Oogenesis is the process by which ova (female gametes) are produced in the ovaries. It begins with primordial germ cells that migrate to the gonads and become oogonia. During fetal development, oogonia undergo mitosis and some become arrested in prophase I of meiosis to become primary oocytes. By birth, around 700,000 primary oocytes are present in the ovaries. At puberty, a few primary oocytes resume meiosis each month to become secondary oocytes. The secondary oocyte undergoes the first meiotic division to form another cell and the first polar body. If fertilization occurs, the secondary oocyte completes the second meiotic division to form the ovum and second polar

1. Ovary & Oogenesis
By
Dr. A.U. Agu
MBBS, MSc, PhD
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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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