The major female reproductive organs are a pair of ovaries, situated in the lower abdomen. At birth, each ovary houses approximately 1 million immature follicles. Each follicle is composed of an egg cell (ovum) surrounded by two layers of cells: an inner layer of granulosa cells and an outer layer of theca cells. During the follicular phase of the female reproductive cycle (Box 11.3), a group of follicles (~20), approximately 5 mm in diameter, are recruited by FSH (i.e. they begin to grow). FSH targets the granulosa cells, prompting them to synthesize oestrogen. The dominant follicle continues to grow to a diameter of 20-25 mm (Figure 11.B2). At this stage, it contains a fluid-filled cavity with the ovum attached to one side. Ovulation is characterized by bursting of the follicle and release of the ovum.
Typically, 400 follicles will mature and fully ovulate during an average woman's reproductive lifetime. The remaining 99.98 per cent of her follicles begin to develop, but regress due to inadequate FSH stimulation. The molecular detail of how FSH (and LH) promotes follicular growth is described in the main body of the text.
Fluid filled sac
Fluid filled sac
Mature follicle prior to ovulation
Prior to puberty, serum FSH levels are insufficient to promote follicular recruitment and development. Subsequent to puberty, as a group of follicles begin to develop at the beginning of a cycle, the one that is most responsive to FSH (i.e. displays the lowest FSH threshold) becomes the first to secrete oestrogen. As one effect of oestrogen is to suppress FSH release from the pituitary, blood FSH levels then plateau or decline slightly. This slightly lower FSH concentration is insufficient to sustain growth of follicles of higher FSH thresholds, so they die, leaving only the single oestrogen-producing dominant follicle (Boxes 11.3 and 11.4) to mature and ovulate.
FSH exerts its molecular effects via a specific receptor on the surface of sensitive cells. This receptor contains a characteristic seven transmembrane-spanning regions and is functionally coupled (via membrane-associated G-proteins) to adenylate cyclase. This generates the second messenger cAMP. FSH itself can promote increased expression of its own receptor in the short term, although longer-term exposure to elevated FSH levels down-regulates receptor numbers. Cloning and analysis of gonadotrophin receptors from several species indicate a high level of homology between the FSH, LH and CG receptors.
LH exhibits a molecular mass of 28.5 kDa. The gene coding for the P-subunit of this glycoprotein hormone is present on human chromosome 19. This subunit exhibits significant amino acid homology to placental CG. Both promote identical biological effects and act via the same 93 kDa cell surface receptor. The LH receptor is present on testicular Leydig cells in males and on female ovarian theca, as well as granulosa, luteal and interstitial cells.
LH promotes synthesis of testosterone, the major male androgen (Box 11.5) by the testicular Leydig cells. FSH sensitizes these cells to the activities of LH, probably by increasing LH receptor numbers on the cell surface. Leydig cells have a limited storage capacity for testosterone (~25 |g), but secrete 5-10 mg of the hormone into the bloodstream daily in young healthy males.
The primary cellular targets of LH in the females are the follicular theca cells, which con-stitutively express the LH receptor. Under the influence of LH, these cells produce androgens. The androgens (principally testosterone) are then taken up by granulosa cells and converted into oestrogens (Box 11.5) by the already-mentioned aromatase complex. Thus, the follicle represents the major female gonadal endocrine unit, in which granulosa and theca cells cooperate in the synthesis of oestrogens. Physiologically, LH in the female plays a major role in maturation of the dominant follicle and appears central to triggering ovulation.
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