Avian sebokeratocytes and marine mammal lipokeratinocytes: Structural, lipid biochemical, and functional considerations

Abstract

In terrestrial mammals, stratum corneum lipids derive from two sources: deposition of lamellar body lipids in stratum corneum interstices and excretion of sebaceous lipids onto the skin surface, resulting: in a two‐compartment (bricks and mortar) system of lipid‐depleted cells surrounded by lipid‐enriched intercellular spaces. In contrast, intracellular lipid droplets, normally not present in the epidermis of terrestrial mammals, are prominent in avian and marine mammal epidermis (cetaceans, manatees). We compared the transepidermal water loss, ultrastructure, and lipid biochemistry of the viable epidermis and tratum corneum of pigeon apterium, fledgling (featherless) zebra finches, painted storks, cetaceans, and manatees to those of humans and mice. Marine mammals possess an even more extensive lamellar‐body secretory system than do terrestrial mammals; and lamellar‐body contents, as in terrestrials, are secreted into the stratum corneum interstices. In cetaceans, however, glycolipicls, but not ceramides, persist into the stratum corneum; whereas in manatees, glycolipids are replaced by ceramides, as in terrestrial mammals. Acylglucosylceramides, thought to be critical for lamellarbody deposition and barrier function in terrestrial mammals, are present in manatees but virtually absent in cetaceans, a finding that indicates that they are not obligate constituents of lamellar‐body‐derived membrane structures. Moreover, cetaceans do not elaborate the very long‐chain, saturated N‐acyl fatty acids that abound in terrestrial mammalian acylglucosylceramides. Furthermore, cold‐water marine mammals generate large, intracellular neutral lipid droplets not found in terrestrial and warm‐water marine mammals; these lipid droplets persist into the stratum corneum, suggesting thermogenesis, flotation, and/or cryoprotectant functions. Avians generate distinctive multigranular bodies that may be secreted into the intercellular spaces under xerotic conditions, as in zebra fledglings; ordinarily, however, the internal lamellae and limiting membranes deteriorate, generating intracellular neutral lipid droplets. The sphingolipid composition of avian stratum corneum is intermediate between terrestrials and cetaceans (= 50% glycolipids), with triglycerides present in abundance. In the midstratum corneum of avians, neutral lipid droplets are released into the interstices, forming a large extracellular, lipid‐enriched compartment, surrounding wafer‐thin corneocytes, with a paucity of both iipid and keratin (“platesand‐platesand‐mortar” rather than the “bricks‐and‐mortar” of mammals). These studies demonstrate remarkable structural and biochemical heterogeneity of epidermal lipids in various homeotherms and provide new insights into the functions of specific lipid‐enriched organelles in these organisms.