Oogenesis Process Overview

Oogenesis

• Oogenesis is a process involving three phases:
  • Multiplication phase: Primordial germ cells in the germinal epithelium multiply through mitotic divisions, producing numerous oogonia. Each oogonium contains a diploid number (2n) of chromosomes.
  • Growth phase: Oogonia increase in size, storing nutritive materials. This process forms primary oocytes, which also contain a diploid number (2n) of chromosomes.
  • Maturation phase: The primary oocyte undergoes meiosis, a two-part cell division.
    • The first meiotic division forms a large secondary oocyte and a smaller first polar body. The secondary oocyte contains a haploid number (n) of chromosomes.
    • The second meiotic division divides the secondary oocyte unevenly, producing a large ovum and a second polar body. The ovum also has a haploid number (n) of chromosomes. The first polar body may or may not divide further.
  • The primary oocyte produces one mature ovum and polar bodies.

Fertilization

• Fertilization is the process where two gametes (egg and sperm) fuse to form a zygote.

Types of Fertilization

• External fertilization: Gametes are released into the surrounding medium (water).
  • Many fishes, bony fishes, and amphibians use external fertilization.
  • A large number of gametes are released to increase the chances of successful fertilization in the presence of many environmental factors such as water currents, predators, or competition.

• Internal fertilization: Eggs remain in the female's body until fertilization by sperm introduced into the female reproductive tract.
  • This method is seen in mammals, birds, and some reptiles.


Types of Animals Based on Fertilization and Development

• Oviparous: Fertilization is internal, but the embryo develops outside the mother's body, nourished by yolk.
  • Examples: Birds, reptiles, many fishes, and insects.
• Viviparous: Fertilization is internal, the embryo develops inside the mother's body, nourished through the placenta.
  • Examples: Mammals
• Ovoviviparous: Fertilization is internal, the embryo develops within the mother's body, nourished by yolks.
  • Examples: Some fishes, sharks, and lizards.


Steps of Embryo Development

• Gamete formation: Sperm and egg cells are produced.
• Fertilization: Sperm and egg fuse.
• Cleavage: The zygote divides repeatedly.
• Blastula formation: A hollow ball of cells is formed.
• Gastrulation: The blastula reorganizes into three primary germ layers (ectoderm, mesoderm, and endoderm).
• Organogenesis: Body organs form and differentiate.
• Growth: The embryo grows and develops to reach maturity.

Types of Eggs

• Isolecithal eggs: These eggs contain a small amount of yolk evenly distributed throughout the cytoplasm. 
• Centrolecithal eggs: These eggs have the yolk concentrated in the center, with the nucleus and cytoplasm at the periphery.
• Mesolecithal eggs: These eggs have a moderate amount of yolk concentrated towards the vegetal pole.
• Telolecithal eggs: These eggs have a large amount of yolk concentrated at the vegetal pole, and the cytoplasm and nucleus migrate to the animal pole.