Avian epidermal lipids: Functional considerations and relationship to feathering

The avian epidermis is composed of unique sebokeratinocytes that elaborate and secrete sebum-like lipids as they cornify. In addition to the lipid dro plets, the avian epidermis elaborates, but rarely secretes, lipid-enriched organelles, the multigranular bodies. The multigranular bodies are analogou s to the lamellar bodies of mammals (Menon et at, 1991), the secretion of w hich results in formation of occlusive lipid bilayers characteristic of mam malian stratum corneum and providing the permeability barrier. However, in contrast to mammals, the avian multigranular bodies form the reserve barrie r mechanism. In the basal state, when multigranular bodies are not secreted , the avian cutaneous barrier is deficient, but allows evaporative cooling for thermoregulation. However, under conditions of water deficit, multigran ular body secretion allows for rapid facultative waterproofing, as shown in zebra finches (Taenyopygia guttata), In certain glabrous regions of the sk in, such as the maxillary rictus, interdigital web, and combs and wattles i n the domestic fowl, there is a high degree of epidermal lipid secretion. A lso specialized feathers such as powder downs elaborate lipid rich material , which can be classified as secretion. Additionally, an inverse relation b etween epidermal lipogenesis and the degree of feathering has been demonstr ated, as in temporarily bare areas (e.g., brood patches) and following perm anent feather loss from the head accompanying attainment of maturity in cer tain ibises and storks. In the latter, the neo-apteria often hold large res erves of carotenoids dissolved in the lipid droplets, possibly related to a n altered gradient of retinoids influencing feather morphogenesis. Unusual secondary functions of epidermal lipids include cosmetic coloration (e.g., in the Japanese Crested mis) and chemical defense (e.g., in the Pitohui).