Skin Sciences Institute, Children's Hospital Research Foundation, University of Cincinnati, Cincinnati, Ohio, USA

Correspondence: Steven B. Hoath, MD, Skin Sciences Institute, 231 Sabin Way, Cincinnati, OH 45267-0541. Email: hoathsb@uc.edu

Received 17 February 2003; Revised 6 May 2003; Accepted 24 June 2003; Published online 8 December 2003.

Abstract

The concept that mammalian epidermis is structurally organized into functional epidermal units has been proposed on the basis of stratum corneum (SC) architecture, proliferation kinetics, melanocyte:keratinocyte ratios (1:36), and, more recently, Langerhans cell: epidermal cell ratios (1:53). This article examines the concept of functional epidermal units in human skin in which the maintenance of phi (1.618034) proportionality provides a central organizing principle. The following empirical measurements were used: 75,346 nucleated epidermal cells per mm², 1394 Langerhans cells per mm², 1999 melanocytes per mm², 16 (SC) layers, 900-m² corneocyte surface area, 17,778 corneocytes per mm², 14-d (SC) turnover time, and 93,124 per mm² total epidermal cells. Given these empirical data: (1) the number of corneocytes is a mean proportional between the sum of the Langerhans cell + melanocyte populations and the number of epidermal cells, 3393/17,778–17,778/93,124; (2) the ratio of nucleated epidermal cells over corneocytes is phi proportional, 75,346/17,778phi³; (3) assuming similar 14-d turnover times for the (SC) and Malpighian epidermis, the number of corneocytes results from subtraction of a cellular fraction equal to 2/phi²the number of living cells, 75,436 – (2/phi²75,346) 17,778; and (4) if total epidermal turnover time equals (SC) turnover timethe ratio of living/dead cells, then compartmental turnover times are unequal (14 d for (SC) to 45.3 d for nucleated epidermis 1/2phi) and cellular replacement rates are 52.9 corneocytes/69.3 keratinocytes per mm² per h 2/phi². These empirically derived equivalences provide logicomathematical support for the presence of functional epidermal units in human skin. Validation of a phi proportional unit architecture in human epidermis will be important for tissue engineering of skin and the design of instruments for skin measurement.