ABSTRACT
In mammals a specialized squamous epithelium, skin, comprises the outermost tissue and functions as the initial site of interaction with the environment. This continually renewing organ forms a protective yet plastic barrier, relaying infor mation from, and responding to, stimuli from the environment. (1,2; for review see 3,4) . The skin employs an array of strategies to defend the organism from damage. Tissue defense mechanisms range from remodeling its architecture in response to external stimuli, which occurs during chemical or ultraviolet light - induced hyperplasia, to reversibly establishing an inflammatory microenviron ment as a defense from microbial invasion (1,2,5,6). Results from numerous re cent studies indicate that nitric oxide plays critical roles in most, if not all, dermal responses to stimuli from the external environment (5-9). Mammalian skin con sists of diverse cell types in the outermost layer of the skin, the epidermis; keratinocytes comprise the predominant cell type populating the tissue (1,2). This review will discuss nitric oxide production by keratinocytes, focusing largely on the biological functions of nitric oxide in the skin. Unlike the majority of im mune and neural cells, epithelial cells express all three isoforms of nitric oxide synthase found in mammals (10-12). In this review we will also address the physiological roles unique to each isoform and the potential significance of the
expression of multiple nitric oxide synthase isoforms in keratinocytes in terms of tissue functioning.
