Chapter 27
The Reproductive System
1043
27
determined by the time she is born, and the time span during
which she releases them extends only from puberty to meno-
pause (about age 51). However, studies done in adult mice
show that egg stem cells are alive and generating little “egglets”
throughout life. More recent reports indicate that human egg
stem cells can arise from epithelial cells of the ovary surface.
Tese findings might seem to overturn the assumption that the
number of oocytes is limited—an idea that has been part of the
bedrock of biology. However, it is still too early to retire the “no
new eggs” doctrine based on these preliminary data.
Oogenesis
Describe the process of oogenesis and compare it to
spermatogenesis.
Meiosis, the specialized nuclear division that occurs in the testes
to produce sperm, also occurs in the ovaries. In this case, it pro-
duces female sex cells in a process called
oogenesis
(o
0
o-gen
9
e-
sis; “the beginning of an egg”). Te process of oogenesis takes
years to complete
(Figure 27.19)
.
First, in the fetal period the
oogonia
, the diploid stem cells
of the ovaries, multiply rapidly by mitosis. Gradually,
primor-
dial follicles
appear as the oogonia transform into
primary
oocytes
and become surrounded by a single layer of flattened
cells called
follicle cells.
Te primary oocytes begin the first
meiotic division, but become “stalled” late in prophase I and
do not complete it.
By birth, a female is presumed to have her lifetime supply
of primary oocytes. Of the original 7 million oocytes, approxi-
mately 1 million escape programmed death and are already in
place in the cortical region of the immature ovary. By puberty,
an endowment of perhaps 300,000 oocytes remain. Over time,
quiescent primordial follicles are recruited into a growing pool
of primary follicles. Tis recruitment process begins during fe-
tal life and continues throughout life until the supply of primor-
dial follicles is depleted, a time called
menopause
(see p. 1058).
±rastuzumab (Herceptin), a drug containing bioengineered
antibodies that jam estrogen receptors to control aggressively
growing cancer cells
±amoxifen, an antiestrogen compound that blocks estrogen’s
effects and significantly improves the outcome for premeno-
pausal women with early- or late-stage breast cancer
Letrozole (Femara), which disables the enzyme needed
to make estrogen and reduces breast cancer recurrence in
women (particularly postmenopausal women) who have ex-
hausted the usefulness of tamoxifen
Until the 1970s, the standard treatment was
radical mas-
tectomy
(mas-tek
9
to-me; “breast cutting”)—removal of the
entire affected breast, plus all underlying muscles, fascia, and
associated lymph nodes. Most physicians now recommend less
extensive surgeries such as
lumpectomy
, which excises only
the cancerous lump, or
simple mastectomy
, which removes
only the breast tissue (and perhaps some of the axillary lymph
nodes).
Many mastectomy patients opt for breast reconstruction to
replace the excised tissue. ±issue “flaps,” containing muscle, fat,
and skin taken from the patient’s abdomen or back, are used for
“sculpting” a natural-looking breast.
Check Your Understanding
27.
Developmentally, mammary glands are modifications of
certain skin glands. Which type?
28.
From what cell types does breast cancer usually arise?
For answers, see Appendix H.
Physiology of the Female
Reproductive System
Gamete production in males begins at puberty and continues
throughout life, but the situation is quite different in females. It
has been assumed that a female’s total supply of eggs is already
(b) Film of normal breast
(a) Mammogram procedure
(c) Film of breast with tumor
Malignancy
Figure 27.18
Mammograms.
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