Cutaneous water loss and sphingolipids covalently bound to corneocytes in the stratum corneum of house sparrows Passer domesticus.

The barrier to water loss from the skin of birds and mammals is localized in the stratum corneum (SC), the outer layer of the epidermis. The SC consists of corneocytes, each surrounded by a protein envelope, and a lipid compartment, formed by an extracellular matrix of lipids and by lipids covalently bound to the protein envelope. In mammals, covalently bound lipids in the SC consist of omega-hydroxyceramides attached to the outer surface of corneocytes. Evidence suggests that covalently bound lipids in the SC might be crucial for the establishment of a competent permeability barrier. In this study we assessed the composition of covalently bound lipids of the avian SC and their relationship to cutaneous water loss (CWL) in two populations of house sparrows, one living in the deserts of Saudi Arabia and the other in mesic Ohio. Previously, we showed that CWL of adult desert sparrows was 25% lower than that of mesic birds. In the present study we characterize covalently bound lipids of the SC using thin layer chromatography and high performance liquid chromatography coupled with atmospheric pressure Photospray ionization mass spectrometry. Our study is the first to demonstrate the existence of sphingolipids covalently bound to corneocytes in the SC of birds. Although omega-hydroxyceramides occurred in the lipid envelope surrounding corneocytes, the major constituent of the covalently bound lipid envelope in house sparrows was omega-hydroxycerebrosides, ceramides with a hexose molecule attached. Sparrows from Saudi Arabia had more covalently bound cerebrosides, fewer covalently bound ceramides and a lower ceramide to cerebroside ratio than sparrows living in Ohio; these differences were associated with CWL.