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).
Follicular
phase
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).
Luteal phase
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
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 2–4% of circulating
testosterone is free and ‘biologically active’. 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
5α-reductase. Both testosterone and DHT exert their effects by binding to
androgen receptors.
Androgen
action
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.
