5. Gametogenesis

5.2. Oogenesis

6.1 Location

·       ogenesis is the process by which female gametes (ova or eggs) develop from oogonia (germ cells) within the ovaries.

·       Occurs in ovaries within ovarian follicles.

·       It involves multiplication, growth, and maturation, resulting in the formation of a single functional ovum capable of being fertilized.

6.2      Stages of Oocyte Development

Unlike spermatogenesis, oogenesis begins before birth (oogonia → primary oocytes), pauses for many years, and resumes at puberty, continuing cyclically until menopause.

  1. Fetal (Pre-natal) Phase
    • The primordial germ cells (PGCs) migrate into the developing ovaries and differentiate into oogonia (immature egg cells)
    • Oogonia (diploid) proliferate (until 5th month of gestation by mitosis → to produce 6 to 7 million germ cells
    • These germ cells develop into primary oocytes (most most of these degenerates before birth - atresia)
    • Primary oocytes begin Meiosis I but are arrested in prophase I until puberty.
    • This arrested state may last 10–50 years, depending on when ovulation occurs.
    • At birth, about 1–2 million primary oocytes are present; by puberty, only about 300,000–400,000 remain.
  2. Puberty to menopause
    • Monthly cohorts resume meiosis
    • One primary oocyte completes Meiosis I → forms a secondary oocyte and a small polar body.
      • The polar body allows for the oocyte to get rid of chromosomes while at the same time taking the least amount of resources (cytoplasm) from the oocyte
    • The secondary oocyte begins Meiosis II but arrests at metaphase II until fertilization.

3. Ovulation

o   Secondary oocyte arrests in meiosis II.

4. Fertilization

    • Completion of meiosis II only if fertilized.

6.3 Folliculogenesis

·       Primordial follicle

·       Primary follicle

·       Secondary follicle

·       Graafian follicle

·       Corpus luteum formation post-ovulation

6.4 Hormonal Regulation

    • FSH
      • Stimulates follicular growth
    • LH
      • Triggers ovulation
      • Supports corpus luteum formation
    • Estrogen and progesterone
      • Regulate endometrium
      • Provide feedback to hypothalamus/pituitary