8.1 Hair Structure, Trichology & Growth Cycles

8.1 Hair Structure, Trichology & Growth Cycles

Quick Answer: Trichology is the study of hair and its diseases. A strand has three layers — cuticle, cortex, and medulla — built mainly of keratin protein with moisture and lipids. Living growth happens in the follicle (bulb, dermal papilla, arrector pili, sebaceous gland). Scalp hair cycles through anagen (growth), catagen (transition), and telogen (rest/shed) and averages about ½ inch per month. Master structure first; analysis, loss, and disorders build on these names.

Hair is the primary canvas for most Ohio cosmetology services. The Ohio Cosmetology Theory TIP weights Anatomy & Physiology at 27% and Hair Care & Services at 22% of the 100-question theory exam. Structure and growth vocabulary appear in both domains: A&P wants correct labels and functions; hair care wants you to apply those labels when porosity, chemical services, and client concerns come up. This section is the foundation for 8.2 (analysis and loss) and 8.3 (disorders).

Trichology: The Science Frame

Trichology is the scientific study of hair and its diseases. You are not a medical trichologist diagnosing systemic disease, but Board theory expects professional-level structure language from standard texts (Milady, Pivot Point, and similar references cited on Ohio’s TIP). When an item asks which layer holds melanin or which structure supplies blood to the follicle, it is testing trichology fundamentals — not salon gossip about “product that grows hair.”

Two truths guide every later decision:

1. The hair shaft above the skin is largely keratinized dead tissue. You can cut, color, and reshape it; you cannot revive it into living cells.
2. The hair root within the follicle is where living activity, blood supply, and growth cycles occur. Health, hormones, nutrition, and disease act here.

The Hair Shaft: Three Layers

From outside in, textbooks describe three concentric layers:

Cuticle

The cuticle is a transparent outer layer of overlapping scales that point from the scalp toward the hair ends (like shingles on a roof). Healthy, compact cuticle reflects light (shine) and slows moisture loss and chemical penetration. Raised or damaged cuticle feels rough, looks dull, and increases porosity — a concept Section 8.2 develops for service planning.

Alkaline products (many permanent waves, relaxers, and high-lift processes when used as designed) can swell the cuticle so solutions reach the cortex. Acidic rinses and conditioners help close and smooth the cuticle after processing. Exam logic: cuticle = protection and gateway; over-processing that shreds cuticle leaves porous, weak hair.

Cortex

The cortex is the fibrous middle layer and the majority of hair mass in most strands. It contains:

• Keratin protein chains arranged in a complex fiber network
• Melanin pigment that determines natural hair color
• Disulfide, salt, and hydrogen bonds that chemical and thermal services rearrange (detailed in later chemical texture and color chapters)

Cortex integrity equals tensile strength and elasticity. When cortex protein is dissolved or over-stretched (harsh chemicals, extreme heat, mechanical abuse), hair becomes brittle, loses spring, and may break. Permanent color, lightening, permanent waving, and relaxing all target cortex chemistry while managing cuticle entry.

Medulla

The medulla is the innermost layer. It may be continuous, interrupted, or missing. Fine hair often lacks a distinct medulla; coarse hair is more likely to show one. For Ohio theory depth, know that the medulla is the core, that it is not always present, and that most day-to-day service chemistry focuses on cuticle and cortex, not medulla management.

Hair Composition: Protein, Moisture, Lipids

Hair is approximately:

Keratin is the hard, fibrous protein that forms hair, skin surface, and nails. Cosmetology products market “keratin” heavily; scientifically, you are coating or temporarily reinforcing a dead keratin fiber, not recreating living follicle biology. Understanding true composition prevents false claims and supports honest client education — and correct exam answers about what hair is.

The Hair Root and Follicle System

The hair root is the portion of hair enclosed within the follicle below the skin surface. Key structures:

Blood supply through the papilla delivers nutrients and oxygen that support anagen growth. Poor systemic health, certain medications, and disease can disrupt that supply and show up later as thinning or shedding — recognition territory for Section 8.2, not cosmetology treatment of medical disease.

Sebum coats the hair and scalp, helping retain moisture and providing a natural protective film. Overactive sebaceous activity contributes to oily scalp presentations; underactivity can accompany dry scalp feel. Product choice (clarifying vs. moisturizing shampoo) responds to these patterns without diagnosing endocrine disorders.

Vellus vs. Terminal Hair (Recognition)

Theory often distinguishes:

• Vellus hair — short, fine, pale, downy hair on much of the body
• Terminal hair — longer, coarser, pigmented hair of the scalp, eyebrows, lashes, and, after puberty, other androgen-sensitive areas

Scalp services focus on terminal hair structure. Hypertrichosis and related conditions appear in Section 8.3; here, know that follicle type and hormone influence help determine vellus vs. terminal character over a lifetime.

Growth Cycles: Anagen, Catagen, Telogen

Hair does not grow continuously forever from one follicle. Each follicle cycles through phases:

At any moment, most healthy scalp follicles are in anagen. Daily shed of a modest number of telogen hairs is normal. Diffuse or patterned excess loss needs evaluation beyond “buy this shampoo” — see alopecia types in 8.2.

Average Growth Rate

A standard theory figure: human scalp hair grows about ½ inch (approximately 1.25 cm) per month, or roughly 6 inches per year, with individual variation (age, genetics, health, season). Growth rate is a living follicle property, not something a leave-in conditioner permanently accelerates beyond physiologic limits. Clients often believe products “grow hair faster”; accurate counseling uses cycle and rate facts without overselling.

Bonds and Structure (Bridge to Chemical Services)

Even before permanent wave and color chapters, structure requires bond awareness at recognition level:

• Hydrogen bonds — weak; broken by water and heat; reform on drying/cooling (wet set, blow-dry shaping)
• Salt bonds — side bonds affected by pH changes
• Disulfide bonds — strong covalent bonds; chemical texture services break and reform them for permanent wave/relaxer change

All of these reside primarily in the cortex. Cuticle condition controls how easily chemicals reach those bonds. That is why structure literacy is not optional trivia — it is safety and result prediction.

Structure-to-Service Map (Ohio Exam Thinking)

Study Strategy for Structure & Cycles

• Draw a strand cross-section and label cuticle / cortex / medulla from memory daily until automatic.
• Recite the follicle list: follicle, bulb, papilla, arrector pili, sebaceous gland with one function each.
• Flashcard the cycle triad: anagen grow, catagen transition, telogen rest/shed plus ½ inch/month.
• Connect every structure name to a service: “If the cuticle is damaged, porosity rises…” (leads into 8.2).

Ohio Board theory rewards precise vocabulary. Say dermal papilla, not “the root thing.” Say cortex when the question is about strength and natural color. Say telogen when the question is about resting-phase shed. Structure mastery makes analysis and disorder sections faster to learn because every condition is a change in these same parts and cycles.