Page 317 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Hormonal Control of Pregnancy and the FAC, where it is converted to dehydroepian-
Birth drosterone (DHEA) and dehydroepian-
drosterone sulfate (DHEA-S). DHEA and DHEA-S
Beside its other functions, the placenta pro- pass to the placenta, where they are used for
duces most of the hormones needed during estrogen synthesis. Progesterone is converted
pregnancy (! p. 220). Ovarian hormones also to testosterone in the testes of the male fetus.
play a role, especially at the start of pregnancy
(! A). Human placental lactogen (hPL = human chorionic
somatomammotropin, HCS) levels rise steadily
Placental hormones. The primary hormones during pregnancy. Like prolactin (! p. 303), hPL
produced by the placenta are human chorionic stimulates mammary enlargement and lactogenesis
Hormones and Reproduction opiomelanocortin (POMC; ! p. 280). hCG is conc. hormone (CRH)
(hCG),
corticotropin-releasing
gonadotropin
in particular and, like GH (! p. 280), stimulates
progesterone,
physical growth and development in general. hPL
hormone
(CRH),
estrogens,
also seems to increase maternal plasma glucose
human placental lactogen (hPL), and pro-
the predominant hormone during the first
Corticotropin-releasing
trimester of pregnancy (3-month period calcu-
secreted by the placenta seems to play a key
lated from the beginning of the last menses).
role in the hormonal regulation of birth. The
Maternal conc. of hPL and CRH-controlled
plasma levels of maternal CRH increase ex-
estrogens rise sharply during the third
ponentially from the 12th week of gestation
trimester (! B). Placental hormones are dis-
tributed to mother and fetus. Because of the
and slower in post-term births. In other words,
the rate at which the CRH concentration rises
11 close connection between maternal, fetal and on. This rise is more rapid in premature births
placental hormone synthesis, they are jointly
seems to determine the duration of the preg-
referred to as the fetoplacental unit (! A). nancy. Placental CRH stimulates the release of
Human chorionic gonadotropin (hCG) (a) ACTH by the fetal pituitary, resulting in in-
stimulates the synthesis of steroids like DHEA creased cortisol production in the adult zone
and DHEA-S by the fetal adrenal cortex (see of FAC; this again stimulates the release of CRH
below); (b) suppresses follicle maturation in (positive feedback). CRH also stimulates lung
the maternal ovaries, and (c) maintains the development and the production of DHEA and
production of progesterone and estrogen in DHEA-S in the fetal zone of FAC.
the corpus luteum (! A1) until the 6th week of The maternal estrogen conc. rises sharply
gestation, i.e., until the placenta is able to pro- towards the end of the pregnancy, thereby
duce sufficient quantities of the hormones. counteracting the actions of progesterone, in-
Most pregnancy tests are based on the fact that cluding its pregnancy-sustaining effect. Estro-
hCG is detectable in the urine about 6–8 days after gens induce oxytocin receptors (! p. 303), α 1-
conception. Since the levels of estrogen and pro- adrenoceptors (! p. 84ff.), and gap junctions
gesterone greatly increase during pregnancy (see in the uterine musculature (! p. 16ff.), and
table on p. 302), larger quantities of these hormones uterine cells are depolarized. All these effects
and their metabolites estriol and pregnanediol are ex- increase the responsiveness of the uterine
creted in the urine. Therefore, their conc. can also be musculature. The simultaneous increase in
measured to test for pregnancy.
progesterone synthesis triggers the produc-
In contrast to other endocrine organs, the tion of collagenases that soften the taut cervix.
placenta has to receive the appropriate precur- Stretch receptors in the uterus respond to the
sors (cholesterol or androgens, ! p. 294) from increase in size and movement of the fetus.
the maternal and fetal adrenal cortex, respec- Nerve fibers relay these signals to the hypo-
tively, before it can synthesize progesterone thalamus, which responds by secreting larger
and estrogen (! A2). The fetal adrenal cortex quantities of oxytocin which, in turn, increases
(FAC) is sometimes larger than the kidneys and uterine contractions (positive feedback). The
consists of a fetal zone and an adult zone. The gap junctions conduct the spontaneous im-
placenta takes up cholesterol and preg- pulses from individual pacemaker cells in the
304
nenolone and uses them to synthesize pro- fundus across the entire myometrium at a rate
gesterone. It is transported to the fetal zone of of approximately 2 cm/s (! p. 70).
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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