The Female Reproductive Cycle
Shows That Timing Is Everything
how eggs are made.
the major events in the female repro-
ductive cycle.
the function of each hormone associ-
ated with the female reproductive cycle.
emales make gametes called egg cells, or
oocytes, in the ovaries through a process
called oogenesis. Let’s look at the ovary
more closely to see how eggs develop.
Oogenesis Begins Before Birth
During early fetal development, cells in the ovaries differ-
entiate into oogonia, which give rise to cells that turn into
primary oocytes. These cells begin meisosis during fetal
development but do not complete it until puberty. At birth,
200,000 to 2,000,000 primary oocytes remain in each ovary.
Of these, about 40,000 remain at puberty, but only 400 go
on to mature and ovulate during a woman’s reproductive
lifetime. The others degenerate. In other words, females
have all the eggs they will ever have by birth.
The surface of the ovary is covered by the germinal
epithelium. Within the ovarian cortex, ovaries contain
many small sac-like structures called ovarian follicles.
Each follicle consists of an oocyte and a number of sup-
porting cells that nourish the developing oocyte and begin
to secrete estrogens as the follicle grows larger. The pri-
mordial follicles grow and differentiate in various stages
within the ovary (Figure 16.9).
The follicle enlarges until it is a mature (graafian)
follicle, a large, fluid-filled follicle that is preparing to
rupture and expel a secondary oocyte. At this stage, the
developing egg is ready to be ejected from the ovary, a pro-
cess called ovulation.
The remnants of an ovulated follicle develop into a cor-
pus luteum (plural is
lu te a ).
If no pregnancy occurs, the cor-
D e v e l o p m e n t o f o v a r i a n f o l l i c l e s
F i g u r e 1 6 . 9
The arrows within the ovary indicate the developmental stages that occur dur-
ing maturation of an ovum during the ovarian cycle. (The stages shown occur
sequentially in the same place on the ovary during a single monthly cycle but
are drawn in different areas on the ovary for demonstration purposes.)
Growing follicles
P rim ary
develop into
secondary follicles
• Follicular cells differentiate into
granulosa cells.
• Protein layer forms between
primary oocyte and granulosa cells.
• Basement membrane forms
specialized cells.
• Fluid-filled
blood vessels
Secondary follicle
develops into
mature follicle
• Cells of basement membrane
differentiate into
• Inner layer of granulosa cells
attaches to protein layer.
• Fluid-filled antrum grows.
I Corpus luteum degenerates
unless pregnancy occurs
and development proceeds
Mature (graafian) follicle
Ruptured follicle
—Mature (graafian) follicle
ruptures to release
into pelvic cavity. Oocyte
gets swept into uterine tube.
( Corpus luteum
develops from remnants of
follicle after ovulation and secretes
progesterone, estrogens, relaxin, and inhibin.
Frontal section
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