The wood frog (Lithobates sylvaticus) achieves its miraculous winter survival through an extraordinary physiological feat: hyper-concentrated glucose synthesis. As ambient temperatures drop below freezing, ice formation begins on the frog's skin. Specialized ice-nucleating proteins in the blood trigger controlled ice crystallization in extracellular spaces.
Sensing this ice formation, the frog's liver immediately undergoes rapid glycogenolysis, converting stored liver glycogen into massive quantities of glucose. This glucose floods the bloodstream and enters intracellular spaces, reaching concentrations up to several hundred millimolar—over 100 times higher than normal physiological levels. Inside the cells, glucose acts as a cryoprotectant by drastically lowering the freezing point of cytoplasm and increasing intracellular osmolality.
This high osmolality prevents water from exiting cells through osmosis into the freezing extracellular matrix, thereby preserving cell volume and preventing catastrophic cellular dehydration. Furthermore, the thick, syrupy glucose solution undergoes vitrification, transitioning into a glass-like solid state rather than forming sharp, cell-piercing ice crystals. During this period of suspended animation, the frog experiences total cardiac arrest, cessation of respiration, and zero measurable metabolic activity.
When ambient temperatures rise in spring, the ice thaws from the inside out, the liver metabolizes the excess glucose back into glycogen or energy, and heart function resumes spontaneously without reperfusion injury.