Fertilization Overview
- External Fertilization
- Gamete Recognition: Female gametes release attractants in water, guiding male gametes via species-specific positive chemotaxis.
- Stages of Fertilization
- Contact: Sperm meets the egg's jelly layer containing key cellular components.
- Acrosomal Reaction:
- Enzymes from the sperm’s acrosome digest the jelly layer.
- Binding to Vitelline Envelope:
- Sperm binds specifically to the egg’s outer envelope.
- Polyspermy Prevention:
- Fast Block: Changes the egg's membrane potential.
- Slow Block: Initiates cortical granule reaction.
- Fusion: The acrosomal process and egg membrane merge, completing fertilization.
- Acrosomal Reaction in Sea Urchin Sperm
- Extension: Acrosomal process extends via microfilaments.
- Binding and Fusion: Acrosomal membrane binds to vitelline envelope receptors and fuses with the egg’s membrane.
Internal Fertilization in Mammals
- Overview
- Location: Occurs in the ampulla of the fallopian tube.
- Duration: From the initial sperm-egg contact to the first division of the zygote, typically within 24 hours.
- Stages of Fertilization
- Contact and Initial Reaction:
- Sperm navigates through the cumulus cell layer to reach the zona pellucida (ZP).
- Contact with ZP triggers the acrosomal reaction:
- Enzyme Release: Hyaluronidase breaks down connections between cumulus cells, clearing the path. Acrosin helps penetrate the ZP to reach the oocyte membrane.
- P Penetration:
- Sperm’s flagellating tail provides constant force to breach the ZP.
- Membrane Fusion and Egg Activation:
- Sperm and egg membranes fuse.
- Egg Activation: Involves metabolic and physical changes.
- Calcium Rise: Increases intracellular Ca2+ concentration.
- Meiotic Completion: Finishes the second meiotic division.
- Cortical Reaction:
- Massive release of enzymes from cortical granules into the ZP.
- Enzymes modify ZP3 to prevent other sperm from binding and harden ZP2 to enhance ZP's structure, preventing polyspermy.
- Post-Fertilization Developments
- Pronuclei Formation: Male and female pronuclei, each containing a haploid set of chromosomes, form and move towards each other.
- Cell Division: The first mitotic spindle arranges, dividing the zygote into two blastomeres.