The Organization of Human Epidermis: Functional Epidermal Units and Phi Proportionality

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 mm 2, 1999 melanocytes per mm2, 16 (SC) layers, 900-μm2 corneocyte surface area, 17,778 corneocytes per mm 2, 14-d (SC) turnover time, and 93,124 per mm 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/ 17,778 ∼ phi3; (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/phi2 × the number of living cells, 75,436 - (2/phi2 × 75,346) ∼17,778; and (4) if total epidermal turnover time equals (SC) turnover time X the 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/phi 2. 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.