Thermal transitions, stratum corneum

Wilkes et al (22.23) coupled calorimetric, dynamic-mechanical and x-ray diffraction techniques to demonstrate crystallization of the lipids was completely reversible in neonatal rat stratum corneum, and only partially reversible in human stratum corneum. Melting regions near 40°C and from 70 to 90°C corresponded to the thermal transitions noted in the calorimetric studies for both species. The crystalline nature of the lipids did not appear to be dependent on the presence of water. X-ray diffraction and infrared spectroscopy studies (23.28-34) have also shown a to p conformational changes occurred in keratin and stratum corneum protein components with hydration or exposure to increased temperatures. Oertel (28) has reported pretreatment with dimethylsulfox-ide, hexylmethylsulfoxide and decylmethylsulfoxide resulted in the formation of p-sheet protein conformations in vitro in human... 

Calorimetric results for the hairless mouse stratum corneum and isolated components confirmed thermal transitions associated with increased mobility occurred in the temperature region where large increases in permeability of lipophilic components also occurred. Thermal transitions for the extracted lipids were in the 27 to 67°C range, while the protein residue sheets (primarily... 

Thermally induced permeability enhancement of the more lipophilic solutes (butanol, octanol and hydrocortisone) through hairless mouse stratum corneum occurred in the temperature range also associated with lipid transitions in the calorimetry studies. Therefore, it seems likely enhanced permeabilities and lipid mobility within the stratum corneum are correlated. However, these macroscopic studies are unable to provide more specific information concerning the molecular origins of the thermal transitions. The studies provide even less information concerning possible irreversible alterations of the keratinized protein components of the stratum corneum. 

Differential scanning calorimetric and infrared spectroscopic investigations of intact stratum corneum, extracted lipids and keratinized protein residue sheets suggested the thermal transitions occurring within the 30 to 70°C region were associated with increased molecular mobility of the lipids. The permeability coefficients of lipophilic molecules through hairless mouse skin increased by several orders of magnitude over the same temperature... 

Recently Al-Saiden et al [181] examined the thermal transitions in desiccated stratum comeum membranes of neonatal rats, rabbits and adult abdominal human skin. Four endothermic transitions were observed at 39-45°C, 55-58°C, 68-74°C and 77-86°C. The transitions at 39-45°, 68-74° and 77-86°C were attributed to phase charges in the intercellular lipid bilayers. A fifth transition at 48° was seen only in the neonatal rabbit stratum corneum and was attributed to lipid melting. 

Numerous studies have attempted to elucidate the effect of skin penetrants by thermal analysis. Kaplun-Frischoff and Touitou [182] showed that methanol decreased the melting peaks of cholesteryl oleate and ceramides and modified the DSC of isolated stratum comeum this alters its barrier properties. Similarly a soybean-lecithin micro-emulsion gel was shown to affect the stratum comeum lipid organisation by FTIR and DSC [186] and hydroxypropyl-P-cyclodextrin can increase the permeability of the stratum corneum, possibly as a result of lipid extraction that induced modest changes in the stratum comeum lipid transition temperature [179]. 

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