Introduction.
The process of formation of gametes is called gametogenesis.
The male gametogenesis is called “Spermatogenesis”.
The female gametogenesis is called “Oogenesis”.
Oogenesis takes place in the following stages,
Prenatal Stage.
Antral Stage.
Pre ovulatory Stage.
Prenatal Stage:
Unlike male spermatogenesis which starts after puberty, the process of oogenesis starts in the embryonic stage only.
The stem cells called Primordial cells divide mitotically and produce Primary oocytes which start Meiosis but get arrested in the Prophase-I only.
These primary oocytes get covered by follicular cells of ovary called granulosa cells, which secrete a glycoprotein layer around the primary oocyte, the layer is called “Zona pellucida”.
Antral Stage:
The fluid-filled gap between granulosa cells combines to produce the antrum, a central fluid-filled region.
Now the primary follicles are called “Secondary Follicles”.
These secondary follicles start further development under the influence of follicle-stimulating hormone and luteinizing hormone during each month's cycle.
The secondary follicles mature to form the “Graafian Follicle”.
Pre ovulatory Stage:
This stage is triggered by a surge of LH, and it is at this point the meiosis-I which was started before the birth is completed.
Two haploid cells of uneven size are formed.
One of the offspring cells that receives less cytoplasm is called the polar body (This cell isn't engaged in the ovum's development, it is produced to discard extra sets of haploid genome produced during meiosis).
The other daughter cell refers to the secondary oocyte.
In the two daughter cells, meiosis-II takes place.
The polar body divides into two polar bodies, whereas the secondary oocyte enters the meiosis II metaphase and stops reproducing.
Ovulation and Fate of Ovum:
On the fourteenth day of the menstrual cycle the LH surge happens which causes the rupture of the graafian follicle releasing the secondary oocyte in the pelvic cavity.
The secondary oocyte is then picked up by fimbriae in the fallopian tube.
If sperm is available, fertilization takes place which triggers the stopped meiosis into the formation of a zygote and a polar body (it disintegrates soon after as it contains very less cytoplasm).
If sperm are not available the ovum survives for 24 hrs then the dead ovum is swept away through the menstrual waste.
Commonly Asked Questions.
Write a short note on female gametogenesis.
Write a short note on Oogenesis.