🥚 Developmental Genetics: Fertilization & Polyspermy Blocks
Fertilization is the union of haploid male and female gametes to restore diploidy and trigger egg metabolic activation while strictly barring polyspermy.
1. Sequence of Events in Fertilization
- Chemoattraction & Capacitation: Sperm respond to soluble egg chemoattractants (e.g. resact peptide in sea urchins, progesterone in mammals). Capacitation in mammalian female reproductive tract removes cholesterol and inhibitory glycoproteins, exposing acrosomal receptors.
- Acrosome Reaction: Contact with the outer egg envelope (ZP3 glycoprotein in mammals; sulfated fucan in sea urchins) induces $Ca^{2+}$ influx and exocytosis of the acrosomal vesicle, releasing hyaluronidase and acrosin to digest the zona pellucida.
- Gamete Membrane Fusion: Mediated by Izumo1 on sperm and Juno/CD9 on oocyte plasma membrane.
2. Dual Blocks to Polyspermy
| Characteristic | Fast Block to Polyspermy | Slow Block to Polyspermy (Cortical Reaction) |
|---|---|---|
| Onset Time | Within 1–3 seconds of sperm fusion | 20–60 seconds after fusion |
| Biophysical Mechanism | Transient electrical depolarization of egg membrane ($V_m$ shifts from -70 mV to +20 mV via rapid $Na^+$ influx) | Intracellular $Ca^{2+}$ wave triggered by $IP_3$ releases ~15,000 cortical granules via exocytosis |
| Duration | Transient (~1 minute) | Permanent mechanical and biochemical shield |
| Molecular Action | Positively charged sperm cannot fuse with positively charged egg membrane | Cortical granule serine proteases (CGSP) cleave ZP3/bindin receptors; ovoperoxidase crosslinks vitelline envelope forming tough fertilization envelope; mucopolysaccharides cause osmotic water swelling |