2. Introduction
• Male gamete is called spermatozoa & the process
of it’s production is spermatogenesis
• Female gamete is called ova & the process of it’s
production is oogenesis
• Spermatogenesis & oogenesis are called
gametogenesis
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3. Primordial germ cells (PGCs)
• Gametes are derived from
PGCs
• The PGCs are formed in the
epiblast during 2nd wk & move
to yolk sac.
• During 4th wk they begin to
migrate from yolk sac to the
gonads & arrives at 5th wk
• Mitotic divisions increase their
number.
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4. Ovary
• Is the female gonad
• Has outer cortex & inner
medulla
• The cortex contains many
rounded large cells called
oogonia
• All the oogonia to be used
throughout fertile life of a
female are produced b/4 her
birth.
• Ova are derived from
oogonia
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5. OOGENESIS
Maturation of Oocytes Begins Before Birth
• As the primordial germ cells arrives in the ovary,
they differentiate into oogonia
• The oogonia undergo mitotic divisions &, by the end
of the 3rd month, are arranged in clusters at the
ovarian cortex & surrounded flat epithelial cells.
• All the oogonia in one cluster are from a single
follicular cells.
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6. • The oogonia continue to divide by mitosis
• Some of them are arrested at prophase of meiosis I
& differentiate into primary oocytes
• During the next few months, oogonia increase
rapidly in number, & by the 5th month, the total
number in the ovary reaches its maximum of 7
million.
• Thereafter, cell death begins, & many oogonia &
primary oocytes become atretic.
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7. • By 7th month, the majority of oogonia have
degenerated except for a few near the surface.
• All surviving primary oocytes enters prophase of
meiosis I, & are individually surrounded by a layer
of follicular cell (flat epithelial cells)
• A primary oocyte, with its surrounding flat epithelial
cells, is called a primordial follicle
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8. Maturation of Oocytes Continues at Puberty
• Close to the time of birth, all primary oocytes have
started prophase of meiosis I, but instead of proceeding
into metaphase, they enter the diplotene stage.
• Primary oocytes remain in prophase & do not finish
their 1st meiotic division b/4 puberty is reached
• This is due to oocyte maturation inhibitor (OMI),
secreted by follicular cells.
• Total number of primary oocytes at birth is about
700,000 to 2 million.
• During childhood most oocytes become atretic & about
400,000 are present by the beginning of puberty
• Less than 500 will be ovulated.
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9. • Thus, some oocytes that reach maturity late in life
have been dormant in the diplotene stage of the 1st
meiotic division for 40 years or more b/4 ovulation.
• The fact that the risk of having children with
chromosomal abnormalities increases with maternal
age indicates that primary oocytes are vulnerable to
damage as they age.
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10. • At puberty, a pool of growing follicles is established &
continuously maintained from the supply of primordial
follicles.
• Each month, 15 to 20 follicles begin to mature, passing
through 3 stages:
– Primary or preantral;
– Secondary or antral (vesicular or Graafian)
– Preovulatory.
• The antral stage is the longest.
• Preovulatory stage is about 37 hours b/4 ovulation
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11. Stages of oogenesis
• With each menstrual cycle, a
few primary oocytes (PO)
begins to mature & one
complete the 1st meiotic
division shortly b/4 ovulation.
• The 1st meiotic division of a
primary oocyte produces 2
unequal daughter cell, each
with haploid number of xme .
– The large cell receives most
cytoplasm & is called 2ndly
oocyte & the small cell is called
1st polar body
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12. • The 2ndly oocyte enters 2nd
meiotic division
• Ovulation takes place while
the oocyte is in the
metaphase
• The 2ndly oocyte remains
arrested in metaphase till
fertilization occurs
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13. • The 2nd meiotic division is completed only if
fertilization occurs.
• This results in another 2 unequal daughter cell.
• The smaller one becomes 2nd polar body the larger
one is the ovum
• If fertilization does not occur, the 2ndly oocyte fails
to complete the 2nd meiotic division & degenerates
within 24 hours after ovulation
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14. Formation of ovarian follicle
• Some cells of the stroma
become flattened &
surround the oocyte
• The flattened cells form the
ovarian follicle & are called
follicular cells
• The primary oocytes,
surrounded by the layers of
the flat epithelial (follicular)
cells & are primordial
follicles
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15. • A layer of glycoproteins
appears b/w oocyte &
follicular cells & is called
the zona pellucida
• The follicular cell proliferate
& form several layers called
granulosa cells
(membrana granulosa)
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16. • A cavity (antrum) appears
within the granulosa forming
a follicle
• The cavity of the follicle
increases in size & the wall of
the follicle becomes thin
• The oocyte now lies
eccentrically surrounded by
granulosa cells called
cumulus oophorus
• Cells that attach it to the wall
of the follicle is called discus
proligerus
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17. • As the follicles expands, the
stromal cells surrounding the
granulosa organize into an
– Inner layer of secretory cells
(theca interna) that secretes
oestrogen
– Outer fibrous capsule (theca
externa).
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18. Differences b/w spermatogenesis & oogenesis
• One primary spermatocyte gives rise to 4 spermatozoa
• One primary oocyte forms only one ovum
• When primary spermatocyte divides, it’s cytoplasm is
equally distributed b/w the 2 spermatocytes
• When primary oocyte divides, almost all it’s cytoplasm
goes to the daughter cell & the polar body gets ½ the
chromosome
• The polar body is used to get rid of the unwanted
chromosomes
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