The human epidermis, far from being a static organ, renews itself constantly. Until recently, the replacement time remained in doubt, with estimates ranging from 7 to 258 days. In 1961 investigators measured the appearance of radioactivity in stratum corneum after intravenous infusion of aminoacetic acid (glycine) labeled with radioactive carbon (14C) and estimated a 13- to 14day renewal time for the stratum corneum and 12 to 15 days for the viable epidermis (basal, prickle, and granular layers).1 A communication in the October issue of the Archives of Dermatology confirms that at least a portion of the human epidermis renews itself every two to three weeks.2 The investigators injected thymidine labeled with radioactive hydrogen (3H) intradermally in 21 volunteers and secured biopsies sequentially for six weeks.
Examination of autoradiograms revealed that 5% of germinative cells incorporated the labeled substance into DNA within 40 minutes of injection. Calculations