Survival of electrical activity of deep frozen fetal mouse hearts after microwave thawing

Hearts removed from 17–19 day fetal mice were frozen in liquid nitrogen and tested for electrical activity after rewarming. After exposure to various cryoprotective agents, hearts were cooled at 0.5–0.7 °C/min. to ?100 °C and then stored in liquid nitrogen for periods between 72 and 216 hr. Exposure to controlled microwaves at 2450 MHz or immersion in a water bath at 25 C was used in thawing. Electrical activity was studied for periods as long as 90 days after subcutaneous implantation into the ear of syngeneic adult mice. Overall, 59% of 54 frozen-thawed fetal hearts showed strong electrical activity after 30 days when the cryoprotective solution that had been used contained 10% ($\text{v}\text{v}$) dimethylsulfoxide (DMSO) and 10% ($\text{v}\text{v}$) fetal calf serum in Hepes buffer. This system consists of a multicellular structure that is nourished by diffusion; it is well suited for the evaluation of different cryoprotective agents and for various thawing techniques.