Biology today

dormant follicle is called the primordial follicle. It remains

like this (the primary oocyte is arrested at the diplotene of the

first meiotic division) till the onset of puberty.

• at puberty, oogenesis is resumed. The squamous follicular
cells around the primary oocyte become cuboidal. This follicle
is known as primary follicle.

• In the primary follicle, development proceeds, the follicular
cells keep dividing to form several layers around the primary
oocyte and at this stage these cells are known as the
granulosa cells.

• The primary oocyte secretes an acellular glycoprotein layer
around itself, called the zona pellucida.

• The granulosa cells are in communication with the growing
oocyte by microvilli on their surface which interdigitate with
those on the surface oocyte to facilitate the exchange of
material between them.

• as the granulosa cells keep on dividing, a small cavity
appears between these cells called the antrum which is
filled with a fluid, liquor folliculi, secreted by the granulosa
cells. at this stage the follicle is known as an antral follicle
or the secondary follicle.

• around the granulosa cells connective tissue of ovarian
stroma get differentiated into two layers: a vascular theca
interna and a fibrous theca externa.

• Blood vessels in the thecal layer cannot transverse the
membrane so the granulosa layer is completely avascular.

• The antrum keeps enlarging with the developing oocyte so
that the oocyte is suspended in this fluid-filled cavity by a
stalk of granulosa cells surrounding this oocyte. These are
known as the cumulus oophorous.

• The innermost layer of granulosa cells around the zona
pellucida consists of columnar cells and is known as the
corona radiata.

Fig.:Development of a human oocyte and ovarian follicle

Antrum

Antrum

Preantral

follicle

Early antral

follicle

Oocyte

Granulosa cells

Theca

Zona pellucida

Oocyte

Fluid

Cumulus

oophorous

Mature follicle

OocyteGranulosa

cells

Zona

pellucida

Granulosa

cells

Fully grown

oocyte

Primary

follicle

Early

theca

Zona

pellucida

Granulosa cells

Theca

Fluid

The progession of some primordial follicles to the preantral

and early antral stages occurs throughout infancy and

childhood, and then during the entire menstrual cycle.

Therefore, although most of the follicles in the ovaries are

still primordial, nearly constant number of preantral and early

antral follicles are also always present. at the beginning of

each menstrual cycle, 10 to 25 of these preantral and early

antral follicles begin to develop into larger antral follicles.

about one week into the cycle, a further selection process

occurs: only one of the larger antral follicles, the dominant

follicle, continues to develop. The exact process by which

a follicle is selected for dominance is not known, but it is

likely related to the amount of estrogen produced locally

within the follicle. (This is probably why hyperstimulation

of infertile women with gonadotropin injections can result

in the development of many follicles). The nondominant

follicles (in both ovaries) that had begun to enlarge undergo

atresia.

dominant follicle

ovulation

•    The fully formed mature dominant follicle with a large

antrum is called Graafian follicle. It has a primary oocyte,

which is still in meiotic arrest. Just before ovulation this

primary oocyte completes its first meiotic division to form a

haploid secondary oocyte and a polar body.

•    Only meiosis I is complete, meiosis II is initiated but arrested

at the metaphase stage.

•    This secondary oocyte, along with its cumulus oophorous, is

released in the peritoneal cavity at ovulation from where it

is picked up by the fimbriae of the oviduct.

•    The ovulated secondary oocyte is carried into the Fallopian

tube where fertilisation occurs.

•    The remnants of the ovulated follicle in the ovary form the

corpus haemorrhagicum that has a blood clot in the centre

due to rupture of the blood vessels supplying the thecal layer.

•    This clot is dissolved later and the granulosa and thecal cells

of the ruptured follicle are transformed and converted into

lutein cells having yellow carotene pigment or lutein. These

transformed cells form corpus luteum which secretes the

hormones progesterone and estrogen.

•    The corpus luteum is maintained for about 20 days.

•    If fertilisation occurs and a conceptus is formed, the corpus

luteum receives a signal from the conceptus and the corpus

luteum is maintained for a long time to support pregnancy.

•    In the absence of any such signal from the conceptus, the

corpus luteum degenerates. This degenerating corpus luteum

is known as the corpus albicans.