Anatomy, Physiology, and Histology of Hair and Skin
What this domain tests
The structure and function of skin and hair, growth cycles, and how anatomy explains service outcomes.
Core rules
- Hair has three layers: cuticle, outer protective scales; cortex, bulk, pigment, and strength; medulla, central core, not always present.
- The follicle contains the bulb, papilla, blood supply, sebaceous gland, and arrector pili muscle.
- Growth cycles: anagen, active growth; catagen, transition; telogen, resting and shedding. Most scalp hair is in anagen at any time.
- Skin layers: epidermis, dermis with collagen, elastin, nerves, and glands, and subcutaneous layer. The dermis provides strength and elasticity.
Worked example
Why does hair feel rough after lightening? Reasoning: lightener lifts the cuticle and removes some cortex pigment. If the cuticle is raised and porous, moisture escapes and the surface feels rough. That is a structural change, not just a product problem. A lower-pH conditioner can smooth the cuticle temporarily, but it cannot rebuild lost cortex. This explains why a strand test and gentle overlap control matter.
Traps
Saying the medulla is always present; confusing the hair root with the shaft; assuming telogen hair means the whole scalp is inactive; blaming products for damage caused by overlapping chemicals.