28
and skin patch are comparable to those
of the pill. Combination birth control
pills with substantially higher hormone
concentrations used for
postcoital
contraception
have a 75% effectiveness.
Taken within three days of unprotected
intercourse, these
morning-after pills
(
MAPs
), or
emergency contraceptive
pills
(ECPs) as they are also called, “mess
up” normal hormonal signals enough to
prevent a fertilized egg from implanting or
prevent fertilization altogether.
Other hormonal approaches to
contraception use progestin-only products
that thicken the cervical mucus enough
to block sperm entry into the uterus,
decrease the frequency of ovulation, and
make the endometrium inhospitable to
implantion. These include match-size
silicone rods implanted just under the
skin that release progestin over a five-year
period (Norplant), and an injectable form
that lasts for three months (Depo Provera).
New intrauterine devices (IUDs) that are
inserted into the uterus provide sustained
local delivery of synthetic progesterone
to the endometrium and are particularly
recommended for women who have
given birth and are nursing or those in
monogamous relationships. The failure
rates of progestin treatments are even less
than that of the “pill.”
Sterilization techniques
permanently
prevent gamete release.
Tubal ligation
or vasectomy (cutting or cauterizing
the uterine tubes or ductus deferentia,
respectively) are nearly foolproof and
are the choice of approximately 33% of
couples of childbearing age in the United
States. Both procedures can be done in
the physician’s office. However, these
techniques are usually permanent, making
them unpopular with individuals who still
plan to have children but want to select
the time.
This summary doesn’t even begin to
touch on the experimental birth control
drugs now awaiting clinical trials, and
other methods are sure to be developed
in the near future. In the final analysis,
however, the only 100% effective means
of birth control is the age-old one—total
abstinence.
A CLOSER LOOK
(continued)
1091
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1.
Te gestation period of approximately 280 days extends from the
woman’s last menstrual period to birth. Te conceptus undergoes
embryonic development for 8 weeks aFer fertilization, and fetal
development from week 9 to birth.
From Egg to Zygote
(pp. 1065–1067)
Accomplishing Fertilization
(pp. 1065–1067)
1.
An oocyte is fertilizable for up to 24 hours; most sperm are viable
within the female reproductive tract for one to two days.
2.
Sperm must survive the hostile environment of the vagina and
become capacitated (capable of reaching and fertilizing the oocyte).
3.
Hundreds of sperm must release their acrosomal enzymes to
break down the egg’s corona radiata and zona pellucida.
4.
When one sperm binds to receptors on the egg, it triggers the
slow block to polyspermy (release of cortical granules).
5.
±ollowing sperm penetration, the secondary oocyte completes
meiosis II. Ten the ovum and sperm pronuclei fuse
(fertilization), forming a zygote.
Events of Embryonic Development:
Zygote to Blastocyst Implantation
(pp. 1067–1074)
Cleavage and Blastocyst Formation
(p. 1067)
1.
Early development consists of cleavage, a rapid series of mitotic
divisions without intervening growth, that begins with the
zygote and ends with a blastocyst. Te blastocyst consists of the
trophoblast and an inner cell mass. Cleavage produces a large
number of cells with a favorable surface-to-volume ratio.
Implantation
(pp. 1067, 1070–1071)
2.
Te trophoblast adheres to, digests, and implants in the
endometrium. Implantation is completed when the blastocyst is
entirely surrounded by endometrial tissue, about 12 days aFer
ovulation.
3.
hCG released by the blastocyst maintains hormone production by
the corpus luteum, preventing menses. hCG levels decline aFer
four months.
Placentation
(pp. 1071–1074)
4.
Te placenta acts as the respiratory, nutritive, and excretory organ
of the fetus and produces the hormones of pregnancy. It is formed
from embryonic (chorionic villi) and maternal (endometrial
decidua) tissues. Te chorion develops when the trophoblast
becomes associated with extraembryonic mesoderm. ²ypically, the
placenta is functional as an endocrine organ by the third month.
Events of Embryonic Development:
Gastrula to Fetus
(pp. 1074–1081)
Formation and Roles of the Extraembryonic Membranes
(pp. 1074–1075)
1.
Te fluid-filled amnion forms from cells of the superior surface
(epiblast) of the embryonic disc. It protects the embryo from
physical trauma and adhesion formation, provides a constant
temperature, and allows fetal movements.
2.
Te yolk sac forms from the hypoblast of the embryonic disc; it is
the source of early blood cells.
3.
Te allantois, a caudal outpocketing adjacent to the yolk sac,
forms the structural basis of the umbilical cord.
4.
Te chorion is the outermost membrane and takes part in
placentation.
Gastrulation: Germ Layer Formation
(pp. 1075–1076)
5.
Gastrulation involves cellular migrations that ultimately
transform the inner cell mass into a three-layered embryo
Chapter Summary
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