638
Chapter 17
for the protection of the newborn, as well as for longer-term
activation of the child’s own immune system. Milk also con-
tains many growth factors and hormones thought to help in
tissue development and maturation, as well as a large num-
ber of neuropeptides and endogenous opioids that may subtly
shape the infant’s brain and behavior. Some of these substances
are synthesized by the breasts themselves, not just transported
from blood to milk. The reasons the milk proteins can gain
entry to the newborn’s blood are that (1) the low gastric acid-
ity of the newborn does not denature them, and (2) the new-
born’s intestinal epithelium is more permeable to proteins than
is the adult epithelium.
Unfortunately, infectious agents, including the virus
that causes AIDS, can be transmitted through breast milk, as
can some drugs. For example, the concentration of alcohol in
breast milk is approximately the same as in maternal plasma.
Breast-feeding for at least the fi rst 6 to 12 months is
strongly advocated. In less-developed countries, where alter-
native formulas are often either contaminated or nutrition-
ally inadequate because of improper dilution or inadequate
refrigeration, breast-feeding signifi cantly reduces infant sick-
ness and mortality. In the United States, effects on infant sur-
vival are not usually apparent, but breast-feeding reduces the
severity of gastrointestinal infections, has positive effects on
mother-infant interaction, is economical, and has long-term
health benefi ts. Cow’s milk has many but not all of the con-
stituents of mother’s milk, but often in very different concen-
trations, and it is very diffi cult to reproduce mother’s milk in a
commercial formula.
Contraception
Physiologically, pregnancy is said to begin not at fertilization
but after implantation is complete, approximately one week
after
fertilization. Birth control procedures that work prior
to implantation are called
contraceptives
(
Table 17–12
).
Procedures that cause the death of the embryo or fetus after
implantation are called
abortifacients
.
Some forms of contraception, such as vasectomy, tubal
ligation, vaginal diaphragms, vaginal caps, spermicides, and
condoms, prevent sperm from reaching the egg. (In addition,
condoms signifi
cantly reduce the risk of
sexually transmit-
ted diseases
[
STDs
]
such as AIDS, syphilis, gonorrhea, chla-
mydia, and herpes.)
Oral contraceptives
are based on the fact that estrogen
and progesterone can inhibit pituitary gonadotropin release,
thereby preventing ovulation. One type of oral contraceptive
is a combination of a synthetic estrogen and a progesterone-
like substance (a progestogen or progestin). Another type is
the so-called mini-pill, which contains only the progesterone-
like substance. In actuality, the oral contraceptives, particu-
larly the minipill, do not always prevent ovulation, but they
are still effective because they have other contraceptive effects.
For example, progestogens affect the composition of the cer-
vical mucus, reducing the ability of the sperm to pass through
the cervix, and they also inhibit the estrogen-induced pro-
liferation
of
the
endometrium,
making
it
inhosp
itab
le
for
implantation.
Another method of delivering a contraceptive progesto-
gen is via tiny capsules that are implanted beneath the skin
and last for fi ve years. Yet another method is the intramus-
cular injection of a different progestogen substance (e.g.,
Depo-Provera
®
or
Lunelle
®
) every 1 to 3 months. Alternate
methods of providing highly effi cacious hormonal contracep-
tion include skin patches and vaginal rings.
The
intrauterine device
(
IUD
)
works beyond the
point of fertilization but before implantation has begun or is
complete. The presence of one of these small objects in the
uterus somehow interferes with the endometrial preparation
for acceptance of the blastocyst.
In addition to the methods used before intercourse (pre-
coital contraception), there are a variety of drugs used within
72 h
after
intercourse (postcoital or emergency contraception).
These most commonly interfere with ovulation, transport of
the conceptus to the uterus, or implantation. One approach
is a high dose of estrogen, or two large doses (12 h apart) of
a combined estrogen-progestin oral contraceptive. Another
approach has used the drug
RU 486
(
mifepristone
),
which
has antiprogesterone activity because it binds competitively
Breasts
Gland cell
stimulation
Contraction of
myoepithelial cells
Milk synthesis
Milk ejection
Plasma prolactin
Anterior pituitary
Prolactin secretion
Plasma dopamine
?
Plasma PRF
(in hypothalamo-pituitary
portal vessels)
Plasma oxytocin
Nipple mechanoreceptor
stimulation
Neural input
to hypothalamus
Suckling
Hypothalamus
Dopamine secretion
?
PRF secretion
Posterior pituitary
Oxytocin secretion
Figure 17–33
Major controls of the secretion of prolactin and oxytocin during
nursing. The importance of PRF (prolactin-releasing factors) is not
known (indicated by ?).
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