The human epidermis

摘要

The human skin represents the largest organ in the human body with an average surface area of 1.8 m~2, where the epidermis is the outermost living layer. Keratinocytes and raelanocytes, in the ratio 36:1, form the epidermal unit. A symbiotic relationship exists between these cells with regard to the processes of signal transduction, metabolism, cell growth, differentiation and defence against damaging oxygen radicals (ROS). Recent results suggest that the epidermis has a metabolic similarity to neuronal tissue, with the capacity to synthesize as well as degrade catecholamines (Schallreuter et al. 1992,1994d). In association with catecholamine synthesis, there is expression of a high density of beta-2-adrenoceptors on keratinocytes and alpha-adrenoceptors on melanocytes (Steinkraus etal. 1991, 1992; Schallreuter et al. 1993). Since undifferentiated keratinocytes (i.e. basal cells) provide the major supply of catecholamines to the total epidermis, it seems feasible that those basal cells in contact with melanocytes are of major importance in signal transduction, metabolism, growth and differentiation. Any local trauma to the epidermis by infection or physical damage can attract infiltrating lymphocytes to produce the biological 'oxygen-burst' leading to a local release of superoxide anion radicals (O_2~-) at high concentration via the activity of NADPH-oxidase (EC 1.6.2.2). In addition, radicals can be produced by u.v. light and other environmental factors. Consequently, the epidermis invests a significant amount of metabolic energy in neutralizing ROS such as Or, H_2O_2 and OH centre dot (Schallreuter & Wood, 1989).