The researchers compared the amount of cryoprotectant inside mouse zygotes after vitrification and slow freezing. Surprisingly, they found that vitrified zygotes had lower cryoprotectant levels than those frozen slowly, even though vitrification solutions had higher cryoprotectant concentrations. This suggests that vitrification may be more efficient at preventing ice crystal formation inside cells. The study involved exposing mouse zygotes to different cryoprotectant solutions and comparing their cryoprotectant levels and survival rates. The results showed that vitrified zygotes had an intracellular cryoprotectant concentration of around 2.14 M before being frozen in liquid nitrogen.