Corneocyte changes after acute UV irradiation and chronic solar exposure.

The effects of UV irradiation on the human stratum corneum (SC) were studied under both artificial and natural conditions. Artificial UV exposure was performed with a solar simulator. Single doses of radiation from 1 to 3 MED were delivered on the backs of volunteers and the measurements were recorded for a period of one month. Corneocytes were harvested from the skin surface by using a "turbine-machine". Their count and projected area were measured by image analysis. The evolution of corneocyte counts showed 3 steps: 1) a background level of desquamation until the 8th day; 2) a sharp increase from day 9 to day 11; and 3) a plateau until the 20th day. No evolution of the corneocyte size occurred at 1 MED, but a decrease was recorded at day 20 for 2 MED and earlier for 3 MED. Natural sun exposure was investigated in 2 different experiments: on a population living in a sunny coastline and on professional racing cyclists. In these natural conditions, the response of the SC to UV led to an increase in corneocyte count associated with a decreased size. Even though natural environment involves both weather and sun influences, results showed similar SC responses. In case of acute irradiation, the skin recovers its initial state one month later, but in the natural environment, it maintains altered features as long as the UV irradiation occurs.