Cryopreservation and Thawing of Gametes and Embryos
Core concepts
Cryopreservation uses cryoprotectants (permeating and non-permeating), controlled cooling, and either slow-freezing or vitrification. Vitrification relies on high cryoprotectant concentrations and rapid cooling to avoid ice crystal formation. Thawing reverses the process with stepwise removal of cryoprotectant to limit osmotic shock.
Key variables
Cooling and warming rates, cryoprotectant concentration and exposure time, carrier device, and seeding for slow-freezing. Understand the difference between intracellular ice (damaging) and extracellular ice (manageable with seeding).
Traps
- Confusing vitrification (no ice) with slow-freezing (controlled ice formation).
- Assuming higher cryoprotectant is always safer; toxicity rises with concentration and exposure time.
- Forgetting that warming rate matters as much as cooling rate.
- Overlooking osmotic shock during cryoprotectant removal.
Worked example
If post-thaw survival is poor despite correct cooling rates, check warming rate and cryoprotectant removal steps. Rapid warming plus stepwise dilution usually improves survival.