ABSTRACT
In an unusual scenario where religion imitates evolution, the various incarnations of the Hindu God represent sequentially the higher forms of vertebratesbeginning with the fish, a turtle, a beast-man, and then human formsevolutionary steps most obvious in the morphological appearance and specializations of the integument. Notwithstanding the ideal separation of church from the “state of science,” one could venture to say that all skins may not be created equal, but all have evolved to suit perfectly the environmental needs of specialized habitats. Today when Homo sapiens are “creating” skin or skin equivalents via tissue engineering for use in medical and research applications, appreciation of an evolutionary and environmental perspective of the primary, secondary, and even tertiary barrier functions of skin could be extremely useful. As stated ad nauseam, the skin is the largest and perhaps the most complex organ system, and its multitudinous true functions continue to stimulate discussions in interdisciplinary scientific forums [1]. The primary function of skin, as commonly agreed, is protection: that is, it functions as the body’s primary defense-a barrier to water loss (preventing death by desiccation) or water influx. Depending on the challenges of the habitat, the barrier function also extends to ice crystal propagation, ultraviolet (UV) radiation, xenobiotics, microbial pathogens, oxidants, biofouling, defense from predators by camouflage or chemical secretions, and so on, as will be discussed in sections to follow. A good measure of the skin’s barrier properties, at least in the terrestrial species, is transepidermal water loss (TEWL). Most factors that affect the barrier properties (such as mechanical stripping of the outermost layers, selective removal of lipids, or acute UV insult) affect TEWL, as well as setting in motion a process of barrier repair or barrier homeostasis. The integumentary response to barrier disruption (or challenge) tends to holistic nature: not selectively upregulating a specific defense, but rather fortifying the entire gamut of barriers. This is manifested when tape stripping of stratum corneum not only leads to restoration of the permeability barrier, but also stimulates melanogenesis (a barrier against UV) and often hyperpigmentation in darker phototypes. The relationship between UVand human skin pigmentation is also considered to be an adaptation that protects sweat glands from UV-induced injury, ensuring the integrity of
somatic thermoregulation, as well as preventing folate deficiency and hence birth defects [2]—a function that is of obvious value to the species.
