JOURNAL OF COSMETIC SCIENCE 442 membrane and the endocuticle (actually the inner layer, a layer about 10- to 40-nm thick (7) between the endocuticle and the cell membrane and similar in composition to the exocuticle (see Figure 6), through bonding that is resistant to chlorine water oxidation (15) (possibly isopeptide linkages). Negri et al. (12) have shown that the Allworden reaction is an effect of the membranous proteins around cuticle cells and that 18-MEA is not required for the formation of All- worden sacs because the sacs can be produced from cuticle in which 18-MEA has been removed by prior treatment with either methanolic KOH or potassium t-butoxide in t- butanol. Because of the bulky nature of the t-butoxide anion, it removes only covalently bound fatty acid at or near the fi ber surface. Furthermore, Negri et al. (12) have shown that removal of the covalently bound fatty acid facilitates the formation of Allworden sacs because the rate of formation of the sacs increases with prior removal of the covalently bound 18-MEA. The separation and analysis of the Allworden membrane by Allen et al. (30), together with the work of Zahn et al. (31), provided indirect evidence for the cuticle cell mem- brane containing an ultra-high sulfur protein, loricrin, and involucrin. This important contribution will be described in more detail later in this paper in the section entitled “Proteins of the CMC.” Bilayers versus monolayers in the cuticle–cuticle CMC. Whether or not the covalently bound lipids of the cuticle–cuticle CMC are bonded to another lipid layer on their hydrophobic end, forming a bilayer, or they are bonded to a hydrophobic protein in the delta layer is still debated, but this author believes the evidence clearly favors the monolayer model (4,7) for the following reasons: O If the beta layers are monolayers, then 18-MEA is linked to the delta layer through short hydrophobic bonds, making the upper beta layer susceptible to failure at the delta layer where it has been shown to occur (4,7,32,33). O Swift (7) has pointed out that a monolayer model fi ts better from the point of view of CMC measurements. O If bilayers exist (a schematic of the bilayer model is shown in reference 4), then there are two options for bonding of the second fatty acid layer to the delta layer. One op- tion is for fatty acids to be covalently bonded to the delta layer, but this option is not plausible because in human hair and wool fi ber 40% to 50% of the covalently bonded fatty acids are 18-MEA (34–36) therefore, there are insuffi cient covalently bound fatty acids in human hair and wool fi ber to account for this type of bonding. The other option is bonding of the second layer of fatty acids through hydrophobic link- ages to the covalently bound fatty acids and bonding to the delta layer through polar attachments and ionic bonding. However, this type of bonding would provide beta– beta failure, not beta–delta failure, and it would allow for solvent removal of the non-covalently bound lipid layer by chloroform/methanol extraction, which has been shown to occur in cortex–cortex CMC but not in cuticle–cuticle CMC, which is more resistant to this type of solvent system (37,38). To provide beta–delta failure from this bilayer model, the new hair surface would form a bilayer consisting pri- marily of hydrophilic acid groups at the very surface, and so this bilayer model is also not plausible. O Negri et al. (39) noted that formic acid removes proteins more readily from the cortex– cortex CMC and that it modifi es CMC junctions of the cortex more than those of the

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