked proliferation and differentiation of the somatic cell components of the follicle. The developing follicle provides the optimal environment for the maturation of the oocyte in readi-

Ovarian folliculogenesis is a dynamic process characterized by a marked proliferation and differentiation of the somatic cell components of the follicle. The developing follicle provides the optimal environment for the maturation of the oocyte in readiness for its subsequent fertilization after ovulation. The primary regulators of folliculogenesis are the gonadotrophins, which regulate ovarian follicular development via classic endocrine mechanisms. In addition to this systemic mechanism, it has become increasingly apparent that additional intraovarian mechanisms are involved in the control of folliculogenesis. The concept of intraovarian control of ovarian function arose in the 1970s and is now recognized as a complex regulatory system involved in modulating the response of follicles to circulating gonadotrophins (Adashi and Roban, 1992). Ovarian follicles are known to produce a range of locally acting peptide/protein growth factors. These can either interact directly with the same cell type from which they are produced (autocrine action) or with other cell types within the developing follicle (paracrine action) to stimulate or attenuate the cellular response to gonadotrophins. Additional mechanisms, such as juxtacrine (activation of receptors on adjacent cells by membrane bound growth factors) and intracrine (stimulation of the cell in which the factor is produced without prior secretion of the factor from the cell) are also recognized as mechanisms through which granulosa and theca cells can control their behaviour. Folliculogenesis is associated with the development of a group of follicles at various stages of development from which a species-specific number of follicles is selected for continued growth (Webb et al., 1992; Fortune 1994; Campbell et al., 1995; Gong and Webb, 1996). According to Goodman and Hodgen (1983) folliculogenesis can be divided into three separate stages: (1) recruitment, the stage during which a pool of growing follicles begin to grow rapidly; (2) selection, a process whereby follicles are selected for further growth; and (3) dominance, the process whereby the dominant follicle(s) undergoes rapid development while the growth of subordinate follicles is suppressed. This pattern of follicle development is associated with changes in expression of mRNA encoding gonadotrophin receptors (Xu et al., 1995a) and steroidogenic enzymes (Xu et al., 1995b) and allows selected follicles, when exposed to the requisite hormonal environment, to ovulate in response to the preovulatory gonadotrophin surge. Endocrine signals (for example, gonadotrophins, inhibins and steroids), as well as locally produced growth factors, are responsible for the control and coordination of these processes. Functional models for follicle selection and the control of ovulation rate in ewes have been described (Scaramuzzi et al 1993) that take some of these concepts into account. In this review, we will describe some of the more recent ideas on the intraovarian control of folliculogenesis. The mechanisms whereby locally produced growth factors control the development of dominance will be described. The disruption of this local control system in subordinate follicles, by putative factors produced by dominant follicles, coupled with alterations in the endocrine signals, will also be discussed. Where possible we will illustrate the mechanisms described using data from experiments in cattle. However, it must be stressed that, although the growth factors described in this review are expressed in the Ovarian follicular dominance: the role of intraovarian growth factors and novel proteins