4.1 Hair structure, growth cycles, analysis & disorders

The Structure of the Hair

Hair is an appendage of the skin composed almost entirely of the protein keratin, and the scientific study of hair is called trichology. Every strand has two defined divisions: the hair root, the portion enclosed within the follicle below the skin's surface, and the hair shaft, the visible portion that extends above the scalp.

The follicle and hair root

The follicle is a tube-like pocket of epidermal tissue that encases the root. At its base the follicle widens into the bulb, a thickened, club-shaped structure that fits over and covers the dermal papilla — a small cone of connective tissue supplied with the blood vessels that nourish the growing hair. Two other structures attach to each follicle: the arrector pili muscle, an involuntary muscle that contracts to create "goose bumps," and the sebaceous (oil) glands, which secrete sebum to lubricate the hair and scalp.

The three layers of the shaft

The hair shaft is built from three concentric layers. The cuticle is the tough, transparent outermost layer of overlapping scales that point away from the scalp toward the ends; a healthy, tightly packed cuticle protects the interior, gives hair its shine, and is the layer that chemical products must lift and penetrate. The cortex is the thick middle layer that accounts for roughly 90 percent of the strand's total weight; it holds the melanin (natural pigment) and is responsible for strength, elasticity, and the changes produced by wet-setting, thermal styling, perming, relaxing, and haircolor. The innermost medulla is a soft, spongy core — sometimes called the pith — that is frequently absent in fine and naturally blond hair and plays essentially no role in salon services.

Chemical Composition and Side Bonds

Keratin is built from amino acids linked end to end by peptide (end) bonds into long polypeptide chains. These coiled chains are cross-linked and held in shape by side bonds. Hydrogen bonds and salt bonds are weak physical bonds: hydrogen bonds break with water or heat, salt bonds break with changes in pH, and both re-form as the hair dries or is neutralized — which is exactly why a wet set or thermal style is only temporary. Disulfide bonds are strong chemical bonds broken and rebuilt only during permanent waving and chemical relaxing, which is why those services create lasting change. Hair is roughly 80 to 90 percent keratin and is made of the five elements carbon, oxygen, hydrogen, nitrogen, and sulfur, remembered by the acronym COHNS.

The Hair Growth Cycle

Scalp hair grows in a repeating three-phase cycle and averages about one-half inch of growth per month. Shedding roughly 50 to 100 hairs a day is normal, not a sign of disease.

Analyzing Hair: The Five Characteristics

Before most services the cosmetologist evaluates five qualities of the hair and scalp:

• Texture — the diameter of a single strand, classified as coarse, medium, or fine.
• Density — the number of hairs per square inch, described as low, medium, or high; density measures how many hairs there are, not how thick each one is.
• Porosity — the hair's ability to absorb moisture. Low-porosity hair resists penetration, while high (over-porous) hair absorbs quickly and processes faster.
• Elasticity — the ability to stretch and return without breaking. Dry hair with normal elasticity stretches about 20 percent of its length; wet hair up to 50 percent.
• Growth pattern (hair stream/direction) — the direction hair flows from the scalp, including whorls and cowlicks, which influence styling and finished shape.

Disorders of the Hair and Scalp

A cosmetologist must know which conditions can be serviced and which require referral to a physician. The guiding rule is simple: if a condition is infectious or inflamed, do not perform the service. Alopecia (hair loss) is non-infectious — androgenetic alopecia (pattern baldness) and alopecia areata (sudden, patchy loss) may be worked on cosmetically. Pityriasis capitis simplex (dry dandruff) can be managed in the salon, while greasy pityriasis steatoides and any inflamed scalp warrant referral. Contagious diseases must never be serviced: tinea (ringworm, a fungal infection), tinea capitis, pediculosis capitis (head lice), scabies (itch mite), and inflamed folliculitis all call for referral to a physician before any further care.

Several other non-infectious conditions may appear and can be improved cosmetically: canities (gray hair, an absence of melanin), hypertrichosis (excessive growth), trichoptilosis (split ends), and fragilitas crinium (brittle hair). Because these are not contagious, hair with them can be conditioned, cut, or styled in the salon. Use this quick reference to decide:

• Service in the salon (non-infectious): alopecia, dry dandruff (pityriasis capitis simplex), gray hair, split ends, and brittle hair.
• Refer to a physician (infectious or inflamed): tinea/ringworm, head lice, scabies, inflamed folliculitis, boils, and any open, crusted, or oozing lesion.