Question

Explain the development of a secondary oocyte (ovum) in a human female from the embryonic stage upto its ovulation. Name the hormones involved in the process.

Answer 1

Hint: Oogenesis involves the maturation of reproductive cells in the ovary. It starts before birth and in the 25 week old fetus, all the oogonia are formed. They are generally ready by the third month of the gestation.

Complete answer:
- The primordial germ cells in the ovaries divide mitotically to form oogonia, during the multiplication phase. Each oogonium divides mitotically to form two primary oocytes.
- Primary oocytes undergo growth, the growth phase during oogenesis is comparatively larger, size of oocytes increases very much. Primary oocytes begin the first development step of meiosis-I and proceed upto diplotene. These oocytes resume their development at puberty.
- The primary oocytes then complete meiosis-I and produce two haploid cells- the secondary oocyte and first polar body.
- The secondary oocyte then forms ovum under the influence of FSH and LH. On the $13_{th}$ day, the LH and FSH peak results in the ovulation after 24 hours of LH peak. Meiosis II in secondary oocytes is arrested till sperm entry in metaphase-II due to metaphase promoting factor.

The hormones involved in the process are: FSH (follicle stimulating hormone) and LH (luteinizing hormone). FSH stimulates follicular growth and maturation of oocytes while LH converts Graaafian follicle into corpus luteum which secretes progesterone.

Note: The oogonia in female babies are generally ready by the third month of the gestation. There are about 600.000 oogonia after two months of gestation, 7 million oogonia in the seventh month of gestation. After mitosis stops, two lakhs to two millions are left at birth and only 40,000-3 lakhs oocytes remain till puberty.