636
Chapter 17
number of oxytocin receptors in the uterus increases
during the last few weeks of pregnancy. Thus, the
contractile response to any given plasma concentration
of oxytocin is greatly increased at parturition.
5.
Throughout pregnancy, progesterone exerts an essential
powerful inhibitory effect upon uterine contractions by
decreasing the sensitivity of the myometrium to estrogen,
oxytocin, and prostaglandins. Unlike the situation in
many other species, however, the rate of progesterone
secretion does not decrease before or during parturition
in women (until after delivery of the placenta, the source
of the progesterone); therefore, progesterone withdrawal
does not play a role in parturition.
These mechanisms are shown in a unifi ed pattern in
Figure 17–31
. Once started, the uterine contractions exert a
positive feedback effect upon themselves via both local facili-
tation of inherent uterine contractions and refl ex stimulation
of oxytocin secretion. But precisely what the relative impor-
tance of all these factors is in
initiating
parturition remains
unclear. One hypothesis is that the feto-placental unit, rather
than the mother, is the source of the initiating signals to start
parturition. That is, the fetus begins to outstrip the ability of
the placenta to supply oxygen and nutrients, and to remove
waste products. This leads to the fetal production of hormonal
signals like ACTH. Another theory is that a “placental clock”
acting via placental production of CRH signals the fetal pro-
duction of ACTH. Either way, ACTH-mediated changes in
fetal adrenal steroid production seem to be an important sig-
nal to the mother to begin parturition.
The actions of prostaglandins on parturition are the last
in a series of prostaglandin effects on the female reproductive
system. They are summarized in
Table 17–11
.
Lactation
The secretion of milk by the breasts, or
mammary glands,
is termed
lactation.
The breasts contain ducts that branch
all through the tissue and converge at the nipples (
Figure
17–32
). These ducts start in saclike glands called
alveoli
(the
same term is used to denote the lung air sacs). The breast
alveoli, which are the sites of milk secretion, look like bunches
of grapes with stems terminating in the ducts. The alveoli and
the ducts immediately adjacent to them are surrounded by
specialized contractile cells called
myoepithelial cells.
Before puberty, the breasts are small with little internal
glandular structure. With the onset of puberty in females, the
increased estrogen causes a marked enhancement of duct growth
and branching but relatively little development of the alveoli, and
much of the breast enlargement at this time is due to fat deposi-
tion. Progesterone secretion also commences at puberty during
the luteal phase of each cycle, and this hormone contributes to
breast growth by stimulating the growth of alveoli.
During each menstrual cycle, the breasts undergo fl uc-
tuations in association with the changing blood concentra-
tions of estrogen and progesterone. These changes are small
Cervical
stretch
Uterine
contraction
Baby’s head
pushed downward
Posterior
pituitary
+
+
+
+
+
Prostaglandins
Oxytocin
Figure 17–31
Factors stimulating uterine contractions during parturition. Note
the positive feedback nature of several of the inputs.
Figure 17–31
physiological
inquiry
If a full-term fetus is oriented feet-fi rst in the uterus, parturition
may not proceed in a timely manner. Why?
Answer can be found at end of chapter.
Table 17–11
Some Effects of Prostaglandins*
on the Female Reproductive System
Site of
Production
Action of
Prostaglandins
Result
Late antral
follicle
Stimulate production of
digestive enzymes
Rupture of
follicle
Corpus
luteum
May interfere with hormone
secretion and function
Death of
corpus luteum
Uterus
Constrict blood vessels in
endometrium
Onset of
menstruation
Cause changes in
endometrial blood vessels
and cells early in pregnancy
Facilitates
implantation
Increase contraction of
myometrium
Helps to
initiate both
menstruation
and parturition
Cause cervical ripening
Facilitates
cervical
dilation during
parturition
*The term
prostaglandins
is used loosely here, as is customary in reproductive physiology, to
include all the eicosanoids.
previous page 664 Vander's Human Physiology The Mechanisms of Body Function read online next page 666 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off