Plunge-cooling of tissue blocks: determinants of cooling rates

Tissue blocks have been plunged into a liquid coolant and the resultant ice-crystal damage is discussed. Some blocks have been frozen without apparent ice-crystal damage; they were not treated with cryo-protectants but were mounted on streamlined foil supports which separated the specimen from the thermal mass of the support pin. Hydrated gelatin specimens of various sizes have been built around thermocouples and plunged at different speeds. Results show that cooling is relatively slow in specimens mounted on solid metal supports. In specimens mounted on foil supports cooling is related to plunge velocity in 0.25 mm diameter specimens but in 1 mm diameter specimens cooling occurs mainly after coming to rest. In 0.5 mm diameter specimens cooling is related to plunge velocity at low immersion speeds but then it decreases with increasing plunge velocity; this trend should allow monitoring of attempts to improve the efficiency of the plunge-cooling technique.