The Structure and Permeability of Integument

SYNOPSIS. The skin is a heterogeneous, multimembrane system with multiple diffusion pathways and potentially numerous rate-limiting barriers for specific molecules that are exchanged with the environment. A broad survey of animals indicates that integumentary coverings are morphologically, biochemically and embryologically diverse but with common themes of adaptation. The evolutionary proliferation of intercellular junctions, fibrous or mineralized protective barriers, and lipoid waterproofing barriers emphasize the generality of diffusion limitations. However, the regulation of specific exchange processes such as the flux rates of respiratory gases entails a complex interaction of multiple factors affecting both diffusion and perfusion limitations. Consideration of specific pathways and rate limitations for diffusion of various substances suggests that the permeation of skin by specific molecules can be partially independent of other exchange processes. Our understanding of regulated permeability is, however, lacking in mechanistic and integrated analysis in most cases. Comprehensive understanding of the integument as a regulatory pathway of communication with the environment will require comparative studies of specific transport pathways, their rate-limiting resistances, and the interactions as well as individual regulation of transported molecules.