Gametogenesis and Oogenesis Process in Humans
G
AMETOGENESIS
OOGENESIS
Dr
.
Samer Alnussairi
Objectives
Oogenesis
(
prenatal
& postnatal)
Ovulation
In
preparation
for
fertilization,
Germ cells
undergo
•
Gametogenesis
(include
meiosis)
.
•
Cytodifferentiation
to
complete
their
differentiation
.
Gametogenesis
•
Is
the
process of
formation
of gametes
from
germ
cells in
the testes
and
ovaries
.
•
Male &
female
germ
cells have the same
origin,
primordial
germ cells
Oogenesis
Is the
process
where by oogonia
differentiate into
mature
oocytes
.
Prenatal
oogenesis
occurs during
embryonic development
before
birth. Postnatal
oogenesis occurs
after
birth.
Oogonia
•
By
the
end
of
the
3
rd
month
,
the
majority of
the
oogonia
continue
to
divide
by
mitosis, but
some of
them
give rise
to
primary oocytes that enter
prophase
of
the
first
meiotic division.
By
the 5
th
month
of
prenatal
development
,
germ cells
in
the ovary
reaches its maximum( 7
million).
7
th
B
y
th
e
mon
t
h
,
•
the majority of oogonia
have
degenerated,
•
All
surviving primary
oocytes have entered
prophase of
meiosis I, and
most
of
them
are
individually
surrounded by
a
layer of flat
epithelial
cells
(
primordial
follicle
).
Maturation
of the
oocytes
-
At birth
and during
childhood
Near
the
time of
birth,
all
primary oocytes
have started
prophase of
meiosis I,
but
instead
of proceeding
into
metaphase,
they
enter
the
diplotene
stage
,
a resting
stage during
prophase.
The
total number
of primary
oocytes
at birth is estimated
to
vary from 700,000 to two
million
remain.
During
childhood,
most
oocytes
become
atretic
.
only approximately
400,000
are present by the
beginning
of
puberty,
and
fewer than 500 will
be
ovulated.
-
At
puberty
•
Each
month, 15 to 20 follicles
begin
to
mature
and
passing through three
stages:
(1)
P
rimary
(Preantal).
(2)
S
econdary
(A
ntral
).
(3)
Tertiary
or mature vesicular
or
Graafia
n
follicle
(Preovulatory)
.
•
Under normal
conditions,
only one of these
follicles reaches full
maturity
and
the
others
degenerate and
become
atretic.
Unilaminar
P
rimary
F
ollicle
•
Primary
oocyte
(
The
total
number of
primary
oocytes
at
birth
is
estimated
to vary
from
600,000 - 800,000).
•
S
urrounded
by a layer
of
cuboidal
follicular
epithelium
cells.
•
Beginning of
zona
pellucida
Multilaminar
P
rimary
F
ollicle
•
The
follicular
cells
change from
flat
epithelial
to
cuboidal
and proliferate
to
several layers called
granulosa layer.
•
Zona
pellucida
.
Secondary
(A
ntral
) F
ollicles
•
F
luid –
filled spaces
appear
between
the
granulosa
cells.
coalescence of these spaces
form the
antrum
which
is
crescent shaped, but with time
it
enlarges
.
•
The
follicle
is
surrounded
by
:
1.
T
heca
I
nterna
.
2.
T
heca
E
xterna
.
Tertiary
F
ollicle
•
Granulosa cells
surrounding the
oocyte form the
cumulus oophorus.
Maturation
of the
oocytes
Maturation
of
the
oocyte
•
Meiosis
I is
completed,
resulting in
formation of
two
daughter cells of unequal size, each with
23
double-
structured chromosomes
.
One
cell
, the
secondary
oocyte,
receives most of
the cytoplasm;
the other
, the first
polar body,
receives
practically
none.
•
The
cell then enters meiosis
II
but
arrests in metaphase
approximately 3
hours before
ovulation.
Ovulation
•
The
oocyte,
in metaphase
of
meiosis
II, is
discharged from ovary
together with
a
large
number
of cumulus
oophorus cells.
•
Some of the cumulus oophorus cells then
rearrange
themselves
around
the
zona
pellucida to
form the
corona radiata
•
Meiosis
II is
completed only
if
the oocyte
is
fertilized; otherwise, the cell
degenerates
approximately 24
hours
after
ovulation.
•
The
first polar body
may
undergo
a
second
division
.
•
With
onset of puberty and under
the
hormonal control the Ovarian Cycle
will
start.
Gametogenesis is the formation of gametes from germ cells in the testes and ovaries, involving meiosis and cytodifferentiation. Oogenesis specifically refers to the differentiation of oogonia into mature oocytes, with prenatal oogenesis occurring before birth and postnatal oogenesis after birth. The process involves the development of primary oocytes and follicles, with only a fraction surviving to be ovulated. Maturation of oocytes occurs at birth and during childhood, with a significant decrease in numbers before puberty. At puberty, follicles begin to mature monthly, leading to the ovulation of one mature follicle.
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Presentation Transcript
GAMETOGENESIS OOGENESIS Dr. Samer Alnussairi
Objectives Oogenesis (prenatal & postnatal) Ovulation
In preparation for fertilization, Germ cells undergo Gametogenesis (include meiosis). Cytodifferentiation to complete their differentiation.
Gametogenesis Is the process of formation of gametes from germ cells in the testes and ovaries. Male & female germ cells have the same origin, primordial germ cells
Oogenesis Is the process where by oogonia differentiate into mature oocytes. Prenatal oogenesis occurs during embryonic development before birth. Postnatal oogenesis occurs after birth.
Oogonia By the end of the 3rd month, the majority of the oogonia continue to divide by mitosis, but some of them give rise to primary oocytes that enter prophase of the first meiotic division.
By the 5th month of prenatal development, germ cells in the ovary reaches its maximum( 7 million). 7th By the the majority of oogonia have degenerated, All surviving primary oocytes have entered prophase of meiosis I, and most of them are individually surrounded by a layer of flat epithelial cells (primordial follicle ). month,
Maturation of the oocytes - At birth and during childhood Near the time of birth, all primary oocytes have started prophase of meiosis I, but instead of proceeding into metaphase, they enter the diplotene stage , a resting stage during prophase. The total number of primary oocytes at birth is estimated to vary from 700,000 to two million remain. During childhood, most oocytes become atretic. only approximately 400,000 are present by the beginning of puberty, and fewer than 500 will be ovulated.
- At puberty Each month, 15 to 20 follicles begin to mature and passing through three stages: (1) Primary (Preantal). (2) Secondary (Antral). (3) Tertiary or mature vesicular or Graafian follicle (Preovulatory). Under normal conditions, only one of these follicles reaches full maturity and the others degenerate and become atretic.
Unilaminar Primary Follicle Primary oocyte (The total number of primary oocytes at birth is estimated to vary from 600,000 - 800,000). Surrounded by a layer of cuboidal follicular epithelium cells. Beginning of zona pellucida
Multilaminar Primary Follicle The follicular cells change from flat epithelial to cuboidal and proliferate to several layers called granulosa layer. Zona pellucida.
Secondary (Antral) Follicles Fluid filled spaces appear between the granulosa cells. coalescence of these spaces form the antrum which is crescent shaped, but with time it enlarges. The follicle is surrounded by: 1. Theca Interna. 2. Theca Externa.
Tertiary Follicle Granulosa cells surrounding the oocyte form the cumulus oophorus.
Maturation of the oocyte Meiosis I is completed, resulting in formation of two daughter cells of unequal size, each with 23 double- structured chromosomes. One cell, the secondary oocyte, receives most of the cytoplasm; the other, the first polar body, receives practically none. The cell then enters meiosis II but arrests in metaphase approximately 3 hours before ovulation.
Ovulation With onset of puberty and under the hormonal control the Ovarian Cycle will start. The oocyte, in metaphase of meiosis II, is discharged from ovary together with a large number of cumulus oophorus cells. Some of the cumulus oophorus cells then rearrange themselves around the zona pellucida to form the corona radiata Meiosis II is completed only if the oocyte is fertilized; otherwise, the cell degenerates approximately 24 hours after ovulation. The first polar body may undergo a seconddivision.
