Anatomy, Physiology, and Histology of Hair, Skin, and Nails
Hair
Study the hair shaft from the outside in: cuticle (overlapping scales that protect and that chemicals must open), cortex (largest layer, holds melanin and the disulfide bonds that perming, relaxing, and coloring change), and medulla (innermost core, which may be absent in fine or light hair). The follicle sits in the dermis with a sebaceous gland, arrector pili muscle, and papilla that supplies nutrients.
Growth cycles: anagen (active, roughly 2-6 years, about 90% of scalp hair), catagen (short transitional phase), telogen (resting, then shedding). Average scalp growth is about half an inch per month.
Skin and nails
Skin layers are epidermis (avascular, outermost), dermis (nerves, glands, follicles, vessels), and subcutaneous tissue. Nail structures to name: nail plate, nail bed, matrix (where growth occurs), lunula, eponychium (cuticle), hyponychium, sidewalls, free edge. The nail grows from the matrix, not the lunula.
Also learn the bones of the skull and face (frontal, parietal, temporal, occipital; zygomatic, mandible, maxillae, nasal), the major facial muscles (frontalis, occipitalis, temporalis, masseter, platysma, orbicularis oculi), and the facial nerve (motor) versus the trigeminal nerve (sensory).
Traps
- Mixing up hair cuticle and nail cuticle.
- Assuming a medulla is always present.
- Naming the cuticle as the layer where color and perms act - that is the cortex.
- Confusing the matrix with the lunula when asked where growth happens.
Rule of thumb
Outside in: Cuticle, Cortex, Medulla - protection, then chemistry, then core.