Current concepts of the roles of follicle stimulating hormone and luteinizing hormone in folliculogenesis.

Around 400 follicles sequentially mature and ovulate during an average woman's reproductive lifetime. From birth to the menopause, the other approximately 99.98% of her follicles begin development but never complete it. Instead they default to atresia due to inadequate stimulation by follicle stimulating hormone (FSH). Follicular growth to the stage of antrum formation (approximately 0.25 mm diameter) is independent of gonadotrophic stimulation. Antrum formation and further growth to the stage at which follicles become potentially able to begin preovulatory development (2-5 mm diameter) require tonic stimulation by FSH. Before onset of puberty, blood concentrations of FSH do not rise sufficiently to sustain development beyond this stage, therefore all antral follicles become atretic. After puberty, as each menstrual cycle begins, FSH concentrations rise beyond a critical 'threshold' and multiple follicles are recruited to begin pre-ovulatory development. Due to increases in its responsiveness to FSH and luteinizing hormone (LH), one of these follicles becomes selected to ovulate while the remainder become atretic. At mid-follicular phase, the dominant follicle reaches > or = 10 mm in diameter and increasingly synthesizes oestradiol. Tonic stimulation by FSH and LH, underpinned by local paracrine signalling, maintains oestrogen secretion by the dominant follicle, which grows to > or = 20 mm in diameter before it ovulates in response to the mid-cycle LH surge. The development-related response to LH shown by the pre-ovulatory follicle raises the possibility that exogenous LH might be used as an adjunct to therapy with exogenous FSH in clinical ovulation induction regimens where the aim is to induce monovulation.