The sensitivity of various stages of preimplantation mouse embryos to UV irradiation in vitro was determined by two criteria of development during in vitro culture after UV irradiation: 1. Development throughout the following 24 h, and 2. the ability to form a blastocyst. In these studies the UV sensitivity was the highest in 1-cell embryos and it decreased with development towards the morula stage as indicated by the ED50 values of the rate of blastulation following UV irradiation: 2-cell embryos <tex-math>$90\ {\rm erg}/{\rm mm}^{2}$</tex-math>, 4- and 8-cell embryos <tex-math>$100\ {\rm erg}/{\rm mm}^{2}$</tex-math>, and morulae <tex-math>$140\ {\rm erg}/{\rm mm}^{2}$</tex-math>. After UV irradiation the in vitro development of the embryos was further impaired by the presence of caffeine in the culture medium at concentration levels which did not interfere with the development of nonirradiated embryos. In 2-cell embryos caffeine treatment was only effective during the first 24 hr after irradiation. These results suggest that early mammalian embryos are able to repair UV-induced DNA lesions by the postreplication repair mechanism.

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