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Oogenesis in mammals

OOGENESIS IN MAMMALS

Oogenesis is the process by which female germ cells (oogonia) undergo a series of cell divisions and differentiations to produce mature eggs (ova or oocytes). This complex process takes place in the ovaries of mammals and is crucial for reproductive success. Oogenesis involves several stages, from the development of primordial germ cells to the formation of mature eggs ready for fertilization. Here is an overview of the key stages of oogenesis in mammals:

1. Formation of Primordial Germ Cells (PGCs):

  • Occurs during early embryonic development.
  • PGCs arise from a subset of cells in the embryonic epiblast.
  • Migrate to the genital ridge and undergo mitotic divisions.

2. Differentiation into Oogonia:

  • PGCs differentiate into oogonia.
  • Oogonia undergo mitotic divisions to increase their numbers.

3. Formation of Primordial Follicles:

  • Oogonia develop into primary oocytes.
  • Surrounding somatic cells (pre-granulosa cells) form a single layer around the oocyte, creating primordial follicles.

4. Meiotic Arrest:

  • Primary oocytes undergo DNA replication (S phase) but arrest in prophase I of meiosis.
  • This meiotic arrest occurs during fetal development.

5. Primordial Follicle Activation:

  • At puberty, a pool of primordial follicles is activated.
  • One or a few primordial follicles develop into primary follicles.

6. Primary and Secondary Follicle Development:

  • Primary Follicle:
    • The primary oocyte within the primary follicle is surrounded by a single layer of granulosa cells.
    • Theca cells begin to differentiate in the surrounding stroma.
  • Secondary Follicle:
    • Granulosa cells proliferate, forming multiple layers.
    • An antrum (fluid-filled cavity) develops within the follicle.

7. Meiosis Resumes:

  • In response to hormonal signals, the primary oocyte within the secondary follicle completes meiosis I.
  • This results in the formation of a secondary oocyte and a polar body.

8. Ovulation:

  • The mature secondary oocyte is released from the ovary during ovulation.
  • The follicle ruptures, releasing the secondary oocyte surrounded by the corona radiata.

9. Meiosis II and Fertilization:

  • If fertilization occurs, the secondary oocyte completes meiosis II, resulting in a mature ovum and another polar body.
  • The sperm penetrates the egg, and fertilization takes place.

10. Formation of the Corpus Luteum:

  • The remaining follicle transforms into the corpus luteum, secreting progesterone to prepare the uterus for pregnancy.

11. Corpus Luteum Regression:

  • If fertilization does not occur, the corpus luteum regresses, leading to the shedding of the uterine lining during menstruation.

The process of oogenesis is highly regulated and involves intricate interactions between the oocyte and surrounding somatic cells. It ensures the production of mature eggs with the potential for fertilization and the initiation of embryonic development. Oogenesis is a dynamic and essential component of the female reproductive system.

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