°ACΔDEMiC℠
Tutorials
Definition
Occurence
OOGENESIS Site
Events
Clinical correlate
Oogenesis can be
defined in simple
terms, as the
DEFINITION formation and
maturation of
oocytes from
primordial germ cells
starts before
birth in females
OCCURENCE continues after
puberty
ends at
menopause
SITE
TAKES PLACE IN THE OVARY
Pre-natal
phase
EVENTS
Post-natal
phase
Week 3: Primordial germ cells
(PGCs) originally derived from
epiblast, migrate via the primitive
streak into endoderm of the yolk-
sac
Pre-natal
phase Week 5: From yolk sac, PGCs
migrate to the developing gonads
of a genetic female, where they
differentiate into oogonia. There, in
the gonads, the oogonia undergo
series of divisions giving rise to many
more oogonia.
Month 3: The oogonia in the
ovaries organize in clusters,
and are surrounded by flat
follicular cells which migrate
from surface epithelium of
Pre-natal ovary.
phase Month 5: Clustered oogonia
undergo mitotic divisions giving
rise to larger cells called
PRIMARY OOCYTES. The total
number of germ cells at this
stage (oogonia + primary
oocytes) in the ovaries are now
about 7 million.
Month 7: Some germ cells in the
ovaries undergo atresia
(degeneration) & surviving
primary oocytes begin MEIOSIS I.
The primary oocytes at this stage,
are surrounded individually by flat
Pre-natal follicular cells.
phase
Note: Primary oocyte,
together with its surrounding
flat follicular cells, constitute -
The PRIMORDIAL FOLLICLE
At birth: About 1 million
primordial follicles are
present in each ovary.
All primary oocytes, have commenced
Pre-natal MEIOSIS I but are halted at prophase I by
phase OMI (oocyte maturation inhibitor) until
after puberty.
NB: OMI is secreted by
follicular cells that surround
the primary oocyte
Post-natal phase
After birth: About 40,000 primary oocytes are present in
the ovaries, because a lot have undergone atresia & no
primary oocytes form after birth
Upon puberty: Every month, about 15 - 20 primordial
follicles begin to mature under the influence of FSH & LH,
but only ONE (1) attains maturity. Eventually, only about
400 oocytes would be ovulated in a woman's lifetime.
Follicle maturation
• Formation of PRIMARY FOLLICLE:
This involves the conversion of
follicular cells of the primordial
follicle from flat to cuboidal cells,
then columnar cells before finally
becoming stratified (granulosa)
cells
• Zona pellucida(ZP): a layer of
amorphous acellular
glycoproteins material which
directly surround the primary
oocyte, is secreted by the primary
oocyte & some granulosa
(follicular) cells
Follicle Maturation
• Cells of the ovarian stroma separate to surround the
primary follicle and are referred to as - Theca of the
primary follicle.
• Fluid filled spaces appear within the granulosa cells.
Coalescence of these spaces form the antrum & the
follicle becomes - Antral Follicle.
• Note: This is also known as the vesicular or SECONDARY
FOLLICLE
Follicle Maturation
• The theca of the secondary follicle, differentiate into:
(a) Theca Externa (protective cells)
(b) Theca Interna (secretory cells)
• Some follicular cells, which surround the primary oocyte re-
organise, to form the cumulus oophorus (meaning 'cloud
around oocyte)
Resumption & completion of
MEIOSIS I occurs in the primary
oocyte, thus, the secondary
oocyte & 1st polar body are
formed. At this stage, the follicle is
Follicle termed - Graafian Follicle
Maturation
Note: This is also known as the
mature vesicular or TERTIARY
FOLLICLE
CLINICAL CORRELATE
• Offspring of older women:
The ideal maternal age is considered to be between 18 - 35
years. The likelihood of chromosomal abnormalities such as
Down syndrome in the embryo, increases with advanced
(40+) maternal age.
Whereby offspring has reduced cognitive ability (mental
retardation)
