Ichiro Iwai

The Human Skin Barrier Is Organized as Stacked Bilayers of Fully Extended Ceramides with Cholesterol Molecules Associated with the Ceramide Sphingoid Moiety

The skin barrier is fundamental to terrestrial life and its evolution; it upholds homeostasis and protects against the environment. Skin barrier capacity is controlled by lipids that fill the extracellular space of the skins surface layer--the stratum corneum. Here we report on the determination of the molecular organization of the skins lipid matrix in situ, in its near-native state, using a methodological approach combining very high magnification cryo-electron microscopy (EM) of vitreous skin section defocus series, molecular modeling, and EM simulation. The lipids are organized in an arrangement not previously described in a biological system-stacked bilayers of fully extended ceramides (CERs) with cholesterol molecules associated with the CER sphingoid moiety. This arrangement rationalizes the skins low permeability toward water and toward hydrophilic and lipophilic substances, as well as the skin barriers robustness toward hydration and dehydration, environmental temperature and pressure changes, stretching, compression, bending, and shearing.

Human skin barrier formation takes place via a cubic to lamellar lipid phase transition as analyzed by cryo-electron microscopy and EM-simulation

ABSTRACT The skins permeability barrier consists of stacked lipid sheets of splayed ceramides, cholesterol and free fatty acids, positioned intercellularly in the stratum corneum. We report here on the early stage of skin barrier formation taking place inside the tubuloreticular system in the secretory cells of the topmost viable epidermis and in the intercellular space between viable epidermis and stratum corneum. The barrier formation process was analysed in situ in its near‐native state, using cryo‐EM combined with molecular dynamics modeling and EM simulation. Stacks of lamellae appear towards the periphery of the tubuloreticular system and they are closely associated with granular regions. Only models based on a bicontinuous cubic phase organization proved compatible with the granular cryo‐EM patterns. Only models based on a dehydrated lamellar phase organization agreed with the lamellar cryo‐EM patterns. The data support that human skin barrier formation takes place via a cubic to lamellar lipid phase transition.