JOURNAL OF COSMETIC SCIENCE 440 diffuse out of the semipermeable cuticle cell membrane. Swelling results from osmotic forces, and the cuticle membrane stretches, producing the so-called Allworden sacs that separate from the underlying proteinaceous intracellular matter. The epicuticle membrane was fi rst isolated and named by Lindberg et al. in 1949 (16,17). Several years later, in 1968, Leeder and Bradbury (18) defi ned the epicuticle as the “thin outer membrane which is raised on the surface of fi bers as sacs by treatment with chlorine water” in the Allworden reaction. The epicuticle provides the protein supporting struc- ture for 18-MEA in cuticle cells (see Figures 2 and 5). It is also attached to the A-layer of cuticle cells of wool and human hair, and together with 18-MEA is perhaps the most thoroughly studied part of the CMC. Leeder and Rippon (19), in 1985, suggested that the epicuticle was proteinaceous and covered with a strongly bound lipid layer that could not be removed by lipid solvents, but could be removed with alcoholic alkali they called this lipid layer the F-layer (see Figures 5 and 6). The F-layer of covalently bound fatty acids, together with the cuticle cell membranes (es- sentially the epicuticle), is analogous to the cornifi ed envelopes or the cellular envelope of Figure 5. Schematic of the hair surface showing the lipid layer (F-layer) and the cell membranes (not drawn to scale). Figure 6. Schematic of a transverse section of a cuticle cell with the CMC (not drawn to scale).

CELL MEMBRANE COMPLEX 441 stratum corneum. As early as 1945, Weitkamp (20) reported 18-MEA in wool wax (de- gras). In 1985, Evans et al. (21) demonstrated that 18-MEA is covalently bonded to the keratin fi ber surface by reacting wool fi ber with anhydrous alkali after solvent-extractable lipids have been removed. The cleavage of 18-MEA with chlorine water by Negri et al. (12) and by hydroxyl amine at neutral pH by Evans and Lanczki (13) support the attach- ment by a thioester linkage rather than an ester or amide linkage, although there is evi- dence for ester and/or amide attachment of some fatty acids (primarily palmitic, stearic, oleic, and others), mainly in the lower beta layer in cuticle–cuticle CMC by Evans and Lanczki (13) and by Korner and Wortmann (22). Essentially all of the MEA is in the upper beta layer of the cuticle–cuticle CMC, as dem- onstrated in a paper by Jones et al. (23). Maple syrup urine disease (MSUD) is a genetic defect in humans and Poll Herford cattle (24) involving 18-MEA. MSUD is caused by a defi ciency in an enzyme involved in the complicated synthesis of 18-MEA. Isoleucine serves as a precursor in the biosynthesis of 18-MEA via the branched chain 2-oxo acid dehydrogenase, which is the enzyme that is defi cient in this genetic defect (10). Jones and Rivett in their TEM studies of MSUD (10,23) found that the structural defect of MSUD in human hair occurs only on the upper surface of cuticle cells (upper beta layer), where 18-MEA is replaced by straight chain C18 and C20 fatty acids, and that the undersides of cuticle cells (lower beta layer) are not affected. These facts confi rm that 18-MEA is at- tached to the top surface of cuticle cells (upper beta layer) and not to the underside. Only a few years after the discovery by Evans et al. (21) that 18-MEA is covalently bound to the keratin fi ber surface, Negri et al. (12) proposed a model for the keratin fi ber surface consisting of a monolayer of 18-MEA covalently bonded to an ultra-high sulfur protein through a thioester linkage at approximately 1-nm spacings, and they suggested that the protein support was in the beta confi guration and that it might be attached to the All- worden membrane. Although widely varying estimates of the thickness of the epicuticle have been made from 5 to 14 nm, one of the more recent and perhaps reliable estimates is by Swift and Smith (25), who examined wool fi ber, human hair, and several other mam- malian hairs using high-resolution TEM and identifi ed that the epicuticle is approxi- mately 13-nm thick and is rich in cystine, and that thioester-bound lipids might be present within its bulk. Swift’s estimate of the epicuticle thickness is consistent with the maximum thickness reported by several other workers (26–28). Leeder and Bradbury (18,29) discovered that the Allworden reaction takes place with isolated cuticle cells from several different animal hairs including wool and human hair fi ber, proving that this proteinaceous membrane material completely surrounds each cu- ticle cell and is not a continuous external membrane on hair fi bers. In this important scientifi c effort, cuticle cells were isolated by shaking fi bers in formic acid and then ex- posing the isolated cells to chlorine water. Formic acid is known to attack and to solubi- lize some proteins believed to be largely from the delta layer of the cell membrane complex, and its effects will be discussed later in the section entitled “Proteins of the CMC.” In the intact fi ber Allworden sacs form over the top of cuticle cells (the exposed surface). Leeder and Bradbury suggested that “the sac always occurs on only one side of the cuticle cell,” i.e., the top of cuticle cells and not the bottom (15,18,29). They ex- plained that this effect occurs because the connecting bonds on the top of cuticle cells are between the epicuticle and the A-layer and therefore are most likely through disul- fi de crosslinks that are vulnerable to chlorine water oxidation (15). Furthermore, they suggested that the connecting bonds on the underside of cuticle cells are between the