Renewal of the Epidermis

The epidermis is continually renewed: keratinocytes form in the basal layer, differentiate, and move toward the surface, where dead corneocytes are gradually shed. A cycle of about 28 days is often quoted, but it is an approximate popular estimate, not a universal timer. Studies report different durations depending on which layer and stage they measure; one classic study found stratum-corneum transit of about 20 days in young adults and more than 30 days in older adults. Renewal varies with age, body site, and skin condition; diseases such as psoriasis can substantially alter the process.