S u m m a r y o f o o g e n e s i s a n d f o l l i c u l a r d e v e l o p m e n t •
F i g u r e 1 6 . 1 0
Age
Oogenesis
Follicular development
Fetal period
Mitosis
2
J I
J Oogonium
X
2 0
Primary oocyte
Meiosis in progress ^
&>,
Primary oocyte
*
' (in prophase I)
o
Primordial follicle
Childhood (no development
of follicles)
Puberty to menopause
each m onth
I
Meiosis I completed
by one primary oocyte
each month
First polar body l^ ^ 1
2
Æ
Pr; mary oocyte~ = - - -
(still in
*■'
prophase I)
I
' 2 i$
' Primary oocyte
\
Meiosis II
of first
polar body
may or may
not occur
i
~<n
n
N l
n
W l
Secondary oocyte
(in metaphase II)
Sperm cell
Meiosis II completed
if fertilization occurs
Primary follicle
Secondary follicle
Mature (graafian) follicle
Ovulation
Ovulated secondary
oocyte
The nuclei of the sperm cell
and the ovum unite, forming
a diploid (2n) zygote.
Second
polar body
All polar bodies degenerate
Ovum
Zygote
pus luteum produces progesterone, estrogens, relaxin, and
inhibin until it degenerates and turns into fibrous tissue.
The series of events in which a follicle develops in the
ovary, releases a secondary oocyte during ovulation, and
becomes a corpus luteum is called the ovarian cycle. This
ovarian cycle occurs about once per month during the fe-
male’s reproductive years. Hormones from the hypothala-
mus (gonadotropin-releasing hormone, GnRH) and the
anterior pituitary gland (luteinizing hormone, LH) and
follicle-stimulating hormone, FSH) drive the cycle.
Like spermatogenesis, oogenesis involves meiosis (Fig-
ure 16.10), a form of sexual reproduction in which each
gamete receives half the normal number of chromosomes
(n). One of the cells produced from meiosis is the second-
ary oocyte; the other is called a polar body and is essential-
ly a packet of discarded nuclear material. Once a secondary
oocyte is formed, it begins meiosis II and then stops. These
events take place in the mature (graafian) follicle. At ovu-
lation, the follicle will rupture and release the secondary
oocyte, which has undergone only the first half of meiosis.
If the secondary oocyte is not fertilized, it deteriorates
and never completes meiosis.
If the secondary oocyte is
penetrated by a sperm in the uterine tubes, however, it will
complete meiosis II and produce another polar body and the
ovum. The ovum and sperm combine their chromosomes in
the fertilized egg, which is the first cell of the new offspring.
The Female Reproductive Cycle Shows That Timing is Everything
481
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