The early effects of ultraviolet light (UV) irradiation on deoxyribonucleic acid (DNA) synthesis in human skin were studied in vivo. Volunteers exposed to 3 minimal erythema doses (MED) (8.2-40.8 × 106 ergs/sq cm) were injected intradermally with tritiated thymidine (TdR-H3), immediately, 15 minutes, 3, 5, and 24 hours afterward, and biopsies processed for light microscopy autoradiography. Comparison of the results with findings in unirradiated human skin indicated: (1) a depression in the number of germinative basal cells synthesizing DNA prior to division at three and five hours after UV, as seen in other systems; (2) a population of sparsely labeled cells not only in the basal layer but also in the malpighian and granular layers and not seen in unirradiated skin or after several other types of acute injury. This aberrant type of TdR-3H incorporation has been seen in other systems and is thought to represent dark reactivation repair of irradiated DNA as occurs in microorganisms.
Epstein WL, Fukuyama K, Epstein JH. Early Effects of Ultraviolet Light on DNA Synthesis in Human Skin in Vivo. Arch Dermatol. 1969;100(1):84-89. doi:10.1001/archderm.1969.01610250090021