C H A P T E R 3 9
Introduction to the reproductive system
601
point, local prostaglandins stimulate uterine contraction
and the onset of labour. Once the fetus and the placenta
have been expelled from the uterus, the hormone levels
plummet towards the non-pregnant state. It often takes
6 to 8 weeks to reverse the effects of these hormones.
This is a time of tremendous adjustment for the body as
it tries to re-achieve homeostasis.
Menopause
The follicles contained in the ovary become depleted
over time, the ovaries no longer produce oestrogen
and progesterone, and
menopause
—the cessation of
menses—occurs. The hypothalamus and pituitary
produce increased levels of GnRH, FSH and LH for a
while in an attempt to stimulate the ovaries to produce
oestrogen and progesterone. If that does not happen, the
levels of these hormones fall back within a normal range
in response to their own negative feedback systems.
Menopause is associated with loss of many of the effects
of these two hormones on the body, including retention
of calcium in the bones, lowered serum lipid levels and
maintenance of secondary sex characteristics.
■■
The female ovary stores ova and produces the sex
hormones oestrogen and progesterone.
■■
The hypothalamus releases GnRH at puberty to
stimulate the anterior pituitary release of FSH and
LH, thus stimulating the production and release of
the sex hormones. Levels are controlled by a series of
negative feedback systems.
■■
Female sex hormones prepare the body for pregnancy
and the maintenance of the pregnancy. If pregnancy
does not occur, the prepared inner lining of the uterus
sloughs off as menstruation in the menstrual cycle.
■■
Menopause occurs when the supply of ova is
exhausted and the woman’s body no longer produces
the hormones oestrogen and progesterone.
MALE REPRODUCTIVE SYSTEM
The male reproductive system consists of two testes,
the vas deferens, the prostate gland, the penis and the
urethra. The hormone that stimulates and maintains
these structures is testosterone.
Structures
The male reproductive system originates from the same
fetal cells as in the female. The major male reproduc-
tive system structure is the testes, the two endocrine
glands that continually produce
sperm
, as well as the
hormone
testosterone
. During fetal development, the
two testes migrate down the abdomen and descend
KEY POINTS
into the scrotum outside the body. There they are pro-
tected from the heat of the body to prevent injury to the
sperm-producing cells. The testes are made up of two
distinct parts: the
seminiferous tubules
, which produce
the sperm, and the
interstitial
or
Leydig cells
, which
produce the hormone testosterone. Other components
include the vas deferens, which stores produced sperm
and carries sperm from the testes to be ejaculated from
the body; the prostate gland, which produces enzymes
to stimulate sperm maturation, as well as lubricating
fluid; the penis, which includes two corpora cavernosa
and a corpus spongiosum, structures that allow mas-
sively increased blood flow and erection; the urethra,
through which urine and the sperm and seminal fluid
are delivered; and other glands and ducts that promote
sperm and seminal fluid development (Figure 39.4).
Hormones
The primary hormone associated with the male repro-
ductive system is testosterone. Testosterone is responsible
for many sexual and metabolic effects in the male. Like
oestrogen, testosterone enters the cell and reacts with a
cytoplasmic receptor site to influence mRNA activity,
resulting in the production of proteins for cell structure
or function. Box 39.3 summarises the effects of testos-
terone on the body.
If the testes are lost before puberty occurs, there will
be no development of the secondary male sex characteris-
tics or the other effects seen when testosterone is released.
Such a person would require testosterone replacement
therapy to develop these characteristics. However, once
puberty and the physical changes brought about by tes-
tosterone have occurred, the androgens released by the
adrenal glands are sufficient to sustain the male char-
acteristics. Androgens are very similar in structure to
testosterone and are able to influence cells to maintain
the changes caused by testosterone. This is important
information for adults undergoing testicular surgery or
chemical castration.
Control mechanisms
The activity of the male sex glands is not thought to
be cyclical like that of the female. The hypothalamus
in the male child is also sensitive to circulating levels of
adrenal androgens and suppresses GnRH release. After
the hypothalamus matures, this sensitivity is lost and
the hypothalamus releases GnRH. This in turn stimu
lates the anterior pituitary to release FSH and LH, or
what is sometimes called interstitial cell–stimulating
hormone (ICSH) in males. FSH directly stimulates the
seminiferous tubules to produce sperm, a process called
spermatogenesis. FSH also stimulates the Sertoli cells in
the seminiferous tubules to produce oestrogens, which
provide negative feedback to the pituitary and hypothal-
amus to cause a decrease in the release of GnRH, FSH
and LH.
