What Makes a Good Barrier? Adaptive Features of Vertebrate Integument

Barrier functions of skin have different meanings to different investigators, depending on their focus of investigation or subspecialty within skin biology. To the majority of investigators in dermatology and the field of transdermal drug delivery, it is synonymous with the permeability barrier, primarily located in the stratum corneum (SC). This paper-thin tissue, which in humans, ranges from 15 to 18 layers of corneocytes embedded in a matrix of hydrophobic lipids (a brick-and-mortar organization), is a composite material with unique functional properties (1). Investigators from multiple disciplines continue to explore this fascinating tissue, which permits the survival of almost all vertebrates on land and in water. Most studies contributing to our present day understanding of the formation and homeostasis of the permeability barrier come from studies on the epidermis of two mammalian species: laboratory mice and humans. However, extrapolating from these models to arrive at a global rationale for how this tissue allows for survival (i.e., when the barrier is good enough) is fraught with hazard. To begin a discussion on this topic, it is appropriate to consider the many different kinds of barriers that exist in the vertebrate integument: from physical barriers (impact resistance, barriers to ultraviolet radiation, ice crystal propagation), chemical barriers (antimicrobial, metabolic defenses, anti-oxidant), immune barriers (initiation of cytokine cascade, immunoglobulins, dendritic cell-mediated), barriers to biofowling (cetacean mammals), to psychological barriers to human relations (xenophobia based on skin color). Adaptations to specific ecological challenges and seasonal changes in habitats add another layer of complexity, where survival depends on the skin’s capacity for ‘‘facultative barrier upregulation,’’ documented in experimental acclimation studies on the lower vertebrates. An overview of the barrier functions of lower vertebrates, as well as homeotherms living under different environments, provides us with a broader understanding of the multifaceted barrier that we often overlook due to our primary focus on the permeability barrier.