ULTRASTRUCTURAL ORGANIZATION OF AVIAN STRATUM-CORNEUM LIPIDS AS THE BASIS FOR FACULTATIVE CUTANEOUS WATERPROOFING

The ultrastructure of naked neck epidermis from the ostrich (Struthio camelus) and ventral apterium from watered, and water-deprived, Zebra finches (Taeniopygia [Poephila] guttata castanotis) is presented. The form and distribution of the fully differentiated products of the lipi d-enriched multigranular bodies are compared in biopsies post-fixed wi th osmium tetroxide or ruthenium tetroxide. The fine structure of ostr ich epidermis suggests it is a relatively poor barrier to cutaneous wa ter loss (CWL). The fine structure from watered, and 16-hr water-depri ved Zebra finches, considered in conjunction with measurements of CWL, confirms previous reports of ''facultative waterproofing,'' and empha sizes the rapidity of tissue response to dehydration. The seemingly co unterintuitive facts that one xerophilic avian species, the ostrich, l acks a ''good barrier'' to CWL, whereas another, the Zebra finch, is c apable of forming a good barrier, but does not always express this cap ability, are discussed. An explanation of these data in comparison to mammals centers on the dual roles of the integument of homeotherms in thermoregulation and conserving body water. It is concluded that birds , whose homeothermic control depends so much on CWL, cannot possess a permanent ''good barrier,'' as such would compromise the heat loss mec hanism. Facultative waterproofing (also documented in lizards) protect s the organism against sudden reductions in water availability. In bir ds, and probably in snakes and lizards, facultative waterproofing invo lves qualititative changes in epidermal cell differentiation. Possible control mechanisms are discussed. (C) 1995 Wiley-Liss, Inc.