Mesophase formation by ceramides and cholesterol: a model for stratum corneum lipid packing?

Previous X-ray diffraction and electron microscopy experiments have suggested that there is an unusual double bilayer structure formed by stratum corneum lipids, with a lamellar spacing of about 131 A (White, S.H., Mirejovsky, D. and King, G.I. (1988) Biochemistry 27, 3725-3732; Hou, S.Y.E., Mitra, A.K., White, S.H., Menon, G.K., Ghadially, R. and Elias, P.M. (1991) J. Invest. Dermatol. 96, 215-223; Bouwstra, J.A., De Vries, M.A., Gouris, G.S., Bras, W., Brussee, J. and Ponec, M. (1991) J. Controlled Release 15, 209-220). Two contradictory models have been proposed for this structure. In the Downing model, used to explain electron microscopy observations, acylceramides are vital, acting as a 'lynch-pin' and holding the lipid layers together (Swartzendruber, W.C., Kitko, D.J., Madison, K.C. and Downing, D.T. (1989) J. Invest. Dermatol. 92, 251-257). Alternatively, to explain X-ray diffraction results from intact corneum, protein intercallation into the lipid bilayers is suggested, since an electron dense region wider than can be accounted for by lipid headgroups alone, is required (Bouwstra, J.A., De Vries, M.A., Gouris, G.S., Bras, W., Brussee, J. and Ponec, M. (1991) J. Controlled Release 15, 209-220). Thus, existing models require the presence of either acylceramides or protein. We describe how a similar structure can be prepared in vitro using mixtures of cholesterol and ceramides. Cholesterol induces a novel double-bilayer structure in ceramides II, and IV. This result is in conflict with the existing literature which cites acylceramides, or protein as instrumental in maintaining the in vivo structure of the phase. Characterisation has been carried out using optical microscopy and synchrotron X-ray diffraction.