Thermal analysis of stratum corneum of hairless mouse with attention to phase transitions near 35 °C

The high-resolution differential scanning calorimetric measurement, in which the baseline was stable, was performed in the stratum corneum of a hairless mouse with the water content of 25 wt%. Near 35 °C two successive phase transitions were found at 32 and 39 °C. The latter is consistent with the transition from the orthorhombic to the hexagonal structure in a region of the intercellular lipid matrix as obtained from the former X-ray diffraction and also the former electron diffraction. To the contrary so far the phase behavior of a region having the hexagonal structure at room temperature was unknown. Based upon the present differential scanning calorimetric measurement, we suggest that this region in the intercellular lipid matrix undergoes a transition from the hexagonal to the liquid structure at 32 °C. The reason why the structural phase transition at 32 °C has not been detected easily might be due to the fact that it lies close to the transition at 39 °C and furthermore the characteristic of the X-ray or the electron diffraction in which the diffraction peak in the liquid structure is broad and frequently undetectable near 35 °C. Finally, we propose that the two domains having the orthorhombic and the low-temperature hexagonal structures at room temperature undergo transitions to the high-temperature hexagonal structure at 39 °C and to the liquid structure at 35 °C, respectively. Besides the above evidence it was firstly obtained that the total transition enthalpy due to the intercellular lipid matrix is 13 J g−1 in the stratum corneum of a hairless mouse.