If the secondary oocyte is fertilized and begins to di-
vide, the corpus luteum does not degenerate; it is rescued
by human chorionic gonadotropin (hCG), a hormone pro-
duced by the embryo beginning about eight days after fer-
tilization. Like LH, hCG stimulates the secretory activity
of the corpus luteum. The presence of hCG in maternal
blood or urine is an indicator of pregnancy, and hCG is the
hormone detected by home pregnancy tests.
In the uterus.
Progesterone and estrogens produced by
the corpus luteum promote growth of the endometrial
glands, which begin to secrete glycogen. The hormones also
promote thickening of the endometrium. These changes
peak about one week after ovulation, at the time a fertilized
ovum might arrive at the uterus.
CONCEPT CHECK
1.
What
is the difference between an oocyte and a
follicle?
2.
What
happens during the preovulatory phase of
the female reproductive cycle?
3.
What
is the function of follicle-stimulating hor-
mone (FSH)?
Fertilization Requires the Egg and Sperm
to Get Very Close to One Another
LEARNING OBJECTIVES
1.
Explain
why timing is so critical for successful
fertilization.
2.
List
the different categories of contraceptives
and explain how they work.
rom a biological standpoint, the purpose of
sexual intercourse is to deliver sperm in close
proximity to the secondary oocyte so that fer-
tilization can take place.
During sexual arousal or excitement, there is altered
blood flow to the genitals and increased glandular secre-
tions; these responses produce an erect penis in the male
for insertion into the female and prepare the female’s va-
gina to receive the penis. Once aroused, the male inserts
his penis into the female’s vagina.
Fertilization Must Occur While Both
the Egg and Sperm Are Still Viable
After ovulation, the secondary oocyte remains functional
for only about 24 hours; contrast this with sperm, which
can remain active for about 48 hours after being delivered
to the female. For a woman to become pregnant, the tim-
ing of sexual intercourse must occur within a narrow win-
dow of time during the female reproductive cycle. Sperm
require nearly 6 hours to journey from their site of delivery
to the egg that is waiting in the uterine tube. Therefore,
sperm delivered up to 2 days before or as late as 18 hours
after ovulation will have the best opportunity to fertilize
the egg.
Some over-the-counter ovulation tests can detect high
levels of LH during the midcycle surge and can predict
ovulation about 1
day in advance. If sensitive thermome-
ters called
basal body tem perature therm om eters
are used daily, a
woman can also estimate when she is ovulating. Ovulation
is usually associated with about a half-degree increase in
body temperature that lasts throughout the luteal phase.
If a woman wishes to become pregnant, she can increase
her sexual activity at this time. If pregnancy is not desired,
intercourse can be avoided during this period, or other
contraceptive measures can be used.
Once semen is delivered into the vagina near the cer-
vix of the uterus during sexual intercourse, it coagulates.
After 10 to 20 minutes, prostate-specific antigen (PSA)
and other enzymes in semen cause it to re-liquefy, and
prostaglandins in semen increase sperm motility. Sperm
must swim from the vagina through the cervix and uterus
to reach the egg in the uterine tube. Depending on the
timing of the female’s reproductive cycle, the mucus with-
in the cervix may present the first obstacle. During the
ovulatory or early postovulatory phase, cervical mucus is
thin and watery, making it relatively easy to penetrate.
During the preovulatory or late postovulatory phase, the
cervical mucus is thick, which makes it more difficult for
sperm to navigate.
Once past the cervix, sperm swim along the walls of
the uterus into the uterine tubes. Contractions of the
walls of the uterus aid the sperm in their passage.
484 CHAPTER 16
The Reproductive Systems
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