Chapter 27
The Reproductive System
1045
27
However, in humans, the secondary oocyte arrests in metaphase
II, and it is this cell (not a functional ovum) that is ovulated. If
a sperm does not penetrate an ovulated secondary oocyte, the
oocyte deteriorates. But, if a sperm penetration does occur, the
oocyte quickly completes meiosis II, yielding one large
ovum
and a tiny
second polar body
(Figure 27.19). Te union of
the egg and sperm nuclei, described in Chapter 28, constitutes
fertilization.
Comparison of Oogenesis and Spermatogenesis
What you should realize now is that the potential end prod-
ucts of oogenesis are three tiny polar bodies, nearly devoid of
cytoplasm, and one large ovum. All of these cells are haploid,
but only the ovum is a
functional gamete
. Tis is quite differ-
ent from spermatogenesis, where the product is four viable
gametes—spermatozoa.
Te unequal cytoplasmic divisions that occur during oo-
genesis ensure that a fertilized egg has ample nutrients for
its six- to seven-day journey to the uterus. Lacking nutrient-
containing cytoplasm, the polar bodies degenerate and die. Since
the reproductive life of a woman is at most about 40 years (from
about age 11 to 51) and typically only one ovulation occurs each
month, fewer than 500 oocytes out of her estimated pubertal
potential of 300,000 are ever released during a woman’s lifetime.
Perhaps the most striking difference between male and fe-
male meiosis is the error rate. As many as 20% of oocytes but
only 3–4% of sperm have the wrong number of chromosomes,
a situation that o±en results from failure of the homologues to
separate during meiosis I. It appears that faced with meiotic
disruption, meiosis in males grinds to a halt but in females it
marches on.
Check Your Understanding
29.
How do the haploid cells arising from oogenesis differ
structurally and functionally from those arising from
spermatogenesis?
For answers, see Appendix H.
The Ovarian Cycle
Describe ovarian cycle phases, and relate them to events of
oogenesis.
Te monthly series of events associated with the maturation
of an egg is called the
ovarian cycle
. Te ovarian cycle is best
described in terms of two consecutive phases. Te
follicular
phase
is the period when the dominant follicle is selected and
begins to secrete large amounts of estrogen. It generally lasts
from the first to the fourteenth day of the ovarian cycle, at which
point ovulation typically occurs. Te
luteal phase
is the period
of corpus luteum activity, days 14–28. Te so-called typical
ovarian cycle repeats at intervals of 28 days, with
ovulation
oc-
curring midcycle.
However, only 10–15% of women naturally have 28-day cy-
cles, and cycles as long as 40 days or as short as 21 days are fairly
common. In such cases, the length of the follicular phase and
timing of ovulation vary, but the luteal phase remains constant:
It is always 14 days from the time of ovulation to the end of the
cycle.
Follicular Phase of the Ovarian Cycle
Maturation of a primordial follicle involves preantral and antral
phases and several events. Te first phase, the gonadotropin-
independent
preantral phase
, is when intrafollicular paracrines
such as cytokines and growth factors control oocyte and fol-
licle development. Phase 2 is the
antral phase
, directed by FSH
and LH. During this phase, the activated follicles grow tremen-
dously and the primary occyte in the dominant follicle resumes
meiosis I.
Let’s look at events of these phases shown in
Figure 27.20
.
A primordial follicle becomes a primary follicle.
When a
primordial follicle
1
is activated (this occurs almost a year
before its possible ovulation) the squamouslike cells sur-
rounding the primary oocyte grow, becoming cuboidal cells,
and the oocyte enlarges. Te follicle is now called a primary
(1°) follicle
2
.
A primary follicle becomes a secondary follicle.
Next, the
follicular cells proliferate, forming a stratified epithelium
around the oocyte. As soon as more than one cell layer is
present, the follicle is called a
secondary (2°) follicle
3a
and
the follicle cells take on the name
granulosa cells
. Te gran-
ulosa cells are connected to the developing oocyte by gap
junctions, through which ions, metabolites, and signaling
molecules can pass. From this point on, bidirectional “con-
versations” occur between the oocyte and granulosa cells,
so they guide one another’s development. One of the signals
passing from the granulosa cells “tells” the oocyte to grow.
Other signals dictate asymmetry (polarity) in the future egg.
Te oocyte grows tremendously during this stage and FSH
receptors begin to appear on the granulosa cells.
A secondary follicle becomes a late secondary follicle.
Late
in stage
3
, a layer of connective tissue and epithelial cells
condenses around the follicle, forming the
theca folliculi
(the
9
kah fah-lik
9
u-li; “box around the follicle”). As the follicle
grows, the thecal and granulosa cells cooperate to produce
estrogens. In response to LH the inner thecal cells produce
androgens, which the granulosa cells convert to estrogens.
At the same time, the oocyte secretes a glycoprotein-rich
substance that forms a thick transparent extracellular layer
or membrane, called the
zona pellucida
(pĕ-lu
9
sid-ah), that
encapsulates it (see Figure 27.20,
4
).
At the end of stage
3
, clear liquid begins to accumulate
between the granulosa cells, producing the
late secondary
follicle
3b
.
A late secondary follicle becomes a vesicular (antral) fol-
licle.
Te late secondary follicle ushers in events of the antral
phase encompassing events from
3b
to
4
. In this phase, the
follicle reaches the critical preovulatory stage and all granu-
losa cells bear FSH receptors. When six to seven layers of
granulosa cell layers are present, the fluid between the granu-
losa cells coalesces to form a large fluid-filled cavity called
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