849
CHAPTER TWENTY-TWO
Reproductive System
The large secondary oocyte represents a future
egg cell
(ovum) that can be fertilized by uniting with a sperm cell.
If this happens, the oocyte divides unequally to produce
a tiny
second polar body
and a large fertilized egg cell, or
zygote
(zi
go
¯t), that can divide and develop into an
embryo
(em
bre-o). An embryo is the stage of prenatal development
when the rudiments of all organs form. The polar bodies
have no further function, and they begin to degenerate F f-
teen hours post fertilization.
±ormation of polar bodies may appear wasteful, but it
has an important biological function. It allows for produc-
tion of an egg cell that has the massive amounts of cyto-
plasm and abundant organelles required to carry a zygote
through the F
rst few cell divisions, yet the right number of
chromosomes.
PRACTICE
32
Describe the major events of oogenesis.
33
What is the function of polar body formation?
Follicle Maturation
At puberty, the anterior pituitary gland secretes increased
amounts of ±SH, and the ovaries enlarge in response. With
each reproductive cycle, some of the primordial follicles
mature
(f
g. 22.22)
. Within each maturing primordial fol-
licle, the oocyte enlarges and the surrounding follicular cells
divide mitotically, giving rise to a stratiF
ed epithelium com-
posed of
granulosa cells.
A layer of glycoprotein, called the
zona pellucida
(zo
nah pel-u
cı˘-dah), gradually separates
the primary oocyte from the granulosa cells; at this stage, the
structure is called a
primary follicle.
Meanwhile, the ovarian cells outside the follicle orga-
nize into layers. The
inner vascular layer
(theca interna)
is largely composed of steroid-secreting cells, plus some
loose connective tissue and blood vessels. The
outer
F
brous layer
(theca externa) consists of tightly packed con-
nective tissue cells.
The follicular cells continue to proliferate, and when
there are six to twelve layers of cells, irregular, fl uid-F lled
spaces appear among them. These spaces soon join to form a
single cavity (antrum), and the primary oocyte is pressed to
one side of the follicle. At this stage, the follicle is about 0.2
millimeter in diameter and is called a
secondary follicle.
About one week into the cycle, one follicle becomes
the
dominant follicle.
Maturation of this follicle takes ten
to fourteen days. The
mature follicle
(preovulatory, or
GraaF
an, follicle) is about 10 millimeters or more in diame-
ter, and its fl
uid-F
lled cavity bulges outward on the surface
of the ovary, like a blister. The secondary oocyte within
the mature follicle is a large, spherical cell, surrounded by
a thick zona pellucida, attached to a mantle of follicular
cells called the
corona radiata.
Processes from these fol-
licular cells extend through the zona pellucida and supply
nutrients to the oocyte
(f
g. 22.23)
.
Early in development, the primary oocytes begin to
undergo meiosis, but the process soon halts and does not
continue until the individual reaches puberty. Once the pri-
mordial follicles appear, no new ones form. Instead, the
number of oocytes in the ovary steadily declines, as many
of the oocytes degenerate. Of the several million oocytes that
formed in the embryo, only a million or so remain at the
time of birth, and perhaps 400,000 are present at puberty. Of
these, probably fewer than 400 or 500 will be released from
the ovary during the reproductive life of a female. Probably
fewer than ten will go on to form a new individual.
A possible explanation for the increased incidence of chromosome
defects in children of older mothers is that the primary oocytes, hav-
ing been present for several decades, had time to be extensively
exposed to damaging agents, such as radiation, viruses, and toxins.
Oogenesis
Oogenesis
(o
o-jen
e
˘-sis) is the process of egg cell formation.
Beginning at puberty, some primary oocytes are stimulated
to continue meiosis. As in the case of sperm cells, the result-
ing cells have one-half as many chromosomes (23) in their
nuclei as their parent cells, constituting one chromosome
set.
Unlike a primary spermatocyte, when a primary oocyte
divides, the cytoplasm is distributed unequally. One of the
resulting cells, called a
secondary oocyte,
is large, and the
other, called the
F
rst polar body,
is small
(f
g. 22.21)
.
FIGURE 22.20
The ovaries are located on each side against the
lateral walls of the pelvic cavity. The right uterine tube is retracted to
reveal the ovarian ligament.
Uterine tube
(retracted)
Ovarian
ligament
Broad
ligament
Fimbriae of
uterine tube
Round ligament
of uterus
Uterus
Left ovary
Suspensory
ligament
of ovary
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