Biology of Reproduction, Vol 17, 527-531, Copyright © 1977 by Society for the Study of Reproduction

Effects of Cooling Rates and Maturity of the Animal on the Recovery and Fertilization of Frozen-Thawed Rodent Eggs

¹ Worcester Foundation for Experimental Biology, Shrewsbury, Massachusetts 01545

Mouse, rat, golden hamster and Mongolian gerbil eggs were cooled to -75°C in efforts to determine an optimum cooling rate for freezing unfertilized rodent eqs. The best cooling rate for the recovery and in vitro fertilization of mouse and rat eggs was 0.33°C/mm from -4° to -45°C followed by 1°C/min from -45° to -75°C. Hamster eggs could withstand a wider variation in the rate of cooling, since large numbers of eggs could be recovered at 0.33°, 0.5° or 1° C/min; and fertilized in vitro. In contrast with the other species gerbil eggs froze poorly at these same rates of cooling. Eggs from mature mice and rats consistently withstood freezing better than those from immature females (P<0.001) when cooled at 0.33° C/min.

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ACKNOWLEDGMENTS We would like to thank Dr. A. F. Parlow and the NIAMDD Rat Pituitary Hormone Distribution Program for providing us with PMSG. This research was supported by a postdoctoral fellowship ( No. 1 F22 HD00716) from NICHD to T. A. P. and a Ford Foundation grant.