The anatomical arrangement of the hair consists primarily of the follicle) hair shaft, and surrounding glands (Figure 6-2). There are around 100,000 follicles covering the human scalp. The hair shaft is a cylindrical structure that protrudes from the hair follicle. The average diameter of a hair shaft is 0.1 mm and each shaft is embedded 3 to 4 mm below the surface of the epidermal epithelium of the scalp. Other structures in close proximity to the hair fiber include oil-producing sebaceous glands of the epidermis that actually release their sebum into the hair follicle sweat glands (apocrine and eccrine), erector muscles attached to the hair fiber pulling them erect at times, a network of nerve endings surround the follicle in the dermal layer, and a plexus of blood capillaries hi).

The hair follicle can be distinguished into distinct growth zones along its axis. The zone linked with the hair bulb is the zone of separation and biological synthesis Cell proliferation and differentiation takes place in the lower region of this zone, while gene expression occurs in the uppermost region. During this synthetic process, the hair shaft is formed by mitotically active matrix cells and hair proteins are maintained in a reduced state with virtually no cross-linkages. The cells change morphologically by elongating and increasing in size and volume. Melanocytes in the lower portion of the bulb produce melanin pigment during the growth process as well. The zone of keratinization is where the hair fiber begins to form cysteine linkages, becoming stronger, more defined, and less hydrated. The final zone of permanent hair, potentially containing all morphological structures, extrudes through the skin surface. The matrix cells at this point are anucleated and dehydrated, forming a condensed, cornified structure.

The morphology of a hair fiber consists of 3 to 4 distinct structures. The outermost layer, referred to as the cuticle, contains a thick, protective layer of overlapping scale-like matrix cells. The cuticle surrounds the cortex, which is the structure accounting for the majority of the mass of the hair shaft. Cortex cells are “spindle-shaped” and contain fibrous proteins that contribute to hair elasticity. The medulla is the innermost porous region of the hair, and this loosely packed structure may or may not be present. Finally, the cell membrane complex is the structure that “glues” or binds the hair cells together with non-protein components of the hair fiber.

A chemical analysis of hair shows the composition to consist of 65 to 95% proteins, 1 to 9% lipids, 15 to 35% water, and smaller amounts of trace elements and polysaccharides. These percentages are influenced by the state of hydration of the hair.

The matrix proteins of hair are together referred to as keratin. They form tough fibers that are rich in sulfur content and resistant to degradation by proteases. These keratinized proteins ale composed of several strands of organized polypeptide chains wound into an α-helical structure known as micro-fibrils, which combine to form macro-fibrils, making up a hair shaft. The amino acids cysteine, lysine, histidine, glutamic acid, and aspartic acid build the framework of the matrix proteins, with cysteine accounting for the 11 to 18% of protein structure.

Many types of bonding take place in the hair protein matrix Strong covalent bonds are formed from cross- links between sulfyhydryl groups. Electrostatic, hydrophobic, and ionic interactions form between small molecules such as drug analytes and the functional groups of keratin including its carboxyl, amine, hydroxyl, and sulfyhydryl groups Alga. These proteinacious bonds may be broken by chemical treatments such as permanent waves and bleaching used for cosmetic purposes and enzymatic or acidic digests used in the laboratory. Likewise, high temperatures can also denature matrix proteins.

Hair lipids include free fatty acids, sulfates, and neutral fats including esters, waxes, hydrocarbons, and alcohols, phospholipids, and cholesterol. Principal lipids in hair extracts using solvents such as ethanol, benzene, ether, or chloroform are derived primarily from sebum. Hair consists of both surface lipids (external) and structural lipids, (internal), contributing to 1 to 9% of the hair’s chemical composition. Although the structural lipids are part of the cell membrane complex, they are not phospholipids as characteristically found in the cell membrane bi-layers. Hair lipid content is influenced by both age and gender; sebum production is low in children and heightens during puberty, and males have greater sebaceous activity than females. Alcohol and ester groups of these lipids may contribute to both specific and non-specific bonding of drug to hair. The contribution of drug binding to the lipid portion of the hair remains unknown, but given its contribution to the overall chemical composition of hair it is considered a minor role.

Hair color is attributed to pigment granules. The most predominant pigment is melanin, a copolymer composed of repeating 5, 6-quinone units linked to form a polymer. Melanin contains many free carboxyl, phenolic, and/or quinnonoid groups, and is a proposed source of binding sites for drug incorporated into hair. The cation-exchange activity of the carboxyl groups and the phenolic groups of melanin has been proposed as the binding site for many basic drugs.

Melanin is synthesized in small organelles known as melanosomes, located in specialized cells called melanocytes commonly found in the hair bulb. The melanosomes are frequently transferred to the keratinocytes within the medulla and cortex regions of the hair shaft. The average number of melanocytes in the scalp ranges from 1,000 to 1,200 melanocytes/mm2. Melanin pigment granules originate from tyrosine, L-dopa, dopamine, and catechol substrates. There are various melanin types, each varying in size, structure, and physiological properties.

Two types of melanin pigment may be found in human hair—eumelanin and pheomelanin. The relative proportions of the types of melanin as well as the size, structure, distribution, and density of the granules deposited in the cortex determine the shade and color of hair. In general, eumelanin produces brown to black shades of hair and pheomelanin is responsible for blond, ginger, and red shades. The lack of melanin leads to graying or non-pigmented hair. In most hair, both melanin types exist and eumelanins are the most prevalent. The melanin content among ethnic groups is known to vary. For example, the eumelanin content of Chinese black hair 3%, European brown hair 1.2%, Irish red hair 0.3%, and Scandinavian blond hair 0.07% varies as much as 40% within a hair type.

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