4.1 Hair Structure, Growth & Trichology
Key Takeaways
- The hair shaft has three layers: cuticle (outer protective scales), cortex (strength, elasticity, melanin), and medulla (innermost, absent in fine hair)
- Hair is composed primarily of keratin; disulfide bonds give permanent shape, hydrogen bonds are temporary and broken by water and heat
- The hair growth cycle has three phases: anagen (2-7 years growth), catagen (2-3 weeks transition), telogen (~3 months resting then shedding)
- Scalp hair grows approximately 1-1.5 cm per month; trichology is the specialized study of hair and scalp
- Follicle shape determines hair texture: round follicles produce straight hair, oval produces wavy, flat/elliptical produces curly
Hair Anatomy: The Three Layers of the Hair Shaft
Quick Answer: Hair is composed of keratin, a fibrous protein, arranged in three shaft layers — the protective outer cuticle, the strong pigment-bearing cortex, and the innermost medulla — and is shaped by three bond types: hydrogen, salt, and disulfide.
Every hair strand on the scalp is built from three distinct layers, each with a specific function that affects how the hair responds to chemical services, heat, and moisture.
1. The Cuticle
The cuticle is the outermost protective layer of the hair shaft. It is made up of overlapping translucent scales — like shingles on a roof — that lie flat and point toward the hair tip when hair is healthy. The cuticle:
- Protects the inner cortex from chemical, thermal, and mechanical damage
- Controls moisture movement in and out of the hair
- Reflects light, giving healthy hair its shine
- Opens (lifts) when exposed to alkaline conditions (pH above 7) and closes (seals) under acidic conditions (pH below 7)
When the cuticle is damaged or lifted, hair feels rough, looks dull, and absorbs moisture quickly — a key indicator of high porosity.
2. The Cortex
The cortex is the middle and largest layer, making up roughly 80-90% of the hair shaft's mass. It is composed of long keratin fibers and contains the pigment that gives hair its color. The cortex provides:
- Tensile strength — the ability to resist pulling
- Elasticity — the ability to stretch and return to original length
- Pigment (melanin) — eumelanin (brown/black) and pheomelanin (red/yellow) determine natural hair color
- The disulfide bonds that determine the hair's natural shape (straight, wavy, curly)
Chemical services (color, relaxers, perms) work by penetrating through the cuticle and altering the cortex.
3. The Medulla
The medulla is the innermost core of the hair shaft. It is a soft, spongy structure that may be:
- Present in thick or coarse hair
- Fragmented in medium hair
- Absent in fine hair
The medulla's function is not well understood, but it is generally considered to contribute little to the hair's strength or behavior. Its absence does not affect the hair's health.
Keratin: The Building Block
Hair is composed primarily of keratin, a strong, fibrous protein also found in skin and nails. Keratin's structure is held together by three types of side bonds, which together determine the hair's shape and how it responds to services:
| Bond Type | Strength | Broken By | Reforms When |
|---|---|---|---|
| Hydrogen bonds | Weak (temporary) | Water, heat | Hair dries or cools |
| Salt bonds | Weak (temporary) | Strong acids/alkalis, pH change | pH returns to neutral |
| Disulfide bonds | Strong (permanent) | Chemical relaxers, perms, bleach | New bonds form during neutralization |
Hydrogen bonds are the most numerous — they break every time hair gets wet and reform as it dries, which is why wet hair can be shaped (set) and holds a new form when dried. Disulfide bonds are far fewer but far stronger; breaking them requires chemical services (perm solution, relaxer) and produces permanent changes to the hair's shape.
The Hair Growth Cycle
Hair grows from follicles in the scalp in a recurring cycle of three phases:
| Phase | Name | Duration | What Happens |
|---|---|---|---|
| Anagen | Growth phase | 2-7 years | Active cell division; hair lengthens ~1-1.5 cm/month; ~85-90% of scalp hair is in this phase at any time |
| Catagen | Transition phase | 2-3 weeks | Follicle shrinks and detaches from the blood supply; growth stops |
| Telogen | Resting phase | ~3 months | Hair rests, then sheds; ~10-15% of scalp hair is in this phase |
After telogen, the follicle re-enters anagen and a new hair begins to grow, pushing the old shed hair out. On average, a person sheds 50-100 hairs per day as part of this normal cycle.
Trichology and the Follicle
Trichology is the specialized study of the hair and scalp, including its structure, function, and disorders. A trichologist focuses on conditions of the hair and scalp but does not diagnose or treat disease — that is medical scope.
The hair follicle is the living structure beneath the scalp that produces the hair shaft. Follicle shape largely determines the texture of the hair that grows from it:
- Round follicle produces straight hair
- Oval follicle produces wavy hair
- Flat or elliptical follicle produces curly or coily hair
The follicle's blood supply, nerve endings, and attached sebaceous (oil) gland all influence hair growth and scalp health.
Why This Matters for the Exam
The Red Seal Hairstylist 332A exam expects you to know the layers of the hair shaft, the types of bonds and what breaks them, the phases of the hair growth cycle, and the meaning of trichology. These concepts form the basis for every later topic — analysis, chemical services, scalp disorders, and treatments.
Which layer of the hair shaft provides tensile strength, elasticity, and contains the melanin pigment?
Which type of side bond is broken by water and heat and reforms when the hair dries or cools?