Page 315 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Estrogens the cervical mucus, making it more conducive
to sperm penetration and survival, especially
Estrogens are steroid hormones with 18 car- around the time of ovulation.
bon atoms. Estrogens are primarily synthe- ! Fertilization. In the female body, estrogens
sized from the 17-ketosteroid androstene- prepare the sperm to penetrate and fertilize
dione, but testosterone can also be a precursor the ovum (capacitation) and regulate the
(! p. 295 A). The ovaries, (granulosa and theca speed at which the ovum travels in the fal-
cells), placenta (! p. 304), adrenal cortex, and lopian tube.
in Leydig’s cells (interstitial cells) of the testes ! Extragonadal effects of estrogen. During
(! p. 306) are the physiological sites of estro- puberty, estrogens stimulate breast develop-
Hormones and Reproduction (E). The potencies of estrone (E l) and estriol (E 3) and axillary hair. Since estrogens increase the
gen synthesis.. In some target cells for testo-
ment, induces changes in the vagina and in the
sterone, it must first be converted to estradiol
distribution of subcutaneous fat, and (together
with androgens) stimulate the growth of pubic
to become active.
Estradiol (E 2) is the most potent estrogen
coagulability of the blood, the administration
of estrogens (e.g., in contraceptives) increases
are relatively low in comparison (E 2 : E 1 : E 3 =
the risk of thrombosis and leads renal salt and
10 : 5 : 1). Most estrogens (and testosterone)
water retention. Estrogens slow longitudinal
circulating in the blood are bound to sex hor-
bone growth, accelerate epiphyseal closure (in
mone-binding globulin (SHBG). Estriol (E 3) is
the chief degradation product of estradiol (E 2).
tivity. Estrogen deficiencies in menopause
Oral administration of estradiol has vir-
tually no effect because almost all of the hor-
consequently lead to the loss of bone mass
11 mone is removed from the bloodstream during men and women) and increase osteoblast ac-
(osteoporosis). Estrogens induce a decrease in
its first pass through the liver. Therefore, other LDL and a rise in VLDL and HDL concentrations
estrogens (with a different chemical structure) (! p. 254ff.), which is why arteriosclerosis is
must be used for effective oral estrogen ther- less common in premenopausal women than
apy. in men. Estrogen also makes the skin thinner
Actions. Although estrogens play a role in and softer, reduces the sebaceous glands, and
the development of female sex characteristics, increases fat deposits in subcutaneous tissue.
they are not nearly as important as the andro- Lastly, estrogen influences a number of central
gens for male sexual development (! p. 306). nervous functions, e.g., sexual response, social
The preparatory action of estrogen is often re- behavior, and mood.
quired for optimal progesterone effects (e.g., in
the uterus; see below). Other important effects Plasma concentrations of estradiol and progesterone
of estrogens in human females are as follows. (ng/mL)
! Menstrual cycle. Estrogens accelerate matu- Phase Estradiol Progesteron
ration of the ovarian follicle during the men- Women
strual cycle (! p. 298 and table). In the uterus, Early follicular phase 0.06 0.3
estrogen promotes the proliferation (thicken- Mid- and late follicular 0.1 ! 0.4 1.0
ing) of the endometrium and increases uterine phase
muscle contraction. In the vagina, estrogen Ovulation 0.4 2.0
thickens the mucosal lining, leading to the Mid-luteal phase 0.2 8–16
increased discharge of glycogen-containing Pregnancy 7–14 40 ! 130
epithelial cells. The liberated glycogen is used Day 1 after parturition 20
for an increased production of lactic acid by Men 0.05 0.3
Döderlein’s bacillus. This lowers the vaginal
pH to 3.5–5.5, thereby reducing the risk of Progesterone
vaginal infection. In the cervix, the mucous
plug sealing the cervical os functions as a bar- Progesterone, the most potent progestational
rier that prevents sperm from entering the (pregnancy-sustaining) hormone, is a steroid
302 uterus. Estrogens change the consistency of hormone (21 C atoms) synthesized from
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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