Female Nutfah
Oogenesis
• The formation of female gametes is called
oogenesis.
• The terms oogenesis refers to the entire
sequence of events by which oogonia are
transformed into ova.
• It is the maturation of female gametes in the
ovaries, the primary sex organs of female
partner.
• This maturation process begins before birth,
but it is not completed until after puberty.
Primordial germ cells
Ooogonia
• Once the primordial germ cells have arrived in
the gonad of a genetic female, they
differentiate into oogonia.
• Oogonium [Gr. oon an egg +
Gr. gone generation] is singular and oogonia
are plural.
• The oogonia are the stem cells from which
other oogenic cells develop.
Month 3
• These oogonia undergo a number of mitotic
divisions, and by the end of the 3rd month of
intrauterine life they form clusters of cells.
• So there are a lot of clusters of oogonia by the
end of 3rd month of intrauterine life.
Primary oocytes
• Majority of oogonia continue to divide by
mitosis, but some of them differentiate into
much larger cells, called primary oocytes.
• The primary oocytes immediately enter into
the prophase of meiosis I.
Month 5
• By the 5th month of intrauterine life the total
number of germ cells in the ovary reaches its
maximum.
• They are now seven million 7,000,000.
• The oogonia also start degenerating.
DDD
• Now oogonia are multiplying by mitosis,
differentiating into primary spermatocytes
and degenerating.
• The division and differentiation along with
degeneration of oogonia continues
throughout intrauterine life.
Month 7
• Majority of oogonia have degenerated by
7th month, except for a few near the surface.
• The primary oocytes are also forming and
degenerating.
• All surviving primary oocytes have entered
first meiotic division, and most of them are
individually surrounded by a layer of flat
epithelial cells forming primordial follicle.
At Birth
• At birth there is not a single oogonium present in
the ovary.
• They have either degenerated or differentiated
into primary oocytes.
• So at birth the cortex of the ovary contains only
primary oocytes.
• The total number of primary oocytes, present in
the cortex of the ovary, at birth is 2 million
(2,000,000).
• Graphic representation of oogonia and primary
oocytes.
• It must be stressed that no primary oocyte
forms after birth, in contrast to the continuous
production of primary spermatocytes in male
after puberty.
Puberty
• All the primary oocytes are in primordial
follicles.
• They have entered into prophase of meiotic-I
and resting at diplotene stage.
• The degeneration of primary oocytes
continues and only 400,000 are present at the
time of puberty.
• Out of them only 500 ovulate.
Ovary
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Epithelium
Tunica albuginea
Cortex
Medulla
Primordial follicle
• At birth the cortex of the ovary is nothing but
cellular connective tissue with almost
2,000,000 primordial follicles.
• At birth, the cortex contains a superficial zone
of primordial follicles; these consist of primary
oocytes about 25 mm in diameter, each
surrounded by a single layer of flat follicular
cells.
• With the onset of puberty, 5 to 12 primordial
follicles begin to mature with each reproductive
cycle.
• They pass through the stages of primary follicle,
secondary follicle, tertiary follicle and Gra’afian
follicle but only one matures fully and ovulates.
• As soon as one ovulates the others start
degenerating.
• Every primary oocyte completes its first
meiotic division shortly before ovulation (36
to 48 hours).
• It is important to realize that some of the
oocytes, reaching maturity late in life, remain
dormant in the dictyotene phase for 40 years
or more.
• Just before ovulation the primary oocytes in
mature follicle resumes its meiosis I and forms
two daughter cells of unequal size, secondary
oocyte and first polar body.
• The secondary oocyte receives most of the
cytoplasm and polar body receives practically
none.
Primary follicle and Secondary follicle
• The first sign of follicular activation
(primordial follicle) is the growth of the follicle
cells from squamous to cuboidal, followed by
their multiplication to form a multilayered
mass, the membrana granulosa, surrounded
by a thick basal lamina (membrana limitans
sterna).
Zona pellucida
• The primary oocyte secretes a thick layer of
deeply staining a cellular, homogeneous
membrane of glycoproteins, between its
surface and the surrounding granulosa cells
called zona pellucida.
Tertiary or Vesicular follicle
Theca folliculi
• As the follicle enlarges, the cellular connective
tissue of the cortex adjacent to the follicle
form theca folliculi which later on becomes
organized into inner layer of spindle-shaped
secretory cells called theca interna and an
outer layer of fibers with fibroblast like cells
called theca externa .
• Theca interna produces estrogen.
• The tertiary follicle continues to enlarge. The
follicular cells proliferate, the follicular antrum
becomes large and the follicle starts pressing
the surface of the ovary.
• The primary oocyte is towards the surface and
it is about to rupture. This fully mature follicle
is called Preovulatory follicle or Gra’afian
follicle.
• Follicle Stimulating Hormone (FSH) induces
early development of follicles, but the final
stages of maturation require Luteinizing
Hormone (LH) as well.
Secondary oocyte
• As soon as the follicle is mature, the primary
oocyte resumes its first meiotic division, leading
to the formation of two daughter cells
(secondary oocyte and first polar body), of
unequal size, but each with 23 (double
structured) chromosomes.
• The surge in the release of LH also seems to
induce resumption of the first meiotic division of
primary oocyte and it is converted into
secondary oocyte.
• Note the correlation of ovulation and surge of
leutinizing hormone.
• Secondary oocytes enter into meiosis II.
Meiosis II is completed only if secondary
oocyte is fertilized.
• Otherwise the cell degenerates within 24
hours after ovulation.
• So the ovum or ootid develops from a
fertilized secondary oocyte.
• The nucleus does not return to its resting
stage after first meiotic division.
• Immediately after completion of first
maturation division the secondary oocyte
enters the second maturation division without
DNA replication.
• The moment secondary oocyte shows the
spindle formation with the chromosomes
aligned on the metaphase plate, ovulation
occurs and the oocyte is shed from the ovary.
• Ovulation is the discharge of a secondary
oocyte from a Gra’afian follicle of ovary.