Lipid composition and molecular interactions change with depth in the avian stratum corneum to regulate cutaneous water loss.

The outermost 10-20 µm of the epidermis, the stratum corneum (SC), consists of flat, dead cells embedded in a matrix of intercellular lipids. These lipids regulate cutaneous water loss (CWL), which accounts for over half of total water loss in birds. However, the mechanisms by which lipids are able to regulate CWL and how these mechanisms change with depth in the SC are poorly understood. We used attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) to measure lipid-lipid and lipid-water interactions as a function of depth in the SC of house sparrows (Passer domesticus Linnaeus) in the winter and summer. We then compared these molecular interactions at each depth with lipid composition at the same depth. We found that in both groups, water content increased with depth in the SC, and likely contributed to greater numbers of gauche defects in lipids in deeper levels of the SC. In winter-caught birds, which had lower rates of CWL than summer-caught birds, water exhibited stronger hydrogen bonding in deeper layers of the SC, and these strong hydrogen bonds were associated with greater amounts of polar lipids such as ceramides and cerebrosides. Based on these data, we propose a model by which polar lipids in deep levels of the SC form strong hydrogen bonds with water molecules to increase the viscosity of water and slow the permeation of water through the SC.