Oogenesis Overview

Oogenesis

• Oogenesis is a three-phase process
• Phase 1: Multiplication Phase
  • Primordial germ cells in the germinal epithelium multiply repeatedly
  • Mitosis produces oogonia, a large number of small cells
  • Each oogonium contains a diploid chromosome number (2n)
• Phase 2: Growth Phase
  • Oogonia increase in size
  • Accumulate nutritive materials
  • Form primary oocytes, which contain the diploid chromosome number (2n)
• Phase 3: Maturation Phase
  • Primary oocyte divides by meiosis (first and second meiosis)
  • First meiotic division produces a large secondary oocyte and a smaller first polar body
  • Secondary oocyte contains the haploid chromosome number (n)
  • Second meiotic division produces an ovum and a second polar body
  • The first polar body may or may not divide further
  • Thus, one primary oocyte produces one mature ovum and several polar bodies

Types of Eggs

• Isolecithal eggs: Contains little yolk, evenly distributed
• Centrolecithal eggs: Yolk located centrally
• Mesolecithal eggs: Moderate amount of yolk, centrally located
• Telolecithal eggs: Large amount of yolk concentrated at the vegetal pole

Fertilization

• Fertilization is the union of two gametes (sperm and egg) to form a zygote
• Two types of fertilization:
  • External fertilization: Gametes released into the surrounding medium. Large numbers of gametes are released to overcome loss. Common in fishes and amphibians, where animals release gametes into the water for the sperm to swim to the egg
  • Internal fertilization: Gametes unite inside the female's body. Common in mammals and other terrestrial animals, where the male places sperm inside the female body

Types of Animals Based on Fertilization

• Oviparous: Lay eggs; nourishment for the developing embryo comes from the yolk
• Viviparous: Embryo develops inside the mother's body and is nourished through a placenta
• Ovoviviparous: Eggs develop and hatch inside the mother's body; nourishment comes from the yolk, no placenta

Embryo Development

• Gamete Formation: Sperm and egg formation
• Fertilization: Fusion of sperm and egg to form a zygote
• Cleavage: Repeated cell divisions of the zygote
• Blastula: A hollow ball of cells
• Gastrulation: Formation of the germ layers (ectoderm, mesoderm, endoderm)
• Organogenesis: Formation of organs and tissues from the germ layers
• Growth: Cells grow, differentiation occurs, and the adult body forms

Explore the fascinating process of oogenesis, which occurs in three distinct phases: Multiplication, Growth, and Maturation. Learn about the transformation of primordial germ cells into mature ova and the roles of polar bodies in this intricate biological process.