C H A P T E R 3 9
Introduction to the reproductive system
599
the fallopian tube and then into the uterus. The ruptured
follicle becomes a functioning endocrine gland called the
corpus luteum
. It will continue to produce oestrogen and
progesterone for 10 to 14 days unless pregnancy occurs.
Fertilisation of the ovum and implantation in the
uterine wall results in the production of human chori-
onic gonadotropin (HCG). This hormone stimulates the
corpus luteum to continue to produce oestrogen and
progesterone until the placenta develops and becomes
functional, producing these hormones at a level high
enough to sustain the pregnancy.
If pregnancy does not occur, the corpus luteum invo-
lutes and becomes a white scar on the ovary. This scar
is called the corpus albicans. Initially, the rising levels
of oestrogen and progesterone produced by the corpus
luteum act as a negative feedback system to the hypo-
thalamus and the pituitary, stopping the production and
secretion of GnRH, FSH and LH. Later in the cycle, the
corpus luteum atrophies, the falling levels of oestrogen
and progesterone stimulate the hypothalamus to release
GnRH and the cycle begins again.
Factors influencing control mechanisms
Because of its position in the brain, the hypothalamus
is influenced by many internal and external factors. For
example, high levels of stress can interrupt the reproduc-
tive cycle. Tremendous amounts of energy are expended
in reproduction, and if the body needs energy for “fight
or flight”, the hypothalamus shuts down the reproduc-
tive activities, stopping the release of GnRH, which
results in no FSH or LH release and no stimulation of the
follicles. This saves a tremendous amount of energy in
the body, energy that the body will use for fight or flight.
In addition to stress, starvation, extreme exercise and
emotional problems are all associated with a decrease
in reproductive capacity related to the controls of the
hypothalamus.
The menstrual cycle
The cyclical nature of the female sex hormones on the
body produces the
menstrual cycle
. The onset of the
menstrual cycle at puberty is called the
menarche
. Each
cycle starts with release of FSH and LH and stimula-
tion of the ovarian follicles. For about the next 14 days,
the developing follicles release oestrogen into the body.
Thus the woman may notice the many effects of oestro-
gen, such as breast tenderness and water retention. In
addition, oestrogen thins cervical mucosa and increases
susceptibility to infections.
By about day 14, the oestrogen levels have caused
the LH surge and ovulation occurs. The woman experi
ences increased body temperature, increased appetite,
breast tenderness, bloating and abdominal fullness and
constipation, among others—the effects associated with
progesterone, which is released into the system when the
follicle ruptures. The uterus becomes thicker and more
vascular as the cycle progresses and develops a prolif-
erative endometrium. After ovulation, the lining of the
uterus begins to produce glucose and other nutrients
Decreased uterine motility (to provide increased chance
that implantation can occur)
Development of a secretory endometrium (to provide
glucose and a rich blood supply for the developing
placenta and embryo)
Thickened cervical mucus (to protect the developing
embryo and keep out bacteria and other pathogens;
this is lost at the beginning of labour as the mucous
plug)
Breast growth (to prepare for breastfeeding)
Increased body temperature (a direct hypothalamic
response to progesterone, which stimulates
metabolism and promotes activities for the developing
embryo; this increase in temperature is monitored in
the “rhythm method” of birth control to indicate that
ovulation has occurred)
Increased appetite (this is a direct effect on the satiety
centres of the hypothalamus and results in increased
nutrients for the developing embryo)
Depressed T-cell function (again, this protects the non-
self cells of the developing embryo from the immune
system)
Anti-insulin effect (to generate a higher blood glucose
concentration to allow rapid diffusion of glucose to
the developing embryo)
■■
BOX 39.2
Effects of progesterone
CNS
Hypothalamus
GnRH
Anterior pituitary
Follicles
Corpus luteum
Oestrogen
Progesterone
Oestrogen
LH FSH
LH
surge
FIGURE 39.2
Interaction of the hypothalamic, pituitary and ovarian
hormones that underlies the menstrual cycle. Dotted lines indicate
negative feedback surge. CNS, central nervous system; FSH, follicle-
stimulating hormone; GnRH, gonadotropin-releasing hormone;
LH, luteinising hormone.
