Oogenesis pdf - D. Saba

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

or 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