Gametogenesis & Fertilization

Animal Reproduction

• Introduction
  • Overview of reproduction in animals
  • Focus on gametes, sperm, and egg development

Gametogenesis

• Process
  • Production of gametes (sperm and egg) through meiosis
  • Spermatogenesis for males and oogenesis for females

Spermatogenesis

• Occurs in males, leads to the formation of sperm
• Stages:
  1. Spermatogonium (2n) - diploid stem cell
  2. Primary spermatocyte (2n) - undergoes meiosis
  3. Meiosis I - produces secondary spermatocyte (n)
  4. Meiosis II - results in spermatids (n)
  5. Mature sperm cells (n) - finalize formation

Structure of a Human Sperm Cell

• Key Components:
  • Acrosome: contains enzymes for penetrating egg layers
  • Nucleus: contains DNA
  • Mitochondrion: provides energy for movement
  • Plasma membrane: surrounds the sperm cell
  • Tail: propels the sperm

Oogenesis

• Occurs in females, leads to formation of eggs

• Stages:

  1. Oogonium (2n) - diploid stem cell
  2. Primary oocyte (2n) - undergoes meiosis
  3. Meiosis I - produces secondary oocyte (n) and first polar body (n)
  4. Suspended in Metaphase II until fertilization
  5. Upon fertilization, completes meiosis II to form mature ovum (n) and second polar body (n)

• Polar Bodies: Degenerate and do not contribute to fertilization

Egg Structure and Layers

• Outer to Inner Coating of Egg:

  • Jelly Layer: provides protection in some species
  • Vitelline Envelope: outer layer from the extracellular matrix
  • Zona Pellucida: specific to mammals, prevents polyspermy

• Specialized Organelles:

  • Cortical Granules: release enzymes during fertilization

Fertilization Requirements

• Conditions for successful fertilization:
  1. Sperm and egg must meet at the same time
  2. Recognition and Binding: Sperm must bind to the egg
  3. Fusion: Sperm and egg membranes must fuse
  4. Trigger Development: Fusion activates egg's developmental processes

Sperm and Egg Interactions

• Acrosomal Reaction:

  • Hydrolytic enzymes from the sperm dissolve the jelly layer
  • Sperm-binding proteins are exposed and bind to the egg's plasma membrane

• Cortical Reaction:

  1. IP3 Release: Initiated upon membrane fusion
  2. Calcium (Ca2+) Wave: Stimulates changes leading to the hardening of the vitelline layer
  3. Exocytosis of Cortical Granules: Contents prevent further sperm from entering (blocks polyspermy)

Timeline of Events

• Spawned changes occur rapidly within seconds after sperm entry