Basic structural and functional characteristics of the epidermal barrier in wild mammals living in different habitats and climates

Based on the combination of standard light and transmission electron microscopy, cryo-SEM, immunohistochemistry and a new sensitive glycolipid histochemical technique (5-hexadecanoylaminofluorescein staining, laser scanning microscopy), including densitometrical evaluation, our approach gives for the first time an overview of the specific biology of the epidermal permeability barrier in wild mammals (20 species from five orders), living under varying (aquatic or moist to dry) habitat conditions. The results obtained emphasised that the barrier region in most of the species studied is a continuous zone (thickness, 0.1 and 3 μm) between the upper cells of the stratum granulosum and the inner cells of the stratum corneum conjunctum, normally present as a homogeneous glycolipid layer originating from fusion of lamellar body contents after exocytotic activities of the granular cells. However, this finding did not apply to all of the species studied, i.e., the Wild boar, the Common seal and the three large species with a very thick vital epidermis, the African elephant, the hippopotamus and the common dolphin, exhibited variations from the basic scheme. Densitometric evaluation of the 5-hexadecanoylaminofluorescein staining revealed that reaction intensity was not only generally related to the habitat conditions but also to vital epidermis thickness and hair density. The immunohistochemical demonstration of Na⁺/H⁺ exchanger 1 corroborated for all wild mammals studied that this important regulator of pH conditions during barrier formation is continuously produced in the epidermis. The variations in barrier biology observed for some species obviously had to be developed in relation to animal size (or body size area) and hair coat density, but, particularly, by the specific adaptation of certain mammalian groups to the aquatic environment. In the latter case, the typical barrier zone system was lost, as in the hippopotamus or the cetaceans.