The role of steroids in follicular growth

Follicular development begins during foetal life with the transformation of primordial germ cells into oocytes and their enclosure in structures called follicles. In most mammals, primordial follicles form either before, or in the first few days after birth. Primordial follicles give rise to primary follicles which transform into preantral (secondary follicles) then antral follicles (tertiary follicles) and finally preovulatory, Graafian follicles, in a co-ordinated series of transitions regulated by hormones and local intraovarian factors. The growth and differentiation of follicles from the primordial population is termed folliculogenesis. With the LH surge, Graafian follicles rupture and oocytes are released, leaving the follicular cells to luteinise and form a corpus luteum.Sex steroids play important roles in the growth and differentiation of reproductive tissues and in the maintenance of fertility. Produced de novo from cholesterol, progestins, androgens and oestrogens are synthesised by the ovary in a sequential manner, with each serving as substrate for the subsequent steroid in the pathway. The two-cell, two-gonadotrophin model describes the role of theca and granulosa cells in the production of steroids, highlighting the cooperation between the two cell types, which is necessary for oestrogen production (Figure 1). Given that signal transduction for these hormones usually requires the binding and activation of a ligand-specific receptor, one cannot easily dissociate these components and assign definitive roles. The steroid hormones signal via nuclear receptors to regulate transcriptional events. These receptors form part of a nuclear receptor superfamily, all of which contain common structural elements [1,2]. These include a highly conserved DNA binding domain (DBD), a moderately conserved ligand binding domain (LBD) and 2 transactivation domains, AF1 located in domain A/B and AF2 in domain E/F (Figure 2). This review will address the roles that steroid ho