As discussed in our previous article, gametogenesis plays a significant role in humans to support the continuation of generations. Furthermore, it is called spermatogenesis in males. Whereas, it is called oogenesis in females. Now that we have studied gametogenesis in males, let us see how this happens in females.
Author – Kshitij Dwivedi
“Oogenesis is the process through which the female gametes or ova, are produced.”
During the reproductive years of females, i.e., between 13–46 years of age, 400–500 primordial follicles develop enough to expel their ova. Further, one ova is expelled each month and the remaining degenerate at the end of reproductive life. At menopause, only a few primordial follicles remain and even this degenerate.
The Ovarian Cycle
The ovarian cycle attributes to the set of changes in the ovary which results in oogenesis, the formation of the female gamete, i.e. the ova.
It depends on gonadotropin hormone (GnRH) secretion which begins at puberty (8-13 years). During the 8th year, a large quantity of GnRH is secreted which initiates the monthly sexual cycle. GnRH shows an increase or decrease in secretion which is responsible for the cyclical changes in the ovary.
There are three phases in the Ovarian cycle, namely:
- Follicular phase/estrogenic phase
- Luteal phase/progestational phase.
During fetal development, ovaries contain seven million primordial follicles. However, many undergo atresia (involution) before birth, and others degenerate after birth.
Furthermore, the normal ova undergo the 1st part of the first meiotic division and enter a stage of arrest in the prophase which persists until adulthood. Atresia continues during development. Consequently, the number of ova in both ovaries at puberty is less than 300,000.
Also, only one of these ova per cycle (500 in normal reproductive life) normally reaches maturity. Whereas, the remaining ova degenerates. The first meiotic division completes, just before ovulation. One of the daughter cells, i.e., the secondary oocyte immediately begins the 2nd meiotic division, but this division stops at metaphase and completes only when a sperm penetrates the oocyte.
Ovulation is the rupturing of the Graffian follicle to release a secondary (2°) oocyte. This rupturing of Graffian follicle, occurs on the 14th day by LH hormone. The 2° oocyte moves towards the surface of the ovary and protrudes through the outer layer. This viscous fluid contains an ovum surrounded by a mass of several thousand small granulosa cells called corona radiata.
Granulosa and theca cells of the follicle begin to proliferate, and the clotted blood is replaced by yellowish, lipid-rich luteal cells, forming corpus luteum. Luteal cells secrete estrogens and progesterone.
Mechanism of Ovulation or Oogenesis
Follicle secretes estrogen which stimulates the hypothalamus and pituitary to release luteinizing hormone or LH and follicle-stimulating hormone or FSH. LH and FSH are essential factors for the final maturation of the follicle. Estrogen in a high dose results in a positive feedback effect, whereas it produces a negative feedback effect, in a moderate dose.
Important points (Oogenesis) for NEET-UG Exam:
- Menarche is the starting of the menstrual cycle in the female. Usually starts when the female hits puberty.
- Menopause is the end of menstrual cycle for female. The female can now no longer produce gametes. Usually occurs in 45-55 years of age.
- LH surge is an acute rise of luteinizing hormone which results in the expulsion of the ovum.
- Corpus luteum persists if the fertilization of ovum occurs. It secretes progesterone which is necessary for the maintenance of pregnancy.
- Placenta when formed, takes over the function of secreting progesterone upto full term of pregnancy.
- Ovum usually permits the entry of only one sperm into it.
- One meiotic division leads to the formation of only one ovum. The polar bodies degenerate.
- In the absence of fertilization of ovum by the sperm, the corpus luteum changes into corpus albicans and then degenerate.
Click here to read Gametogenesis (Part I), where we have discussed gametogenesis in males, i.e., spermatogenesis in detail.
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