CELL MEMBRANE COMPLEX 443 cuticle, which is consistent with covalent and hydrophobic bonding of the cuticle– cuticle CMC, as shown by the monolayer model in Figure 2, rather than a bilayer model. Another point of contention concerning the CMC is whether or not the delta layer con- tains globular proteins or glycoproteins. Allen et al. (40) found evidence for glycoproteins in several different animal hairs in formic acid extracts, which they suggested could be from the CMC however, they also suggested that these materials could be remains of cell membrane glycoproteins from the follicle or that they could be functional adhesive ma- terials in the CMC. I believe the current evidence favors globular proteins in the delta layer as functional adhesive materials for the following reasons: O The delta layer resists solubilization by aqueous reducing or oxidizing agents and by acids and alkalies (5). If the CMC contains globular proteins similar to those in many other membranes containing large domains of hydrophobic amino acids on their sur- faces (41), that would provide the ideal delta layer surface for the hydrophobic ends of the covalently bound fatty acids to adhere to, and this type of globular protein should be resistant to aqueous reagents, as Bryson et al. (5) found. O Bryson et al. in 1995 (5) isolated lipid-soluble lipoproteins from the delta layer of cortex–cortex CMC and not glycoprotein. O The delta layer stains with phosphotungstic acid (PTA). This is either a reaction of hydroxyl groups of a polysaccharide or of a primary amine function. Swift (7) has ex- plained that this reaction is blocked with fl uro dinitro benzene (FDNB) therefore, it is more likely a reaction involving primary amine groups, consistent with a globular protein. O The delta layer reacts with periodic acid/silver methenamine (7), a method for polysac- charides however, Swift (7) has also pointed out that since cystine interferes with this reaction, it is still consistent with a globular protein in the delta layer. Thus, the globular protein model is consistent with the currently known reactivity of the cuticle–cuticle CMC and with the proposed structure in Figure 2, and therefore the gly- coproteins that Allen et al. (40) found were most likely remains of cell membrane mate- rial from the follicle. CORTEX–CORTEX CMC Wertz and Downing (35) found in fi ve different mammalian hairs, including those of sheep, humans, dogs, pigs, and cattle, that the percentage of 18-MEA relative to the total amount of covalently bound fatty acids varied from 38% to 48%. Table I summarizes a tabulation of analyses of the covalently bound lipids of wool and human hair from several different laboratories. These results were all obtained after the fi bers had been exhaus- tively extracted with chloroform/methanol to remove the non-covalently bound fatty ac- ids and then by saponifying the residue with methanolic alkali, showing that 18-MEA accounts for about 50% of the covalently bound fatty acids in these wool fi bers and about 40% in human hair. Covalently bound internal lipids of animal hairs. Korner and Wortmann (22) (Table I) ana- lyzed covalently bound fatty acids in isolated wool cuticle and found 55% 18-MEA, 25% stearic acid, and 20% palmitic acid, with “only traces of other straight and odd number carbon chain fatty acids.” For wool fi ber Wertz and Downing (35) found 48% 18-MEA, 17% palmitic acid, 10% stearic acid, and 5% oleic acid, and the remaining covalently

JOURNAL OF COSMETIC SCIENCE 444 bound fatty acids ranged from C16 through C20, with 6% uncharacterized. For human hair, Wertz and Downing (34) found 41% 18-MEA, 18%, palmitic acid, 7% stearic acid, and 4% oleic acid, and the remaining small percentages of fatty acids ranged from C16 through C20, with 9% uncharacterized. Negri et al. (42) found 72% 18-MEA, 8% pal- mitic acid, 6% stearic acid, and 5% oleic acid in wool fi ber. The variation in these data from different laboratories is quite large. Part of the variance has been suggested to be related to fi ber diameter, which determines the number of layers of covalently bound fatty acids in the fi bers however, certainly part of the variance is due to experimental error. The bottom line is that somewhere in the vicinity of 50% ± at least 10% of the covalently bound fatty acids in most keratin fi bers is 18-MEA (attached mainly on the top surface of cuticle cells). Furthermore, hair fi bers from sheep, humans, dogs, pigs, and cattle, and likely most keratin fi bers, contain the remaining nearly one half of the covalently bonded fatty acids, primarily as palmitic, stearic, and oleic acids attached mainly on the bottom of cuticle cells. In 1990, Kalkbrenner et al. (43) demonstrated with isolated cuticle cells that 18-MEA is essentially all in the cuticle. Since 18-MEA represents more than 40% of the total cova- lently bound fatty acids in human hair and about 50% in wool fi ber, and since 18-MEA is confi ned to the upper beta layer of the cuticle (23,24) while most (essentially an amount equal to the 18-MEA) of the other covalently bound fatty acids are confi ned to the lower beta layer, then most of the covalently bound fatty acids in wool and hair fi ber must be in the cuticle–cuticle CMC, with some in the cuticle–cortex CMC (to be described later) and virtually none in the cortex–cortex CMC. Therefore, if most of the covalently bound fatty acids are in the cuticle–cuticle CMC, then most of the lipids of the cortex–cortex CMC must be bound to the membranes on one side and to the delta layer on the other side by non-covalent bonds. The fact that most of the remaining lipids can be removed by solvent extraction confi rms that this is the case, as will be shown below and in the sec- tion entitled “Lipids in the CMC.” Leeder et al. (44) fi rst found that there are virtually no phospholipids in keratin fi bers. This fact was confi rmed by Schwan and Zahn (45) and by Rivett (46), casting doubt on whether lipid bilayers could be involved in the cell membranes of keratin fi bers (44). However, Wertz et al. (47) demonstrated that liposomes (lipid bilayers and a presumed precursor to the formation of lipid bilayers in the CMC of keratin fi bers) can form in the absence of phospholipids if an acid species such as cholesterol sulfate is present with other lipids. Table I Covalently Bound Fatty Acids in Wool and Human Hair Fiber Data for wool fi ber Data for human hair Fatty acid [13] [43] [44] [36] [22] Averages [35] 16:0 8 11 8 17 20 12.8 18 18:0 8 12 6 10 25 12.2 7 18:1 7 8 5 5 0 5 4 MEA 51 43 72 48 55 53.8 41 Others 26 26 9 20 trace 16.4 30 Data are expressed as percentages.