Chapter 27
The Reproductive System
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27
Hormonal interactions between the hypothalamus, anterior pi-
tuitary gland, and gonads, a relationship called the
hypothalamic-
pituitary-gonadal (HPG) axis
, regulate the production of gam-
etes and sex hormones. Let’s take a look.
The Hypothalamic-Pituitary-Gonadal (HPG) Axis
Te sequence of regulatory events involving the HPG axis,
shown schematically in
Figure 27.10
, is as follows:
1
Te hypothalamus releases
gonadotropin-releasing hor-
mone (GnRH)
, which reaches the anterior pituitary cells
via the blood of the hypophyseal portal system. GnRH con-
trols the release of the two anterior pituitary gonadotro-
pins:
follicle-stimulating hormone (FSH)
and
luteinizing
hormone (LH)
, both named for their effects on the female
gonad.
2
GnRH binds to pituitary cells (gonadotropic cells), prompt-
ing them to secrete FSH and LH into the blood.
3
FSH stimulates spermatogenesis indirectly by stimulating
the sustentocytes to release
androgen-binding protein
(ABP)
. ABP keeps the concentration of
testosterone
in
the vicinity of the spermatogenic cells high, which in turn
stimulates spermatogenesis. In this way, FSH enhances tes-
tosterone’s stimulatory effects.
4
LH binds to the interstitial endocrine cells in the so± con-
nective tissue surrounding the seminiferous tubules, prod-
ding them to secrete testosterone (and a small amount of
estrogen). Locally, rising testosterone levels serve as the fi-
nal trigger for spermatogenesis.
5
²estosterone entering the bloodstream exerts a number of
effects at other body sites. It stimulates maturation of sex
organs, development and maintenance of secondary sex
characteristics, and libido (sex drive).
6
Rising levels of testosterone feed back to inhibit hypotha-
lamic release of GnRH and act directly on the anterior pi-
tuitary to inhibit gonadotropin release.
7
Inhibin
(in-hib
9
in), a protein hormone produced by the
sustentocytes, serves as a “barometer” of the normalcy of
spermatogenesis. When the sperm count is high, more
inhibin is released, inhibiting anterior pituitary release of
FSH and hypothalamic release of GnRH. (Te inhibitory
effect of testosterone and inhibin on the hypothalamus is
not illustrated in Figure 27.10.) When the sperm count falls
below 20 million/ml, inhibin secretion declines steeply.
As you can see, the amount of testosterone and sperm pro-
duced by the adult testes reflects a balance among the three in-
teracting sets of hormones that make up the HPG axis:
GnRH, which indirectly stimulates the testes via its effect on
FSH and LH release
Gonadotropins (FSH and LH), which directly stimulate the
testes
Gonadal hormones (testosterone and inhibin), which exert
negative feedback controls on the hypothalamus and ante-
rior pituitary
Te sustentocytes:
Provide nutrients and essential signals to the dividing cells,
even telling them to live or die
Move the cells along to the lumen
Secrete
testicular fluid
(rich in androgens and metabolic acids)
that provides the transport medium for sperm in the lumen
Phagocytize faulty germ cells and the excess cytoplasm
sloughed off as the spermatids transform into sperm
Produce chemical mediators (inhibin and androgen-binding
protein) that help regulate spermatogenesis
Given hospitable conditions, spermatogenesis—from forma-
tion of a primary spermatocyte to release of immature sperm
into the lumen—takes 64 to 72 days. Sperm in the lumen are
unable to “swim” and are incapable of fertilizing an egg. Te
pressure of the testicular fluid pushes them through the tubular
system of the testes into the epididymis, where they gain in-
creased motility and fertilizing power.
Homeostatic Imbalance
27.3
Roughly one in seven American couples seek treatment for
infertility, mostly because of problems with sperm quality or
quantity. According to some studies, a gradual decline in male
fertility has been occurring in the past 50 years.
Some believe the main culprit is
xenobiotics
, alien molecules
that have invaded our lives in a variety of forms—environmental
toxins, PVCs, phthalates (oily solvents that make plastics flexible),
pesticides and herbicides, and especially compounds with estro-
genic effects. Tese estrogen-like compounds, which block the
action of male sex hormones as they program sexual develop-
ment, are now found in our meat supply as well as in the air.
Common antibiotics such as tetracycline may suppress sperm
formation, and radiation, lead, marijuana, lack of selenium, and
excessive alcohol can cause abnormal (two-headed, multiple-
tailed, etc.) sperm to be produced. Male infertility may also be
caused by the lack of a specific type of Ca
2
1
channel (Ca
2
1
is
needed for normal sperm motility), anatomical obstructions,
hormonal imbalances, and oxidative stress (which fragments
sperm DNA). Termal-related events that inhibit sperm matu-
ration include fever and overusing hot tubs.
Check Your Understanding
13.
What is the final outcome of meiosis?
14.
Describe the major structural and functional regions of a
sperm.
15.
What is the role of sustentocytes? Of interstitial endocrine
cells?
For answers, see Appendix H.
Hormonal Regulation of Male Reproductive
Function
Discuss hormonal regulation of testicular function and the
physiological effects of testosterone on male reproductive
anatomy.
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