Developmental Zoology
(ZOO 228.1.0)
Gametogenesis
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What is Oogenesis?
• Formation, maturation and development
of an ovum.
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You are born with your destiny
•  Primoridal germ cells differentiate into
oogoina (stem cells)
•  At 2 – 7 months of gestation, ~ 1000
oogonia divide to form ~ 7 million germ
cells
3
You are born with your destiny
• 
From 7 – 9 months, most oogonia die
•  the remaining enter the mitotic division to
become primary oocyte
• 
The primary oocytes remain resting
within small follicles until puberty
4
You are born with your destiny
• 
All the eggs (oocytes) are stored in the
ovaries at birth (It s your hormones that
make them mature and available for any
sperm that may come their way)
• 
• 
Release starts with puberty- (the hormone
fluctuations can be very dramatic for a
year or two!)
Release stops with menopause
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Oogenesis
Maturation inhibiting
factor arrest the
growth at birth
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Oogenesis
Oogenesis
1st polar body
Primary
oocyte
arrested in
diplotene of
prophase I
Completion
of meiosis I
entry into
meiosis II
Secondary
oocyte
arrested in
metaphase II
1st polar body
Completion
of meiosis II
2nd polar body
Activation or
Fertilization
Mature egg
Menstrual cycle
• 
At birth there about 2,000,000 oocytes
• 
At puberty only 40,000.
• 
Only about 300 – 400 develops during a
reproductive life
• 
Cyclic physiological changes under endocrine
control. In primates this is the menstrual cycle.
In other mammals there is an Oestrus cycle.
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Oogenesis
Each oocyte surrounded by
follicle layer which secretes a
membrane; membrana propia
& nourishes developing
Primordial follicle
a single
basement
(protect
egg) =
With development, single follicle layer
becomes many layered = many layers
(granulosa cells) + primary oocyte
= Primary follicle
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secondary oocyte
first polar body
4.Follicle matures
Graffian
follicle
3.Fluid
filled
antrum
develops
Ovarian
cycle
2.Zona
pellucida
develops
1.Immature
cell
5.Ovulation.
Mature follicle ruptures
and releases the
secondary oocyte and
the first polar body
along with granulosa
cells
6.A corpus luteum forms
from remnants of the
ruptured follicle.
primordial
follicle
7.When no pregnancy
occurs, the corpus
luteum degenerates.
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Video Clip
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Pre ovulatory phase (1-13)
About 6 primordial cells starts to develop
Primary oocyte
FSH
increases diameter(20200,400µm)
At the same time
follicular
cells surrounding oocyte oocyte
condense; granulosa cells
Oocyte secretes a glycoprotein layer; ZP
Follicular cells on the membrane propia condense to
form thecal cell layer
12
Pre ovulatory phase
Thecal cell layer divide into 2 layer
Glandular, vascular
theca interna
Fibrous
theca externa
At the same time, further proliferation of
granulosa cells results in secretion of
viscous fluid; follicular fluid
Now the dense mass of granulosa cells
surrounding the oocyte; cumulus oophorus
13
Ovulatory phase
At day 14 ovulation occurs due to increase of antral
cavity
Mature follicle secretes collagenase which breaks
connective tissues
Breakdown products create an inflammatory
reaction attracting leukocytes that secretes
prostaglandins into follicle
Just before ovulation, it enters 2nd meiotic division &
at metaphase arrested due to action of C-mos
protein
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Ovulatory phase
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Post ovulatory phase (15-28)
Following ovulation, collapsed follicle walls become
Corpus luteum
Membrana propia breaks down & blood vessels
invade. Both granulosa cells & theca interna
contributes to Corpus luteum
Granulosa cells cease dividing & hypertrophy to
form lutein cells with mitochondria, smooth ER,
lipid droplets, Golgi bodies & lutein pigment
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secondary oocyte
first polar body
4.Follicle matures
Graffian
follicle
3.Fluid
filled
antrum
develops
Ovarian
cycle
2.Zona
pellucida
develops
1.Immature
cell
5.Ovulation.
Mature follicle ruptures
and releases the
secondary oocyte and
the first polar body
along with granulosa
cells
6.A corpus luteum forms
from remnants of the
ruptured follicle.
primordial
follicle
7.When no pregnancy
occurs, the corpus
luteum degenerates.
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Summary of ovarian cycle
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Oogenesis
Prophase I
Uterine cycle; proliferative phase
• 
5-14 days of the normal 28 day cycle
• 
Estrogen produced by theca cells of ovarian
follicle
• 
Endometrium consists of just gland bases
• 
Cells of gland bases proliferate forming
simple columnar epithelium
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Uterine cycle; proliferative phase
• 
Connective tissue cells proliferate in lamina
propria
• 
2-3 mm of endometrium form with glands of
straight tubes with narrow lumens
• 
Coiled arteries grow into regenerated lamina
propria
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Proliferative phase
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Secretory phase
• Begins after ovulation, days 15-28
• Corpus luteum forms and produces
progesterone
• Glands develop further, become coiled
and begin to secret
• Endometrium reaches 5 mm thick
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Secretory phase
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Menstrual phase
• If fertilization does not occur, corpus luteum
stops secreting hormones after about 14 days
• Progesterone and estrogen decrease causing
coiled arteries to constrict cutting off blood flow
to the functional layer of endometrium
• Endometrial cells die and the functional layer is
sloughed off
• Vessels distal to constrictions are shed with the
functional layer causing some bleeding
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Developmental Stages
Ø Early Development
•  Fertilization
•  Cleavage
•  Gastrulation
•  Neurulation
Ø Later Development
•  Organogenesis
•  Larval molts
•  Metamorphosis
•  Aging
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