The stratum corneum is viewed currently as a layer of protein-enriched corneocytes embedded in a lipid-enriched, intercellular matrix , the so-called bricks and mortar model. The bricks are corneocytes surrounded by a cornified cell envelope made up of proteins, mainly loricrin, filaggrin, and involucrin, and covalently bound to the hydroxyceramide molecules of a lipid envelope. These bricks are embedded in a mortar of lipid bilayers [2-4]. The so-called mortar contains a variety of intercellular lipids including, ceramides, free sterols and sterolesters, cholesterolsulfate, and free fatty acids. The stratum corneum continually renews itself, and there is a steady state between the proliferation and differentiation process of keratinocytes and desquamation of corneocytes.
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