Fertilization Processes in Mammals

Fertilization

• Occurs in the oviduct of mammals
• Ova remains viable for 12-24 hours after ovulation
• Sperm remains viable for up to 72 hours inside the female reproductive tract

Mechanisms of Fertilization

• Sperm undergoes a process called capacitation to become competent to fertilize the egg
• Capacitation involves changes to sperm motility and structure occurring in the female reproductive tract
• Sperm undergoes a process called the acrosome reaction to penetrate the zona pellucida surrounding the egg

Capacitation

• Occurs within the female reproductive tract
• Involves interactions between sperm and mucosal surfaces
• Removal of glycoprotein coat and seminal plasma proteins from the sperm plasma membrane
• Capacitated sperm exhibit increased motility and force
• Capacitated sperm possess hyaluronidase enzymes to penetrate the extracellular matrix of cumulus cells
• Sperm exhibit the ability to sense temperature differences and preferentially swim towards warmer sites
• Capacitated sperm sense a thermal gradient of 2 degrees Celsius
• Sperm also sense molecules secreted by the oocyte and cumulus cells, such as progesterone
• Progesterone binds to receptors on the sperm tail, activating calcium channels and increasing sperm activity

Acrosome Reaction

• Triggered after sperm binds to the zona pellucida (ZP)
• Release of enzymes such as acrosin and trypsin-like substances from the acrosome
• Occurs in three phases:
  • Penetration of the corona radiata
  • Penetration of the zona pellucida
  • Fusion of the oocyte and sperm membranes

Phases of Fertilization

• Phase 1:
  • Millions of sperm reach the fertilization site
  • Capacitated sperm pass through corona cells
• Phase 2:
  • The zona pellucida (ZP) is a glycoprotein shell facilitating sperm binding and inducing the acrosome reaction
  • Binding and the acrosome reaction involve interaction with ZP3
• Phase 3:
  • Release of acrosin allows sperm to penetrate the zona pellucida, establishing contact with the oocyte membrane
  • Changes in zona pellucida permeability occur when the sperm head contacts the oocyte surface
  • Release of lysosomal enzymes from cortical granules lining the oocyte plasma membrane (zona reaction)
  • Only one sperm typically penetrates the egg, despite other sperm being embedded in the zona pellucida

Gamete Fusion

• Initial adhesion of sperm to the oocyte involves interaction between integrins on the oocyte and their ligands, disintegrins, on the sperm
• Fusion of the sperm and oocyte plasma membranes occurs, forming a fertilization cone
• The plasma membrane covering the acrosomal head cap disappears during the acrosome reaction
• Fusion of the acrosomal head cap and the oocyte plasma membrane leads to the formation of a fertilization cone

Polyspermy Prevention

• Polyspermy, the fertilization of an egg by multiple sperm, is prevented by the fast and slow blocks:
  • Fast block: Changes in the electric potential of the egg plasma membrane
  • Slow block:
    • Cortical granule reaction, where enzymes are released to modify the zona pellucida, preventing further sperm binding
    • Zona reaction involves the inactivation of species-specific receptor sites on the zona pellucida
    • The Juno protein plays a role in slow block by preventing sperm fusion with the oocyte membrane

Fusion of Genetic Material

• The male pronucleus enlarges while the oocyte completes its second meiotic division
• Both pronuclei lose their nuclear envelopes and become indistinguishable
• The centrosome from the male pronucleus produces asters and contacts the female pronucleus
• Each pronucleus migrates toward the other, replicating its DNA during the migration
• The nuclear envelopes break down
• Glutathione in the egg reduces disulfide bonds and uncoils sperm chromatin
• The sperm contributes the pronucleus, mitochondria, and centriole
• Mitochondria from the sperm are degraded in the egg cytoplasm
• Chromatin condenses into chromosomes which orient themselves on a common mitotic spindle

Calcium Role in Embryo Development

• Calcium release activates metabolic reactions involved in respiration and protein synthesis
• Increased respiration and protein synthesis contribute to embryo development

Summary

• The male nucleus enters the egg cytoplasm and becomes the male pronucleus
• The fusion of the sperm and egg triggers the oocyte nucleus to complete its second meiotic division, forming a polar body