1044
UNIT 5
Continuity
27
all the cytoplasm of the primary oocyte, is the
secondary oocyte
.
Tose maturing follicles not selected undergo atresia.
Te events of this first maturation division ensure that the
polar body receives almost no cytoplasm or organelles. Notice
in Figure 27.19 (le±) that a spindle forms at the very edge of the
oocyte. A little “nipple” also appears at that edge, and the polar
body chromosomes are cast into it.
Te first polar body may continue its development and un-
dergo meiosis II, producing two even smaller polar bodies.
Oogenesis After Puberty
Before puberty, all of the primordial follicles that are recruited un-
dergo atresia (apoptosis, or programmed cell death). Beginning
at puberty, FSH rescues a small number of growing follicles from
that fate each month. In each cycle, one of the rescued follicles is
“selected” to become the
dominant follicle
and continue meiosis
I, ultimately producing two haploid cells (each with 23 replicated
chromosomes) that are quite dissimilar in size. Te smaller cell is
called the
first polar body
. Te larger cell, which contains nearly
Meiotic events
Follicle development
in ovary
Before birth
Infancy and
childhood
(ovary functionally
inactive)
Primary oocyte
Primary oocyte (still
arrested in prophase I)
Vesicular (antral)
follicle
Primary follicle
Primordial follicle
Primordial follicle
Oocyte
Ovulated secondary
oocyte
In absence of fertilization,
ruptured follicle becomes
a corpus luteum and
ultimately degenerates.
Degenerating
corpus luteum
Secondary follicle
Primary oocyte
(arrested in prophase I;
present at birth)
Oogonium (stem cell)
Each month from
puberty to menopause
Meiosis I
(completed by
one primary oocyte each
month in response to
LH surge)
First polar body
Mitosis
Meiosis II of polar body
(may or may not occur)
Polar bodies
(all polar bodies
degenerate)
Ovum
Second
polar body
Meiosis II
completed
(only if sperm
penetrates
oocyte)
Sperm
Ovulation
Secondary oocyte
(arrested in metaphase II)
Follicle cells
Spindle
2n
2n
2n
2n
n
n
n
n
n
Figure 27.19
Events of oogenesis.
Left, flowchart of meiotic events. Right, correlation with
follicle development and ovulation in the ovary. At various stages, unsuccessful oocytes undergo
atresia (not shown). Corresponding illustrations of meiotic events and follicle development lie
opposite each other.
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