Pronuclear DNA synthesis in mouse eggs: An autoradiographic study

Abstract

A total of 4738 eggs, collected at 2 h intervals, were obtained from mated CF1 (Carworth) female mice between 15 and 31 h following human chorionic gonadotropin (HCG) administration. Eggs pooled from several females were exposed to 3H-thymidine in vitro for 1 h. Pronuclear DNA synthesis began in some eggs as early as 21 h after HCG, approx. 5 h after pronuclear formation, and was completed in all zygotes by 31 h. The maximum number of eggs with labeled pronuclei occurred at 25 h. Some cumulus cells and second polar bodies were also labeled. Pronuclear incorporation of 3H-thymidine indicates DNA synthesis since no labeled pronuclei occurred following exposure to deoxyribonuclease, and labeled first cleavage metaphase chromosomes were obtained when pronuclear eggs were pulsed with 3H-thymidine during the S period. In each zygote the S period is about 4 h with DNA synthesis beginning and terminating slightly earlier in the male pronucleus. Cytoplasmic factors and not pronuclear volume per se are implicated in controlling the onset of pronuclear DNA synthesis. Pronuclear labeling was not affected by removal of cumulus cells or zona pellucida. When 3H-thymidine was injected into the Fallopian tube, the timing and frequency of labeled pronuclei corroborated the findings obtained in vitro.