750 Chapter 20
CONCLUDING REMARKS
We shall not cease from exploration
And the end of all our exploring
Will be to arrive where we started
And know the place for the first time.— T.S. Eliot, Little Gidding, VIt may seem strange to end a textbook on physiology with
the topics of pregnancy and parturition. This is done in part
for practical reasons; these topics are complex, and to
understand them requires a grounding in subjects covered
earlier. Also, it seems appropriate to end at the beginning,
at the start of a new life. Although generations of research-
ers have accumulated an impressive body of knowledge,
the study of physiology is still young and rapidly growing.
I hope that this introductory textbook will serve students’
immediate practical needs as a resource for understanding
current applications, and that it will provide a good founda-
tion for a lifetime of further study.
Linda was given injections of a GnRH analogue that stim-
ulated the GnRH receptors in her anterior pituitary in a
constant, rather than in a pulsatile, fashion. This caused
desensitization of these receptors, reducing her anterior
pituitary secretion of FSH and LH, and thereby reducing
the secretion of estradiol and progesterone from her ova-
ries. She took birth control pills containing estrogen and
progesterone to compensate. She may have had uterine
fibroids, because these are very common, but her symp-
toms of painful menstruation, thinning hair on her head,
and growth of body hair are more indicative of polycystic
ovarian syndrome (PCOS). The ovarian cysts would have
been seen in the ultrasound to confirm this diagnosis.
Her physician advised Linda to lose weight and exercise,
because women with PCOS are more likely to develop
type 2 diabetes and metabolic syndrome. Her pregnancy
test detected hCG, a hormone secreted by the embry-
onic trophoblast, using monoclonal antibodies against
the beta subunit of hCG.
See the additional chapter 20 Clinical Investigations on
Male Pituitary Adenoma and Polycystic Ovarian Syndrome
in the Connect site for this text.Clinical Investigation SUMMARY
SUMMARY
1. The secretion of FSH and LH is stimulated by
gonadotropin-releasing hormone (GnRH), which is
secreted by the hypothalamus.
2. The secretion of FSH and LH is also under the control
of the gonads by means of negative feedback exerted
by gonadal steroid hormones and by a peptide called
inhibin.
B. The rise in FSH and LH secretion that occurs at puberty may
be due to maturational changes in the brain and to decreased
sensitivity of the hypothalamus and pituitary gland to the
negative feedback effects of sex steroid hormones.
C. The pineal gland secretes melatonin. This hormone has an
inhibitory effect on gonadal function in some species of
mammals, but its role in human physiology is presently
controversial.
D. The human sexual response is divided into four phases: exci-
tation, orgasm, plateau, and resolution. Both sexes follow a
similar pattern.20.3 Male Reproductive System 712
A. In the male, the pituitary secretion of LH is controlled by
negative feedback from testosterone, whereas the secretion
of FSH is controlled by the secretion of inhibin from the
testes.
1. The negative feedback effect of testosterone is
actually produced by the conversion of testosterone to
5 a -reduced androgens and to estradiol.20.1 Sexual Reproduction 702
A. Sperm that bear X chromosomes produce XX zygotes when
they fertilize an ovum; sperm that bear Y chromosomes pro-
duce XY zygotes.
- Embryos that have the XY genotype develop testes;
those without a Y chromosome produce ovaries. - The testes of a male embryo secrete testosterone and
müllerian inhibition factor. MIF causes degeneration of
female accessory sex organs, and testosterone promotes
the formation of male accessory sex organs.
B. The male accessory sex organs are the epididymis, ductus (vas)
deferens, seminal vesicles, prostate, and ejaculatory duct. - The female accessory sex organs are the uterus
and uterine (fallopian) tubes. They develop when
testosterone and müllerian inhibition factor are absent. - Testosterone indirectly (acting via conversion to
dihydrotestosterone) promotes the formation of male
external genitalia; female genitalia are formed when
testosterone is absent.
C. Numerous disorders of embryonic sexual development can
be understood in terms of the normal physiology of the
developmental processes.
20.2 Endocrine Regulation of Reproduction 708
A. The gonads are stimulated by two anterior pituitary
hormones: FSH (follicle-stimulating hormone) and LH
(luteinizing hormone).