Activation of Eggs by Hypothermia in Rats and Hamsters

Abstract

Hamster eggs appear unique among known mammalian eggs in that nearly 80% were found to undergo spontaneous activation, extruding the second polar body, within 24 hr. of ovulation. Often the eggs showed spontaneous early parthenogenesis-- the development of one or occasionally two nuclei and the formation of a haploid first cleavage spindle took place in a manner that closely resembled the corresponding stages in fertilization. Time relations, too, were much the same. Rarely, however, does the cleavage spindle pass into mitosis, but instead it breaks up and the chromosomes become scattered. Only one normal-looking 2-cell egg was found. Rat and hamster eggs were very resistant to activation by hypothermia while still in the ovary. After ovulation, however, 100% of rat eggs were activated by body temperatures between 0 and 1° C. and about 60-70% of hamster eggs by temperatures between 0 and 16° C. In view of the high incidence of spontaneous activation in hamster eggs, the effect of chilling in this animal can be regarded as merely that of hastening the change. Hypoxia alone activated 36% of rat eggs. Relative tissue anoxia is therefore probably the immediate effective agent in activation by anaesthetics and is partly responsible for activation by low temperatures. Cold-shock itself, however, is evidently a more potent activator. Hamster eggs were not activated by hypoxia, probably because a less severe tissue anoxia was induced. Rat eggs activated by hypothermia showed less tendency to early parthenogenesis than those activated by local chilling of the Fallopian tube. Nucleus and spindle formation was rather better in artificially than in spontaneously activated hamster eggs, but the prospects of extensive parthenogenetic development did not seem in any way improved. Only two 2-cell eggs resembling fertilized eggs were seen. Nuclei in activated hamster eggs achieved a volume similar to that of male or female pronuclei at full development. Total nucleolar volume was equal to the sum of the nucleolar volumes of male and female pronuclei.