4.1 Hair and Scalp Structure and Growth

Why hair anatomy shows up on the New Jersey theory exam

The NIC New Jersey Cosmetologist-Hair Stylist theory exam weights Scientific Concepts at 30 of 110 scored items. Subtopic 1.B.1.a is the function and structure of the hair and scalp. This section is not a disease chapter. You need the parts of the hair and scalp well enough to shampoo, cut, color, perm, relax, braid, and shave without destroying the fiber or injuring living tissue.

Hair is an appendage of the skin. The hair root is the portion below the surface, inside the follicle. The hair shaft is the keratinized portion that projects above the skin or scalp. Almost every hair service billed under a New Jersey cosmetologist-hairstylist license is performed on that dead shaft. The living structures in the follicle still decide whether the next shaft grows oily, dry, coarse, fine, or not at all.

If you remember only one clinical split, remember this: cutting does not hurt because the shaft is dead; growth continues because the papilla and bulb are alive. Chemical services do not "feed" the hair. They rearrange or remove what is already in the cortex.

The follicle and the five structures of the hair root

The hair follicle is a tube-like pocket in the skin or scalp that contains the hair root. Follicles slant. That slant is why a hair stream has a direction and why a shave that fights follicle angle nicks the epidermis or drives a hair back under the surface. Follicles are absent on the palms, soles, and lips.

Five structures travel with the root. Learn them as a set, because exam items mix location and function.

If the dermal papilla is destroyed, that follicle will not produce a new hair. If the papilla is intact, coloring, cutting, or a close shave does not stop future growth. New Jersey laser hair removal and other medical destruction of the papilla sit outside this license; you service the shaft, not the blood supply of the bulb.

Three layers of the hair shaft

From the outside in, the shaft has up to three layers:

1. Cuticle. Transparent, overlapping, scale-like cells that point toward the tip (imbricated). A compact, acidic cuticle gives shine and low porosity. Alkaline chemical services, harsh shampoo, heat, and friction lift or chip the cuticle. Once those scales stand up, the cortex is exposed to water, pigment, and reducing agents.
2. Cortex. The middle layer and, in standard theory teaching, about 90 percent of the hair's weight. Elongated cells packed with keratin and melanin. Eumelanin reads brown to black; pheomelanin reads red to yellow. Mechanical strength, elasticity, natural color, and the lasting result of color, lightener, permanent wave, and relaxer all live in the cortex.
3. Medulla. The innermost pith. It may be absent in fine or naturally blonde hair. It is not the target of chemical services and it is not what makes hair "thick" in the density sense.

A healthy cuticle lies flat like shingles on a roof. You see that as shine. You feel it as smoothness. High-porosity hair feels dry and tangles because the shingles are lifted or broken, not because the medulla vanished.

Keratin, COHNS elements, and the bonds chemical services actually break

Hair is approximately 90 percent protein. Textbook COHNS elements are carbon, oxygen, hydrogen, nitrogen, and sulfur. Amino acids join with peptide (end) bonds into polypeptide chains. Breaking peptide bonds destroys the fiber — that is the logic of chemical depilatories, not of a perm.

Between chains sit side bonds:

• Hydrogen bonds are weak physical bonds. There are many of them. Water and heat break them; drying and cooling rebuild them. Wet sets, wrapping, and blow-drying rearrange hydrogen bonds.
• Salt bonds are physical ionic bonds. They are sensitive to pH. Strong acid or alkali breaks them.
• Disulfide bonds are strong chemical bonds between cysteine sulfur atoms. They are fewer than hydrogen bonds but they account for a large share of lasting strength. Permanent waving uses thio chemistry to reduce disulfides, then a neutralizer to rebuild them in a new shape. Hydroxide relaxers convert disulfides to lanthionine bonds that do not reform, which is why hydroxide and thio services are not stacked on the same hair.

Those reactions happen in the cortex, after the cuticle has been lifted. New Jersey's 1,200-hour curriculum puts heavy hours into tinting, bleaching, permanent waving, and chemical relaxing. Domain 2 will ask procedures. Domain 1 asks whether you know which tissue those procedures enter.

Growth cycles of the follicle

Scalp hair does not grow continuously. Each follicle cycles.

Textbook average scalp growth is about 1/2 inch (1.25 cm) per month. Rate varies with age, hormones, season, and nutrition. These figures are standard theory numbers used on NIC-style items. They are not measurements published by the New Jersey State Board of Cosmetology and Hairstyling.

A shed club hair in the comb is telogen completing, not proof the papilla died. Diffuse shedding as a disease pattern belongs in the disorders chapter. Here you only need the cycle names and what the follicle is doing in each phase.

Texture, porosity, elasticity, and density

Four analysis words are not synonyms. Exam stems love to swap them.

Texture is the diameter of one hair: fine, medium, or coarse. Coarse hair has a larger cross-section and often a more resistant cuticle. Fine hair has less cortex mass, less tensile strength, and often processes faster — overprocessing is the risk, not "not enough product."

Porosity is the hair's ability to absorb moisture. It is a cuticle story. Low porosity: compact cuticle, products sit on top, chemical services may need more time or heat. Average porosity: healthy intact cuticle. High porosity: raised or damaged cuticle, hair grabs color quickly, tangles, and snaps. A strand that drinks toner in a minute is high porosity, not "thirsty medulla."

Elasticity is the ability to stretch and return without breaking. Standard teaching: dry hair stretches about 20 percent; wet hair about 40–50 percent. Low elasticity means broken side bonds. Tight perm rods on low-elasticity hair break fiber; they do not magically add disulfide bonds.

Density is hairs per square inch, not diameter. A client can have fine texture and high density (many skinny hairs) or coarse texture and low density (few thick hairs). "Thin hair" in consultation language usually means density. Classic textbook illustrations also note that naturally blonde scalps often carry more hairs than red scalps; treat those counts as teaching examples, not lab numbers printed on the New Jersey exam form.

Vellus hair, terminal hair, and growth patterns

Vellus hair is short, fine, and often unpigmented (peach fuzz). It covers much of the body and helps regulate temperature.

Terminal hair is longer, coarser, and usually pigmented: scalp, brows, lashes, underarms, pubic hair, and the beard. New Jersey shaving theory and the large practical shaving block in N.J.A.C. 13:28-6.29 (81 hours of shaving) are about terminal facial hair. Removing the shaft does not convert terminal hair to vellus. Medical laser destruction of the follicle is not a cosmetology service in this state.

Growth patterns on the scalp:

• Hair stream — follicles sloping the same way so hair flows in one direction.
• Whorl — circular pattern, often at the crown.
• Cowlick — a tuft that stands more upright.

Haircutting, clipper work, and natural styling must respect these patterns or the design collapses when the hair dries. On the face, follicle direction plus arrector pili tension is why a with-the-grain first pass is the safer shaving-theory start.

Put the New Jersey day together: shampoo manages sebum and the cuticle; chemical texture and color enter the cortex; cutting follows density, texture, and growth patterns; shaving follows follicle angle on terminal hair. None of those services work below living scalp the way a medical procedure would, and none of them replace the anatomy you just named.