A model of the epicuticle membrane of keratin fibers shows that it is a heavily crosslinked protein containing approximately 25% by weight of fatty acid, predomi nantly 18-methyleicosanoic acid, acylated to the protein as a thioester. The conclusion that acylated fatty acids reside on the surface of and completely surround individual cuticle cells is supported by an analysis of the amount of fatty acid removed by alcoholic alkali against treatment time and the observed decreasing amount of bound fatty acid found as fiber diameters increase. Allworden sacs form only under acidic conditions, which also cleave bound fatty acids. Prior removal of bound fatty acids facilitates the rapid formation of Allworden sacs.

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Models for the surface of cuticle cells in hair fibers consist of a monolayer of fatty acids covalently bound to the underlying protein membrane by thioester linkages. The most prominent of these fatty acids is 18-methyleicosanoic acid (C21a), the synthesis of which requires the oxidative decarboxylation of isoleucine. Maple syrup urine disease (MSUD) is caused by an inherited deficiency in the enzyme branched chain 2-oxo acid dehydrogenase, which leads to the accumulation of branched chain alpha-keto-acids derived from the amino acids, leucine, isoleucine, and valine. Transmission electron microscopy studies of developing hair fibers show a structural defect in the fiber shaft in hair from patients with MSUD. This defect is confined to the cuticle of the fiber, where the cuticle membrane directly apposes the intercellular material. Thus, the defect indicates that C21a is located exclusively on the upper surface of fiber cuticle cells. Lipid analysis of MSUD hairs has demonstrated significant changes in the relative abundance of the covalently bound fatty acids and an almost complete absence of C21a, whereas there was little difference in the amino acid composition compared with normal hair. These results provide further evidence for the existence of the surface lipid monolayer and its crucial role in cellular adhesion.

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