2. • Oogenesis is the process by which the female
gametes (ovum) are created.
• Site - Ovary
• Ploidy – Haploid (n)
• Number- one egg in a month
OOGENESIS
3. • Oogenesis is initiated in the EMBRYONIC STAGE.
• Oogenesis starts in fetus
• 6-7 week- start oogenesis
• 20 weeks – 6/7 million oogonia
• Birth – about 2million ogonia
• Puberty- 60,000- 80,000 oogonia in the form of
oocytes
OOGENESIS
5. oogonia
Germinal epithelium in ovary divide by mitosis,
producing a couple of million oogonia (2n)
The oogonia multiply by mitotic divisions
forming egg nest.
One egg from egg nest grows and become the
primary oocytes.
No more oogonia are formed or added after
birth.
6. Meiotic arrest I
• Oogonium develops into priamry oocyte (oocytogenesis)
• These primary oocyte remains arrested at Meiosis I (at
prophase I/ Diplotene)
• The number of primary oocyte remains 60000- 80000 at
puberty.
• Out of these only about 400 primary oocyte will enter in the
menstrual cycle. And out of these 400 only one will form
ovum during each menstrual cycle.
• Meiosis I completed about 36- 48 hr prior to ovulation.
7.
8. MEIOTIC ARREST II
• After puberty
• Meiosis I continues
• One polar body and one secondary oocyte formed
• Secondary oocyte again enters in Meiotic arrest II
(at methaphase II state)
• It remains arrested till fertilization with sperm.
9.
10. Reason of Meiotic Arrest
In menstrual cycle ovary releases
secondary oocyte
It reamin arrested at this stage as it
does not have centriole.
When sperm fuses with egg it
transfer its centriole from the tail
region to the egg.
After getting centriole Meiotic II
begins.
In this way second polar body and
ovum/
ootid is formed.
11. Growth phase
.This phase of the primary oocyte is very long.
It may extend over many years.
The oogonium grows into a large primary oocyte
by taking food from the surrounding follicle
cells.
It happens after puberty.
13. Development of follicles
Secondary follicle
- Secondary oocyte surrounded by two layers cell (Theca externa
and Theca enterna)
- Cavity becomes large (ANTRUM)
14. • Tertiary follicle (Graffian
follicle) Contains
secondary oocyte
• Large Antrum filled with
fluid
• Graffian follicle rupture
due to LH surge and
seconadry oocyte
released.
Development of follicles
17. ell type
ploidy/chromoso
mes
chromati
ds
Process
Time of
completion
Oogoniu
m
diploid/46(2N) 2C
Oocytogenesis (
mitosis)
third trimester
primary
Oocyte
diploid/46(2N) 4C
Ootidogenesis (m
eiosis I)
(Folliculogenesis)
Dictyate in
prophase I for
up to 50
years
secondar
y Oocyte
haploid/23(1N) 2C
Ootidogenesis (m
eiosis II)
Halted in
metaphase II
until
fertilization
Ootid haploid/23(1N) 1C
Ootidogenesis (m
eiosis II)
Minutes after
fertilization
