Oogenesis

• Oogenesis and ovulation


  1. Meiotic changes in oocytes

  2. Formation and function of the zona pellucida

  3. Follicular growth

  4. Pre-ovulatory and post ovulatory atresia

  5. Ovum maturation
• Origin and growth of follicles

  1. Ova arise by proliferation of the germinal epithelium

  2. Cells sink into the ovarian cortex and continue to multiply as oogonia

  3. Indifferent cells encase the oogonia to produce primary follicles

  4. Formation of human oogonia comes to a halt shortly after birth

  5. No advance beyond the primordial follicle stage until puberty

  6. Larger follicles in various stages of ripening evident during reproductive period
• Oogonium


  Measures 0.02mm in diameter, with a single layer of flattened epithelial cells as follicular coverings
• Primary oocyte


  Oogonium increases in diameter sevenfold (to 0.135mm), with cuboidal follicle cells that proliferate and form a stratified epithelium
• Follicle growth


  1. Follicle cells become cuboidal and proliferate

  2. Follicle ripening hormone (prolan A) from anterior pituitary causes follicular fluid-filled cavity (antrum)

  3. Follicular fluid contains estrogen hormone

  4. Oogonium becomes located more eccentrically in the cumulus oophorus

  5. Theca folliculi (connective tissue sheath) differentiates around the follicle
• Vesicular (Graafian) follicle


  Enlarged follicle with fluid-filled cavity (antrum)
• Growth of the follicle is slow at start, but rapid in the last day or two before rupture
• The mature follicle is millions of times bulkier than the primary follicle, with a final diameter of 8mm or more
• Egg maturation


  1. Two mitotic divisions without reconstitution of resting nucleus

  2. Whole chromosomes pass into daughter cells

  3. Results in one large ripe ovum and three polar bodies/polocytes

  4. Ovum retains all cytoplasm and yolk, while polar bodies degenerate

  5. Ovum nucleus reconstitutes as female pronucleus, ready to unite with sperm pronucleus
• Oogenesis


  Differentiation of the primordial germ cells (oogonia) that are present in the cortex of the ovary into mature ova
• Oogenesis


  1. Prenatal maturation

  2. Postnatal maturation
• Prenatal maturation


  1. Primordial germ cells (oogonia) undergo mitotic divisions

  2. Arranged in clusters surrounded by a layer of flat epithelial cells (follicular cells)

  3. Near birth, all the primary oocytes have started first meiotic division then they are arrested till puberty

  4. Number of primary oocytes at birth ranges from 700,000 to 2 million

  5. At puberty only 400,000 primary oocytes remain & about 400 only will be ovulated during the female life time
• Postnatal maturation


  1. With the onset of puberty, primordial follicles begin to mature with each ovarian cycle

  2. The primary oocyte begins to increase in size

  3. The primordial follicle after differentiating becomes primary follicle

  4. Its granulosa cells rest on a basement membrane separating them from the stromal cells of the ovary that form the theca folliculi

  5. The granulosa cells and the oocyte secrete the zona pellucida

  6. Fluid-filled spaces appear between the granulosa cells then they coalesce together forming the follicular antrum, now called the secondary follicle

  7. The follicle enlarges and is called the Graafian follicle

  8. The primary oocyte continues the first meiotic division leading to two unequal daughter cells with 23 chromosomes each (22 autosomes +X)

  9. The secondary oocyte and first polar body, enter the second meiotic division without DNA replication

  10. The second meiotic division is completed only if fertilization occurs to give fertilized oocyte and a second polar body that soon degenerates
• Mature ovum


  • A large oval cell that varies from 117 -142 u in diameter
  • It has two membranes; the inner thin vitelline membrane and the outer zona pellucida
  • The corona radiata is two or three layers of cells surrounding the zona pellucida externally when the ovum is shed from the follicle