6.1 Hair, Skin & Nail Structure

6.1 Hair, Skin & Nail Structure

Every decision a Michigan cosmetologist makes at the chair—how long to leave a lightener on, whether a client's skin can tolerate an exfoliating treatment, how aggressively to shape a nail—traces back to structure. PSI Topic 2 (Client Consultation and Hair, Skin, and Nail Analysis, roughly 19% of the 100 scored theory items) tests this anatomy directly, and it is the foundation the next two sections build on: you cannot analyze a condition or select an appropriate service without first knowing what a healthy structure looks like and what each layer does.

Hair Structure

A single hair has two parts: the hair shaft, the visible portion above the skin, and the hair root, the living portion below the skin that includes the follicle (the tube-shaped depression the hair grows from), the bulb (the swollen area at the base of the root), and the dermal papilla (a cone-shaped area of blood vessels and nerves at the base of the follicle that nourishes the bulb). Because the dermal papilla supplies the blood-borne nutrients hair needs to grow, permanent injury to it can stop that follicle from producing hair again. A small band of involuntary muscle, the arrector pili, attaches to the follicle; when it contracts (from cold or fear) it pulls the hair upright, producing “goose bumps.”

The shaft itself has up to three layers:

• Cuticle — the outermost layer, made of overlapping, translucent, scale-like cells (imagine roof shingles) that point away from the scalp. A tight, flat cuticle reflects light and feels smooth; a raised or damaged cuticle looks dull and feels rough. The cuticle is the hair's protective layer and the main gatekeeper for how easily chemicals penetrate the shaft.
• Cortex — the thick middle layer, made of elongated cells bonded together by physical side bonds (hydrogen and salt bonds, broken by water or heat) and chemical disulfide bonds (broken by chemical reduction). The cortex holds melanin, the pigment that gives hair its natural color, in two forms: eumelanin (black/brown pigment) and pheomelanin (red/yellow pigment). Because color and texture services work primarily by changing the cortex, this layer is where haircoloring, lightening, and chemical texture chemistry does its permanent work.
• Medulla — the innermost layer, a soft, thin core of round cells. It is often absent in fine hair and present in coarse hair; it has little practical effect on services but is tested as a landmark for “innermost layer.”

Hair is composed largely of keratin, a fibrous protein (roughly 90% of the hair shaft), along with moisture, lipids, and pigment. Growth moves through three phases: anagen (the active growth phase, lasting years on the scalp, when most follicles are actively producing hair), catagen (a brief transitional phase when growth stops and the follicle shrinks), and telogen (the resting phase before the old hair sheds and a new anagen cycle begins). At any given time, the large majority of scalp follicles are in anagen, which is why normal daily shedding of resting-phase hairs is expected and not a disorder.

Skin Structure

Skin is the body's largest organ and has three primary layers. The epidermis is the thin, outermost, avascular (no blood supply of its own) layer that cosmetology services touch directly. It is built of five sublayers, from the surface down: stratum corneum (the horny, continuously shedding outer layer of dead, keratinized cells that forms the primary barrier); stratum lucidum (a clear layer found mainly on thick skin such as palms and soles); stratum granulosum (granular cells beginning the keratinization process); stratum spinosum (spiny-shaped cells that add strength and flexibility); and stratum germinativum (also called the basal layer), the deepest epidermal layer where new cells are continuously produced through mitosis and where melanocytes manufacture melanin that determines skin color and protects against UV exposure.

Below the epidermis lies the dermis, a thicker, vascular layer of connective tissue that contains collagen and elastin fibers (structural proteins responsible for strength and elasticity), blood vessels, nerve endings, hair follicles, and the glands discussed below. The dermis has two sublayers: the papillary layer, just beneath the epidermis, and the deeper reticular layer. Beneath the dermis sits the subcutaneous tissue (adipose/fatty layer), which cushions and insulates the body and gives the face its contours.

Skin's appendages include the sebaceous (oil) glands, which secrete sebum into the follicle to lubricate skin and hair, and the sudoriferous (sweat) glands, which help regulate body temperature and, together with sebum, form the skin's slightly acidic acid mantle—a natural barrier that helps resist bacteria and moisture loss. Overly harsh cleansing strips this mantle and can trigger the skin to overcompensate with more oil.

Nail Structure

The nail unit is a specialized skin appendage made of tightly packed, hardened keratin. The nail plate is the visible, hardened body of the nail. It rests on the nail bed, a layer of epithelial tissue richly supplied with blood vessels and nerves (the pink color seen through a healthy plate comes from this vascular bed). Beneath the base of the plate is the matrix, the only part of the nail unit that generates new nail plate cells—damage here can permanently deform future growth. The lunula is the visible, whitish, half-moon-shaped portion of the matrix at the base of the nail.

Surrounding tissues protect the unit: the eponychium is the living skin at the base of the plate that overlaps it slightly (often loosely called the “cuticle,” though technically the cuticle is the thin layer of dead, colorless tissue attached to the plate itself); the perionychium is the skin surrounding the entire nail border; the hyponychium is the slightly thickened skin under the free edge that seals out debris and bacteria; and the nail folds (lateral and proximal) are the skin folds that frame and cradle the sides and base of the plate. The free edge is the portion of the plate extending past the fingertip. Fingernails grow at roughly 1/8 inch per month on average—faster than toenails—though rate varies with age, health, season, and finger.

Knowing these names cold, and knowing which structures are living (dermal papilla, matrix, epidermal basal layer) versus dead/keratinized (hair shaft above the scalp, stratum corneum, nail plate, nail cuticle), is the exact foundation the next section uses to analyze condition and choose services.