Oogenesis

• Oogenesis is the process by which female germ cells, called oocytes, develop into mature eggs (ova) in the ovaries.
• It involves a series of cell divisions and developmental stages that result in the production of a single mature ovum from a primordial germ cell.

Stages of Oogenesis

• Oogenesis can be divided into the following stages:

1. Oogonial Proliferation (Fetal Development)

   • During fetal development, primordial germ cells in the ovaries differentiate into oogonia, which are diploid (2n) cells.
   • Oogonia undergo several mitotic divisions to increase their numbers.
   • Some oogonia enter meiosis and become primary oocytes.
   • By birth, all oogonia have entered prophase I of meiosis I and are arrested at this stage, remaining as primary oocytes.

2. Formation of Primordial Follicles

   • Primary oocytes are surrounded by a single layer of flattened granulosa cells, forming primordial follicles.
   • These primordial follicles remain dormant in the ovaries from birth until puberty.

3. Follicular Development (After Puberty)

   • With the onset of puberty, during each menstrual cycle, a group of primordial follicles is activated to begin growth and development.
   • Primary oocytes within these follicles complete meiosis I, forming two cells:
     • A large secondary oocyte (haploid, 1n).
     • A smaller polar body, which contains excess genetic material and eventually degenerates.
   • The secondary oocyte enters meiosis II but is arrested in metaphase II and remains there until fertilization.

4. Ovulation

   • The mature Graafian follicle, containing the secondary oocyte, ruptures during ovulation, releasing the oocyte into the fallopian tube.
   • If fertilization does not occur, the secondary oocyte degenerates within approximately 24 hours.

Hormonal Regulation of Oogenesis

1. Gonadotropin-Releasing Hormone (GnRH): Released by the hypothalamus, it stimulates the anterior pituitary to release FSH and LH.
2. Follicle-Stimulating Hormone (FSH): Promotes growth and maturation of ovarian follicles and oocytes.
3. Luteinizing Hormone (LH): Triggers ovulation and the formation of the corpus luteum, which secretes hormones that sustain the uterine lining.
4. Estrogen and Progesterone: Produced by the follicles and corpus luteum, they regulate the menstrual cycle, promote endometrial growth, and prepare the body for pregnancy.

Significance of Oogenesis

1. Genetic Diversity: Meiosis during oogenesis contributes to genetic variation through recombination.
2. Fertility and Reproduction: Oogenesis determines the fertility window and the release of the secondary oocyte for potential fertilization.
3. Hormonal Balance: Oogenesis regulates the menstrual cycle and hormonal environment crucial for female reproductive health.

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