Reproduction
617
otic division, the daughter cells each receive 23 chromosomes,
each with a single chromatid. Once again, one daughter cell,
now called an ovum, retains nearly all the cytoplasm. The other
daughter cell, the second polar body, is very small and nonfunc-
tional. The net result of oogenesis is that each primary oocyte
can produce only one ovum (see Figure 17–15). In contrast,
each primary spermatocyte produces four viable spermatozoa.
Follicle Growth
Throughout their life in the ovaries, the eggs exist in structures
known as
follicles.
Follicles begin as
primordial follicles,
which consist of one primary oocyte surrounded by a single
layer of cells called
granulosa cells.
Further development from
the primordial follicle stage (
Figure 17–16
) is characterized
by an increase in the size of the oocyte, a proliferation of the
granulosa cells into multiple layers, and the separation of the
oocyte from the inner granulosa cells by a thick layer of mate-
rial, the
zona pellucida.
The granulosa cells secrete estrogen,
small amounts of progesterone just before ovulation, and the
peptide hormone inhibin.
Despite the presence of a zona pellucida, the inner layer
of granulosa cells remains closely associated with the oocyte
by means of cytoplasmic processes that traverse the zona pel-
lucida and form gap junctions with the oocyte. Through these
gap junctions, nutrients and chemical messengers are passed
to the oocyte. For example, the granulosa cells produce one or
more factors that act on the primary oocytes to maintain them
in meiotic arrest.
As the follicle grows by mitosis of granulosa cells, con-
nective tissue cells surrounding the granulosa cells differentiate
and form layers known as the
theca,
which play an important
role in estrogen secretion by the granulosa cells. Shortly after
this, the primary oocyte reaches full size (115
µ
m in diame-
ter), and a fl
uid-fi lled space, the
antrum,
begins to form in the
midst of the granulosa cells as a result of fl
uid they secrete.
The progression of some primordial follicles to the
preantral and early antral stages (see Figure 17–16) occurs
throughout infancy and childhood, and then during the entire
menstrual cycle. Therefore, although most of the follicles in
Mons pubis
Urethral
opening
Labia
minora
Labia
majora
Vaginal
opening
Anus
Vestibule
Clitoris
Hymen
Figure 17–14
Female external genitalia.
Birth
Childhood
Puberty
First
polar
body
Adult
reproductive
life
Fetal life
Oogonia
Mitosis
Differentiation
Primary oocyte
Secondary oocyte
2nd meiotic division
(completed after fertilization)
1st meiotic division
(begins in utero,
completed prior to
ovulation)
Ovum
Second
polar
body
Chromosomes
per
cell
46
46
23
23
Chromatids
per
chromosome
2
2
2
1
Figure 17–15
Summary of oogenesis. Compare with the male pattern in Figure 17–7. The secondary oocyte is ovulated and does not complete its meiotic
division unless it is penetrated (fertilized) by a sperm. Thus, it is a semantic oddity that the oocyte is not termed an egg or ovum until after
fertilization occurs. Note that each primary oocyte yields only one secondary oocyte, which can yield only one ovum.
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