Follicular development through preantral stages: signalling via growth factors

Although gonadotrophins, particularly FSH, are the primary pituitary regulators of ovarian folliculogenesis, the involvement of ovarian-derived growth factors in follicular growth and maturation has become increasingly apparent over the past decade. Regulators of ovarian somatic cells can be broadly divided into (1) mitogenic factors, and (2) differentiation-induction factors, based on two primary cellular requirements, that is, proliferation and differentiation that are fundamental to folliculogenesis and ovarian functions. In this article, direct as well as indirect evidence is presented that highlights the roles of epidermal growth factor--a mitogen, and transforming growth factor beta--a differentiation-induction factor, in modulating gonadotrophin action in the ovary, particularly in the preantral follicles. An exquisitely timed and regionalized expression of these two types of peptide factor, along with their membrane receptors, may determine the differential success of follicle development, hence, allowing selection of the best oocytes for fertilization and subsequent development.