Organization of lipids in avian stratum corneum: Changes with temperature and hydration

In response to increases in ambient temperature (Ta), many animals increase total evaporative water loss (TEWL) through their skin and respiratory passages to maintain a constant body temperature, a response that compromises water balance. In birds, cutaneous water loss (CWL) accounts for approximately 65% of TEWL at thermoneutral temperatures. Although the proportion of TEWL accounted for by CWL decreases to only 25% at high Ta, the magnitude of CWL still increases, suggesting changes in the barrier function of the skin. The stratum corneum (SC) is composed of flat, dead cells called corneocytes embedded in a matrix of lipids, many of which arrange in layers called lamellae. The classes of lipids that comprise these lamellae, and their attendant physical properties, determine the rate of CWL. We measured CWL at 25, 30, 35, and 40 °C in House Sparrows (Passer domesticus) caught in the winter and summer, and in sparrows acclimated to warm and cold lab environments. We then used Fourier transform infrared spectroscopy to measure lipid-lipid and lipid-water interactions in the SC under different conditions of temperature and hydration, and correlated these results with lipid classes in the SC. As CWL increased at higher temperatures, the amount of gauche defects in lipid alkyl chains increased, indicating that lipid disorder is partially responsible for higher CWL at high temperatures. However, variation in CWL between groups could not be explained by the amount of gauche defects, and this remaining variation may be attributed to greater amounts of cerebrosides in birds with low CWL, as the sugar moieties of cerebrosides lie outside lipid lamellae and form strong hydrogen bonds with water molecules.