Oogenesis pdf - D. Sumeya

Slide 1

Oogenesis

Dr. Sumeya

Slide 2

Primary Oocyte

Secondary

oocyte

Meiosis

morphogenesis

Polar Body

Ootid

Polar

body

Polar

bodies

Ovum

Die

Oogensis

Slide 3

Oogenesis

Is the process whereby oogonia differentiate into mature oocytes.

Maturation of Oocytes Begin Before Birth

• Once PGCs have arrived in the gonad of a genetic female, they

differentiate into oogonia .

• By the end of the third month: they are arranged in clusters

surrounded by a layer of flat epithelial cells(follicular cells) originate
from surface epithelium covering the ovary .

• All the oogonia in one cluster are properly derived from a single cell.
• Oogonia: majority: continue to devide by mitosis.

some: enter meiosis and arrest their cell division in prophase

MI forming primary oocytes .

Slide 4

• 5

month: total number of germ cells(oogonia + primary oocytes)

reach maximum(7 millions) .

•

At this time: cell death begins leading to many oogonia and primary

oocytes degenerate and become atretic .

•

7th month: the majority of oogonia ----- degenerate except few near

a surface.

• All surviving primary oocytes ------ entered prophase of miosis 1 ,

and most of them are individually surrounded by a layer of flat
follicular epithelial cells.

• A primary oocyte, together with its surrounding flat epithelial cells, is

known as a

primordial follicle

Slide 5

Slide 6

Slide 7

Maturation of Oocytes

Continues at Puberty

• Near birth time: all primary oocytes ------- started prophase of

meiosis I, but instead of proceeding into metaphase, they enter
the

diplotene stage

, a resting stage during prophase that is

characterized by a lacy network of chromatin

• Primary oocytes remain arrested in prophase and do not fi nish

their fi rst meiotic division before puberty is reached.

• This arrested state is produced by

oocyte maturation

inhibitor (OMI),

a small peptide secreted by follicular cells.

• The total number of primary oocytes at birth is estimated to vary

from 600,000 to 800,000. During childhood, most oocytes
become atretic; only approximately 40,000 are present by the
beginning of puberty, and fewer than 500 will be ovulated.

Slide 8

• Some oocytes that reach maturity late in life have been dormant

in the diplotene stage of the fi rst meiotic division

for 40 years

more before ovulation------- vulnerable to damage with age and
increase chromosomal abnormalities.

Slide 9

Slide 10

• At puberty, 15 to 20 primordial follicles monthly begin to mature

passing in 3 stages:1-Primary or preantral

2-Secondary or antral or vesicular

3-Preovulatory(Graafian follicle)

• Some of these die, while others begin to accumulate fluid in a

space called the antrum, thereby entering the

antral

vesicular stage

•

Fluid continues to accumulate such that, immediately prior to

ovulation, follicles are quite swollen and are called mature

vesicular follicles or Graffi an follicles

. The antral stage is the

longest, whereas the mature vesicular stage encompasses
approximately 37 hours prior to ovulation.

Slide 11

• As primordial follicles begin to grow, surrounding follicular cells

change from flat to cuboidal and proliferate to produce a
stratified epithelium of granulosa cells, and the unit is called a

primary follicle .

• Granulosa cells: rest on a basement membrane separating

them from surrounding ovarian connective tissue (stromal cells)
that form the theca folliculi.

• Also, granulosa cells + the oocyte secrete a layer of

glycoproteins on the surface

of the oocyte, forming the

zona pellucida

Slide 12

• As follicles continue to grow, cells of the theca folliculi organize

into an inner layer of secretory cells, the

theca interna

, and an

outer fibrous capsule, the

theca externa.

• Also, small, finger-like processes of the follicular cells

extend across the zona pellucida and interdigitate with
microvilli of the plasma membrane of the oocyte. These
processes are important for transport of materials from
follicular cells to the oocyte.

Slide 13

• As development continues, fluid-filled spaces appear between

granulosa cells. Coalescence of these spaces forms the

antrum, and the follicle is termed a vesicular or an antral
follicle

•

Initially, the antrum is crescent-shaped, but with time, it

enlarges. Granulosa cells surrounding the oocyte remain intact
and form the

cumulus oophorus

• At maturity, the

mature vesicular (Graafian) follicle

may be 25

mm or more in diameter. It is surrounded by the theca interna,
which is composed of cells having characteristics of steroid
secretion, rich in blood vessels, and the theca externa, outer
fibrous capsule layer which gradually merges with the ovarian
connective tissue

Slide 14

• With each ovarian cycle, a number of follicles begin to develop,

but usually only one reaches full maturity. The others
degenerate and become atretic.

• When the secondary follicle is mature, a surge in

luteinizing

hormone (LH) i

nduces the preovulatory growth phase. Meiosis I

is completed, resulting in formation of two daughter cells of
unequal size, each with 23 double-structured chromosomes

Slide 15

• One cell, the

secondary oocyte,

receives most of the

cytoplasm; the other, the

first polar

body, receives practically

none. The fi rst polar body lies between the zona pellucida and the
cell membrane of the secondary oocyte in the perivitelline space

The cell then enters meiosis II but arrests in metaphase
approximately 3 hours before ovulation. Meiosis II is completed
only if the oocyte is fertilized; otherwise, the cell degenerates
approximately 24 hours after ovulation. The fi rst polar body may

undergo a second division