Gonadal Steroids and Their Control
Gonadal Steroids and
Their Control
The ovaries produce two kinds
of steroid sex hormones (GPH)— estrogens and progesterone (Figure
7-14). There are three kinds of estrogens:
estradiol, estrone and estriol, of
which estradiol is secreted in the highest
amounts during reproductive
cycles. Estrogens are responsible for
development of female accessory sex
structures (oviducts, uterus, and
vagina) and for stimulating female
reproductive activity. Secondary sex
characters, those characteristics that
are not primarily involved in formation
and delivery of ova (or sperm in the
male), but that are essential for behavioral
and functional success of reproduction,
are also controlled or maintained
by estrogens. These include
characteristics such as distinctive skin
or feather coloration, bone development,
body size and, in mammals, initial
development of the mammary
glands. In mammals, both estrogen
and progesterone are responsible for
preparing the uterus to receive a developing
embryo. These hormones are
controlled by pituitary gonadotropins:
follicle-stimulating hormone
(FSH), and luteinizing hormone
(LH) (Figure 7-15). The two
gonadotropins are in turn governed by gonadotropin releasing hormone
(GnRH) produced by neurosecretory
cells in the hypothalamus. Through this control
system environmental factors such as
light, nutrition, and stress may influence
reproductive cycles.
The male sex steroid, testosterone (Figure 7-14), is manufactured by the interstitial cells of the testes. Testosterone, and its metabolite, dihydrotestosterone (DHT), are necessary for the growth and development of the male accessory sex structures (penis, sperm ducts, and glands), development of secondary male sex characters (such as bone and muscle growth, male plumage or pelage coloration, antlers in deer, and, in humans, voice quality), and male sexual behavior. Development of the testes and secretion of testosterone is controlled by FSH and LH, the same pituitary hormones that regulate the female reproductive cycle, and ultimately by GnRH from the hypothalamus. Testosterone and DHT feedback to the hypothalamus and anterior pituitary to keep the secretion of GnRH and FSH and LH in check (see Chemical Coordination, for a discussion of negative feedback of hormones). The testes also secrete a second hormone, the peptide inhibin, which is secreted by the sustentacular cells (or Sertoli cells). This hormone is an additional regulator of the secretion of FSH from the anterior pituitary in a negative feedback manner.
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| Figure 7-14 Sex hormones. These three sex hormones show the basic four-ring steroid structure. The main female sex hormone, estradiol (an estrogen) is a C18 (18-carbon) steroid with an aromatic A ring (first ring to left). The main male sex hormone testosterone (an androgen) is a C19 steroid with a carbonyl group (C=O) on the A ring. The female sex hormone progesterone is a C21 steroid, also bearing a carbonyl group on the A ring. |
The male sex steroid, testosterone (Figure 7-14), is manufactured by the interstitial cells of the testes. Testosterone, and its metabolite, dihydrotestosterone (DHT), are necessary for the growth and development of the male accessory sex structures (penis, sperm ducts, and glands), development of secondary male sex characters (such as bone and muscle growth, male plumage or pelage coloration, antlers in deer, and, in humans, voice quality), and male sexual behavior. Development of the testes and secretion of testosterone is controlled by FSH and LH, the same pituitary hormones that regulate the female reproductive cycle, and ultimately by GnRH from the hypothalamus. Testosterone and DHT feedback to the hypothalamus and anterior pituitary to keep the secretion of GnRH and FSH and LH in check (see Chemical Coordination, for a discussion of negative feedback of hormones). The testes also secrete a second hormone, the peptide inhibin, which is secreted by the sustentacular cells (or Sertoli cells). This hormone is an additional regulator of the secretion of FSH from the anterior pituitary in a negative feedback manner.
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