Physiology Of The Reproductive System
Physiology of female reproduction
The menstrual cycle
The average menstrual cycle lasts 28 days and is usually only interrupted by pregnancy or terminated by the menopause. It consists of two alternating phases: the follicular and luteal phases. The follicular phase takes up the first half of the cycle and is characterised by developing follicles which produce oestrogen. The luteal phase follows ovulation and is characterised by the presence of the corpus luteum which synthesises progesterone and oestrogen (Figure 23.1).
At the beginning of the cycle, FSH and LH rise because of GnRH release. Rising FSH and LH stimulate oestrogen release from the developing follicle which stimulates follicle development (Figure 23.2). Moderate oestrogen levels inhibit FSH, but not tonic LH secretion from the anterior pituitary. Moderate oestrogen levels also stimulate positive feedback on the follicle, causing oestrogen levels to rise significantly. High oestrogen levels stimulate the anterior pituitary to produce large amounts of LH, the LH surge, which induces ovulation (Figure 23.3).
The ruptured follicle forms the corpus luteum, which produces large amounts of progesterone and oestrogen. Progesterone and oestrogen inhibit LH and FSH release from the anterior pituitary. In the absence of fertilisation, the corpus luteum degenerates, leading to a significant fall in progesterone and oestrogen levels. FSH and LH levels then rise as they are no longer inhibited, and a new cycle commences.
Oestrogen and progesterone action
Oestrogen released from the developing follicle leads to proliferation of the endometrium, comprising uterine glands and epithelium. Progesterone released by the corpus luteum stimulates the uterine glands to secrete ‘uterine milk’ which is high in protein and glycogen and provides a suitable environment in the event of fertilisation. In pregnancy, oestrogen is also important for uterine muscle development whereas progesterone dampens uterine contractility and stimulates breast growth. Oestrogen is also important for the development and maintenance of female secondary sexual characteristics after puberty, and in preserving bone mineral density.
Physiology of male reproduction
Hypothalamic and pituitary hormones
GnRH is secreted in a pulsatile manner by the hypothalamus, which stimulates the secretion of LH and FSH by the anterior pituitary. LH binds to its receptor in testicular Leydig cells to stimulate testosterone synthesis and secretion. FSH binds to its receptor in testicular Sertoli cells to regulate spermatogenesis, a process that takes >70 days. LH and FSH secretion is regulated positively by GnRH pulses and by negative feedback from testosterone, which inhibits both LH and FSH, and inhibin, which inhibits FSH only (Figure 23.4).
Testosterone is the principal androgen produced by the Leydig cells. It has a circadian rhythm, with peak levels reached at around 08.00. A small amount of oestradiol is also made by the testes, but most is generated by conversion (aromatisation) of androgens to oestradiol in adipose tissue. Only 2â€“4% of circulating testosterone is free and â€˜biologically activeâ€™. The remainder is bound to proteins, especially SHBG and albumin. Testosterone is metabolised in target tissues to the more potent androgen dihydrotestosterone (DHT) by the enzyme 5Î±-reductase. Both testosterone and DHT exert their effects by binding to androgen receptors.
Androgens have an important role in prenatal male sexual differentiation, in the development and maintenance of male secondary sexual characteristics after puberty, in spermatogenesis and in normal male behaviour.