Successive alteration and recovery of epidermal differentiation and morphogenesis after specific UVB-damages in skin reconstructed in vitro

The sequence of events affecting skin morphogenesis occurring after a single exposure to UVB was investigated on a model of human skin reconstructed in vitro. The biologically efficient dose (BED) able to induce the early UVB-DNA damages such as pyrimidine dimers, sunburn cells, and apoptotic keratinocytes was determined as 50 mJ/cm2. The subsequent changes induced during a period of 14 days following irradiation were analyzed. Up to Day 3, an epidermal disorganization led to a parakeratotic epidermis characterized by nucleated horny layers, as well as the down regulation of major markers of keratinocyte differentiation such as keratin 10, loricrin, filaggrin, and the keratinocyte transglutaminase (type I). On the contrary, the expression of involucrin and spr1 seemed to be unaffected, indicating distinct responses to UVB of proteins involved in keratinocyte differentiation. A progressive regeneration of normal epidermal morphogenesis begins from Day 4 leading to the normalization of keratinocyte differentiation at Day 10 to 14. In parallel, epidermal proliferation was increased. Taken together, these findings show that in skin reconstructed in vitro, UVB exposure leads to major epidermal developmental changes characterized by (i) an early apoptotic process, (ii) a subsequent down-regulation of specific keratinocyte differentiation markers, and (iii) the recovery of both the early and delayed effects resulting in normal epidermal morphogenesis.