Scientific Concepts · 12 min read
Structure of the Hair, Skin, and Nails
Hair, skin, and nails form the integumentary system, the body covering on which every salon service is performed. All three are built largely from keratin, a fibrous protein. Each structure has a name, a position, and a function, and a small set of figures recurs: growth rates, cycle lengths, and percentages. Trichology is the scientific study of hair, its diseases, and its care.
Structures of the Hair Root
A mature strand of hair has two parts. The hair root lies below the surface of the epidermis; the hair shaft projects above it. Five structures make up the root and its attachments. Follicles cover the entire body except the palms and the soles. Hair protects the head from heat, cold, and injury and serves as adornment. The scalp is the skin of the head; its follicles and sebaceous glands are larger and more numerous than those elsewhere on the body.
Layers of the Hair Shaft
| Layer | Structure | Role in services |
|---|---|---|
| Cuticle | Outermost: one layer of transparent, overlapping, scale-like cells pointing away from the scalp | Protects the cortex; products must lift it to enter |
| Cortex | Middle: fibrous protein core, about 90% of the hair’s weight; holds the melanin | Strength and elasticity; color, waving, and relaxing occur here |
| Medulla | Innermost: round cells; often absent in very fine and naturally blond hair | No part in salon services |
A healthy cuticle lies tight and flat and reflects light, which gives hair its shine; a raised or missing cuticle leaves hair dull, rough, and overly absorbent. Alkaline products such as permanent haircolor, waving solution, and relaxers raise the cuticle scales so that the product can reach the cortex; acidic products close them.
Keratin, COHNS Elements, and Side Bonds
Hair is about 90 percent protein. The protein is built from amino acids joined end to end by peptide bonds (end bonds) into long polypeptide chains, which coil into a helix. Living cells form in the bulb, move up the follicle, fill with keratin, lose their nucleus, and die. This process is keratinization; the shaft that leaves the scalp is nonliving fiber.
- Carbon: 51%
- Oxygen: 21%
- Hydrogen: 6%
- Nitrogen: 17%
- Sulfur: 5%
| Side bond | Type | Broken by | Re-formed by |
|---|---|---|---|
| Hydrogen | Weak physical bond; most numerous | Water or heat | Drying or cooling |
| Salt | Weak physical bond | Strong alkaline or acidic solutions (a change in pH) | Return to normal pH |
| Disulfide | Strong chemical bond joining two sulfur atoms | Thio waving lotion; thio and hydroxide relaxers | Thio: neutralizer. Hydroxide: never re-formed |
Side bonds link neighboring polypeptide chains, and each of the three types accounts for about one-third of the hair’s strength. Wet setting and thermal styling rearrange hydrogen bonds only, so the result lasts until the hair is wet again. Chemical waving and relaxing break disulfide bonds, so the change is lasting (see Chapters 5 and 12).
Melanin and Natural Hair Color
Natural hair color comes from melanin, grains of pigment in the cortex produced by melanocytes in the hair bulb. Eumelanin gives brown and black color. Pheomelanin gives colors from red and ginger to yellow and blond. Every natural shade results from the ratio of the two together with the number and size of the pigment granules. Gray hair appears as melanin production declines, and white hair contains no melanin.
The Hair Growth Cycle
Scalp hair grows an average of ½ inch (1.25 cm) per month in repeating cycles of three phases. Each follicle cycles on its own schedule, so hairs in all three phases are present at any moment and the hair is never shed all at once. Genetics, hormones, nutrition, health, medication, and age all influence growth.
| Phase | What happens | Duration | Share of scalp hair |
|---|---|---|---|
| Anagen (growth) | New hair is produced; cells divide rapidly in the bulb | About 3 to 5 years; can reach 10 | About 90% |
| Catagen (transition) | Follicle shrinks; bulb detaches from the papilla and forms a rounded club | 1 to 2 weeks | Less than 1% |
| Telogen (resting) | Hair rests until shed or pushed out by a new anagen hair | About 3 to 6 months | About 10% |
- The longer the anagen phase, the longer the hair can grow.
- Shaving, clipping, or cutting does not make hair grow back faster, darker, or coarser.
- Normal daily shedding averages 35 to 40 hairs; some references give 40 to 100 hairs a day.
Hair Types, Wave Pattern, and Growth Patterns
| Type | Description | Location |
|---|---|---|
| Vellus (lanugo) | Short, fine, soft, unpigmented; almost never has a medulla | Forehead, eyelids, bald scalp, and other “hairless” areas |
| Terminal | Long, coarse, pigmented unless gray; usually has a medulla | Scalp, legs, arms, and body |
Wave pattern is the shape of the strand: straight, wavy, curly, or extremely curly (coily). It is set by genetics. As a general rule, straight hair is round in cross-section and grows from a straight follicle, wavy and curly hair is oval, and extremely curly hair is flattened or elliptical and grows from a curved follicle.
| Pattern | Description |
|---|---|
| Hair stream | Hair flowing in one direction because the follicles slope the same way; two opposing streams form a part |
| Whorl | Hair that grows in a circular pattern, most often at the crown |
| Cowlick | Tuft of hair that stands straight up, most noticeable at the front hairline |
| Property | Meaning | Standard values |
|---|---|---|
| Texture | Diameter of the individual strand | Coarse, medium, or fine |
| Density | Number of hairs per square inch of scalp | Average 2,200 per square inch; about 100,000 on the head |
| Porosity | Ability to absorb moisture; depends on the cuticle | Low (resistant), average, or high |
| Elasticity | Ability to stretch and return without breaking | Normal wet hair stretches up to 50%; dry hair about 20% |
Layers of the Skin
The skin is the largest organ of the body. It is thinnest on the eyelids and thickest on the palms and soles. It has two main divisions, the epidermis and the dermis, which rest on a layer of subcutaneous tissue. Dermatology is the medical study of the skin.
| Layer | Also called | Key facts |
|---|---|---|
| Stratum corneum | Horny layer | Top layer; flat, keratin-filled cells that are continually shed and replaced |
| Stratum lucidum | Clear layer | Clear, transparent cells; thickest on the palms and soles |
| Stratum granulosum | Granular layer | Cells fill with keratin granules and die as they are pushed upward |
| Stratum spinosum | Spiny layer | Just above the basal layer; the process of skin cell shedding begins here |
| Stratum germinativum | Basal cell layer | Deepest layer; cells divide by mitosis; contains the melanocytes |
The epidermis contains no blood vessels but has many small nerve endings. The dermis, also called the derma, corium, cutis, or true skin, is the underlying support layer and is about 25 times thicker than the epidermis. It holds the blood and lymph vessels, nerves, sweat and oil glands, hair follicles, and arrector pili muscles.
| Layer | Position | Contents and role |
|---|---|---|
| Papillary layer | Outer dermis, directly under the epidermis | Dermal papillae with looped capillaries and touch receptors (tactile corpuscles) |
| Reticular layer | Deeper dermis | Supplies oxygen and nutrients; holds glands, follicles, vessels, nerve endings |
| Subcutaneous tissue | Below the dermis; also called adipose tissue or hypodermis | Fat that gives contour, stores energy, and cushions |
Collagen, Melanin, Glands, and Nerves of the Skin
Collagen is the fibrous protein that gives the dermis form and strength; it makes up about 70 percent of the dermis. Elastin is the protein that lets skin stretch and regain its shape. Sun exposure and aging weaken both. Skin color comes from melanin made by melanocytes in the basal layer; darker skin produces more melanin, which helps protect against ultraviolet light.
| Feature | Sudoriferous (sweat) glands | Sebaceous (oil) glands |
|---|---|---|
| Structure | Coiled base in the dermis; tube-like duct ends at a sweat pore | Small sacs whose ducts open into hair follicles |
| Product | Perspiration | Sebum |
| Function | Regulate body temperature; eliminate waste | Lubricate skin and hair |
| Location | Most numerous on palms, soles, forehead, underarms | All areas except palms and soles |
| Fiber | Action |
|---|---|
| Motor | Carry impulses to the arrector pili muscles, causing goose bumps |
| Sensory | React to heat, cold, touch, pressure, and pain |
| Secretory | Regulate perspiration and sebum flow; part of the autonomic nervous system |
| Function | How the skin performs it |
|---|---|
| Protection | Barrier against injury and bacteria; sebum and sweat form the acid mantle |
| Sensation | Nerve endings respond to heat, cold, touch, pressure, and pain |
| Heat regulation | Sweat and blood flow hold body temperature near 98.6°F (37°C) |
| Excretion | Sudoriferous glands excrete perspiration, removing salt and other wastes |
| Secretion | Sebaceous glands secrete sebum, which lubricates and slows moisture loss |
| Absorption | Limited; some ingredients enter through follicles, pores, and between cells |
The Natural Nail Unit
The natural nail (technical term onyx) is the hard protective plate at the end of each finger and toe; the natural nail unit is the plate together with the tissues that form and hold it. A healthy nail is smooth, shiny, and translucent pink with a whitish free edge. The plate is hard keratin with a water content of 15 to 25 percent, and it is porous.
| Part | Description |
|---|---|
| Nail plate | Hardened keratin plate of about 100 layers of nail cells; slides across the bed as it grows |
| Free edge | Part of the plate that extends past the fingertip |
| Nail bed | Living skin under the plate; its blood supply gives the pink color |
| Bed epithelium | Thin tissue that attaches the plate to the bed |
| Matrix | Where plate cells are formed; contains nerves, lymph, and blood vessels |
| Lunula | Whitish half-moon at the base of the plate; the visible part of the matrix |
| Cuticle | Dead, colorless tissue attached to the plate |
| Eponychium | Living skin at the base of the plate that covers the matrix area |
| Hyponychium | Slightly thickened skin under the free edge; seals out microorganisms |
| Specialized ligaments | Tough fibrous bands that attach the nail bed and matrix bed to the bone |
| Nail folds | Skin folds around the plate that form the nail grooves; the side fold is the sidewall |
- Average adult growth is about 1/10 to 1/8 inch (2.5 to 3 mm) per month.
- A fingernail is replaced in about 4 to 6 months; a toenail in 9 months to 1 year.
- Nails grow faster in summer and in children; the middle fingernail is fastest, the thumbnail slowest.
- A longer matrix produces a thicker plate; the shape of the matrix sets the shape of the plate.
- Root structures: follicle, bulb, dermal papilla, arrector pili muscle, sebaceous glands. Shaft layers: cuticle, cortex, medulla.
- Hydrogen bonds break with water or heat, salt bonds with a change in pH, disulfide bonds with chemical waving and relaxing.
- Anagen is growth (about 90% of hair, 3 to 5 years); catagen is transition (1 to 2 weeks); telogen is rest (about 10%, 3 to 6 months).
- Epidermis from the surface: corneum, lucidum, granulosum, spinosum, germinativum. The dermis, or true skin, has a papillary and a reticular layer.
- Nail plate cells form in the matrix; the lunula is its visible part. The cuticle is dead tissue; the eponychium is living skin.
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