The Three Layers of a Hair Strand
Every hair strand is made up of three concentric layers. Knowing each layer's function and location is a reliable exam question category.
- Cuticle: The outermost layer. Composed of overlapping, scale-like cells pointing toward the ends. Protects the inner layers and controls moisture absorption. A raised cuticle means porous, damaged hair; a flat cuticle means smooth, healthy hair.
- Cortex: The middle layer and the largest part of the hair shaft. Contains the melanin granules responsible for natural hair color. The cortex is where all chemical services (color, perms, relaxers) work. It determines hair's strength, elasticity, and texture.
- Medulla: The innermost layer, sometimes called the pith. Present in coarse hair; often absent in fine or very light hair. Its function in humans is not fully understood and is rarely chemically relevant in salon services.
Chemical services (perms, relaxers, color) act on the cortex. The cuticle must be opened (by alkaline pH) to allow chemicals to reach the cortex. This is why conditioners close the cuticle to lock in results.
Hair Follicle Anatomy
The follicle is the tube-shaped pocket in the skin from which the hair grows. Several structures attach to or surround it:
- Dermal papilla: Located at the base of the follicle. Contains blood vessels that supply nutrients and oxygen to the growing hair. It is the control center of hair growth.
- Sebaceous gland: Attached to the follicle wall. Secretes sebum (the skin's natural oil) that lubricates the hair shaft and scalp. Overactive sebaceous glands produce oily scalp conditions.
- Arrector pili muscle: A small muscle attached to the follicle. Contracts in response to cold or fear, causing "goosebumps" and making the hair stand upright.
- Bulb: The rounded base of the follicle that surrounds the dermal papilla. Contains the actively dividing cells that form the hair shaft.
The Three Phases of Hair Growth
Hair grows in a cycle with three distinct phases. Exam questions often test the names, order, and duration of each phase.
| Phase | Name | Duration | What Happens |
|---|---|---|---|
| 1 | Anagen | 2 to 7 years | Active growth phase. Cells in the bulb divide rapidly. Hair grows approximately 1/2 inch per month. |
| 2 | Catagen | 2 to 3 weeks | Transition phase. Cell division stops. The follicle shrinks and detaches from the dermal papilla. |
| 3 | Telogen | 3 to 4 months | Resting and shedding phase. The old hair is pushed out as a new anagen hair begins forming beneath it. |
At any given time, approximately 85 to 90% of scalp hairs are in the anagen phase. Only about 10 to 15% are in telogen. Losing 50 to 100 hairs per day is considered normal because of ongoing telogen shedding.
Hair Texture and Density
Texture refers to the diameter of an individual hair strand (fine, medium, coarse). Density refers to the number of hairs per square inch on the scalp. These are separate measurements and are both assessed during a client consultation before any chemical service.
Students often confuse texture with density. A client can have fine-textured hair with high density (many thin strands), or coarse-textured hair with low density (few thick strands). Always assess both before formulating chemical services.