Chapter 27
The Reproductive System
1047
27
absorbed. Te remaining granulosa cells enlarge, and along with
the internal thecal cells they form a new, quite different endo-
crine structure, the
corpus luteum
(“yellow body”) (Figure 27.20,
stage
6
). It settles right into its role and begins to secrete proges-
terone and some estrogen.
If pregnancy does not occur, the corpus luteum starts de-
generating in about 10 days and its hormonal output ends. In
this case, all that ultimately remains is a scar called the
corpus
albicans
(al
9
bĭ-kans; “white body”) as shown in Figure 27.20,
7
. Te last two or three days of the luteal phase, when the en-
dometrium is just beginning to erode, is sometimes called the
luteolytic
or
ischemic phase
.
On the other hand, if the oocyte is fertilized and pregnancy
ensues, the corpus luteum persists until the placenta is ready to
take over its hormone-producing duties in about three months.
Check Your Understanding
30.
How do identical twins differ developmentally from fraternal
twins?
31.
What occurs in the luteal phase of the ovarian cycle?
For answers, see Appendix H.
Hormonal Regulation of the Ovarian Cycle
Describe the regulation of the ovarian and uterine cycles.
Ovarian events are much more complicated than those occur-
ring in the testes, but the hormonal controls set into motion
at puberty are similar in both sexes. Gonadotropin-releasing
hormone (GnRH), pituitary gonadotropins, and, in this case,
ovarian estrogen and progesterone interact to produce the cy-
clic events occurring in the ovaries.
However, in females another hormone plays an important
role in stimulating the hypothalamus to release GnRH. Te on-
set of puberty in females is linked to the amount of adipose
tissue, and the messenger from fatty tissue to the hypothalamus
is leptin. If blood levels of lipids and leptin (better known for
its role in energy production and appetite) are low, puberty is
delayed.
Establishing the Ovarian Cycle
During childhood, the ovaries grow and continuously secrete
small amounts of estrogens, which inhibit hypothalamic release
of GnRH. Provided that leptin levels are adequate, the hypotha-
lamus becomes less sensitive to estrogen as puberty nears and
begins to release GnRH in a rhythmic pulselike manner. GnRH,
in turn, stimulates the anterior pituitary to release FSH and
LH, which prompt the ovaries to secrete hormones (primarily
estrogens).
Gonadotropin levels continue to increase for about four
years. During this time, pubertal girls are still not ovulating and
cannot become pregnant. Eventually, the adult cyclic pattern is
achieved, and hormonal interactions stabilize. Tese events are
heralded by the young woman’s first menstrual period, referred
to as
menarche
(mĕ-nar
9
ke;
men
5
month,
arche
5
first). Usu-
ally, it is not until the third year postmenarche that the cycles
become regular and all are ovulatory.
the
antrum
(“cave”), an event that distinguishes the early
ve-
sicular follicle
from the late secondary follicle and all prior
follicles (
preantral follicles
).
Te antrum continues to expand with fluid until it iso-
lates the oocyte, along with its surrounding capsule of granu-
losa cells called a
corona radiata
(“radiating crown”), so the
oocyte is “sitting proudly” on a stalk on one side of the follicle.
When a follicle is full size (about 2.5 cm, or 1 inch, in diame-
ter), it bulges from the external ovarian surface like an “angry
boil.” Tis usually occurs by day 14.
As one of the final events of follicle maturation, the primary
oocyte completes meiosis I to form the secondary oocyte and
first polar body (see Figure 27.19). Once this has occurred, the
stage is set for ovulation. At this point, the granulosa cells send
another important signal to the oocyte that says, in effect, “Wait,
do not complete meiosis yet!” It is estimated that follicle growth
from the primordial stage to this stage takes about a year. So
each follicle that ovulates actually began to grow some 10–12
ovarian cycles earlier.
Ovulation
Ovulation
(stage
5
in Figure 27.20) occurs when the balloon-
ing ovary wall ruptures and expels the secondary oocyte, still
surrounded by its corona radiata, into the peritoneal cavity.
Some women experience a twinge of pain in the lower abdomen
when ovulation occurs. Te precise cause of this pain, called
mittelschmerz
(mit
9
el-shmārts; German for “middle pain”), is
not known, but possible reasons include intense stretching of
the ovarian wall during ovulation and irritation of the perito-
neum by blood or fluid released from the ruptured follicle.
In the ovaries of an adult female, there are always several
follicles at different stages of maturation. As a rule, one follicle
outstrips the others and is at the peak stage of maturation when
the hormonal (LH) stimulus is given for ovulation. FSH is a
survival factor for antral follicles and plays a role in selecting
the so-called
dominant follicle
, but how this follicle is selected,
or selects itself, is still uncertain. It is probably the one that adds
the most gonadotropin receptors and so attains the greatest FSH
sensitivity the quickest. Te others degenerate (undergo apo-
ptosis) and are reabsorbed.
In 1–2% of all ovulations, more than one oocyte is ovu-
lated. Tis phenomenon, which increases with age, can result
in multiple births. Since, in such cases, different oocytes are
fertilized by different sperm, the siblings are
fraternal
, or non-
identical, twins.
Identical twins
result from the fertilization of
a single oocyte by a single sperm, followed by separation of
the fertilized egg’s daughter cells during early development.
Additionally, it now appears that in some women oocytes may
be released at times unrelated to hormone levels. Tis timing
may help to explain why a rhythm method of contraception
sometimes fails and why some fraternal twins have different
conception dates.
Luteal Phase of the Ovarian Cycle
A±er ovulation, the ruptured follicle collapses and the antrum
fills with clotted blood. Tis
corpus hemorrhagicum
is eventually
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