Page 319 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Androgens and Testicular Function the functions of the genitalia, prostate and
seminal vesicle (see below), testosterone also
Androgens (male sex hormones) are steroid induces the secondary sex characteristics that
hormones with 19 C atoms. This group in- occur in males around the time of puberty, i.e.,
cludes potent hormones like testosterone (T) body hair distribution, physique, laryngeal size
and 5α-dihydrotestosterone (DHT) and less (voice change), acne, etc. In addition, testos-
potent 17-ketosteroids (17-KS) such as DHEA terone is necessary for normal sex drive (li-
(! p. 294). In males, up to 95% of testosterone bido), procreative capacity (fertility) and coital
is synthesized by the testes (! A2) and 5% by capacity (potentia coeundi) in the male. Testos-
the adrenal cortex (! Al). The ovaries and terone also stimulates hematopoiesis and has
Hormones and Reproduction proteins (albumin and sex hormone-binding e.g., cause aggressiveness.
anabolic properties, leading to increased
adrenal cortex synthesize testosterone in
muscle mass in males. It also has central
females. The plasma testosterone conc. in
males is about 15 times higher than in females,
nervous effects and can influence behavior—
but decreases with age. Up to 98% of testos-
terone circulating in blood is bound to plasma
Sexual development and differentiation. The
genetic sex (! B) determines the development of the
globulin, SHBG; ! A2).
sex-specific gonads (gamete-producing glands). The
germ cells (spermatogonia; see below) then migrate
The testes secrete also small quantities of DHT and
into the gonads. The somatic sex is female when the
estradiol (E 2). Larger quantities of DHT (via 5-α-re-
ductase) and estradiol are synthesized from testos-
entiation occurs in the absence of testosterone
terone (via aromatase) by their respective target
(! C). Male development requires the presence of
11 cells. A portion of this supply is released into the subsequent somatic sex development and sex differ-
testosterone in both steps (! C) with or without the
plasma. DHT and testosterone bind to the same in-
tracellular receptor. Estradiol influences many func- aid of additional factors (e.g., calcitonin gene-related
tions in the male, e.g., epiphyseal cartilage and peptide, CGRP?) in certain stages of development
ejaculate formation and pituitary and hypothalamic (e.g., descent of testes into scrotum). High conc. of
testosterone, either natural or synthetic (anabolic
activity.
steroids), lead to masculinization (virilization) of the
Testosterone secretion is regulated by female (! C).
luteinizing hormone (= LH, also called ICSH, Testicular function. Spermatogenesis occurs in
! p. 269), the pulsatile secretion of which is several stages in the testes (target organ of tes-
controlled by Gn-RH at 1.5- to 2-hourly inter- tosterone) and produces sperm (spermatozoa)
vals, as in the female. LH stimulates the release (! A3). Sperm are produced in the seminifer-
of testosterone from Leydig’s cells (interstitial ous tubules (total length, ca. 300 m), the
cells) in the testes (! A2), whereas testos- epithelium of which consists of germ cells and
terone and estradiol inhibit LH and Gn-RH Sertoli cells that support and nourish the sper-
secretion (negative feedback). matogenic cells. The seminiferous tubules are
Gn-RH also induces the release of FSH, strictly separated from other testicular tissues
which stimulates the secretion of inhibin and by a blood–testis barrier. The testosterone re-
induces the expression of androgen-binding quired for sperm maturation and semen pro-
protein (ABP) in Sertoli cells of the testes duction (! p. 308) must be bound to andro-
(! A3). Testosterone cannot induce spermato- gen-binding protein (ABP) to cross the barrier.
genesis without the help of ABP (see below).
FSH also induces the formation of LH receptors Spermatogonia (! B) are primitive sex cells. At
in the interstitial cells of Leydig. Testosterone, puberty, a spermatogonium divides mitotically to
DHT, estradiol and inhibin inhibit the secretion form two daughter cells. One of these is kept as a life-
of FSH (negative feedback; ! A). Activin, the time stem cell reservoir (in contrast to oogonia in the
female; ! p. 298). The other undergoes several divi-
physiological significance of which is still un- sions to form a primary spermatocyte. It undergoes
clear, inhibits FSH secretion. a first meiotic division (MD1) to produce two second-
Apart from the important effects of testos- ary spermatocytes, each of which undergoes a sec-
terone on male sexual differentiation, sper- ond meiotic division (MD2), producing a total of four
306 matogenesis and sperm growth as well as on spermatids, which ultimately differentiate into sper-
matozoa. After MD1, the spermatocytes have a
single (haploid) set of chromosomes.
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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