Follicular Development
• During the course of maturation, the primary oocyte become surrounded by a single layer of squamous follicular cells, to
form basal lamina called primordial follicle. It remains arrested at the diplotene stage of the first meiotic division till the
onset of puberty.• At puberty, oogenesis is resumed and the basal lamina around the primary oocyte becomes cuboidal to form primary follicle.
• As the development proceeds in primary follicle, the follicular cells keep dividing to form several layers around the primary
oocyte and are known as granulosa cells.• The primary oocyte secretes an acellular glycoprotein layer around itself, called the zona pellucida. The innermost layer of
granulosa cells around the zona pellucida consists of columnar cells and is known as the corona radiata.• 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 gets 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 cumulus oophorous.• 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. 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 rupturing 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, then 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 and leads to menstruation.
- Only one ovarian follicle matures and ovulates in an adult woman in every menstrual cycle, alternatively by the two
ovaries. So, only about 450 of the total follicles mature during the entire reproductive span. The rest of them degenerate
at different times. The degenerating process is called follicular atresia. This is an example of programmed cell death or
apoptosis. - Due to this developmental pattern, eggs ovulated near age 50 are 35 to 40 years older than those ovulated just after puberty.
Certain chromosomal defects or abnormalities among children born to older women may be the result of ageing changes in
the eggs.
Follicular growth and atresis