Anatomy, Physiology, and Histology of Hair and Skin
This domain tests the structures of skin and hair and their functions, plus growth cycles and head anatomy. Questions often ask you to connect a structure to its job rather than simply label a diagram.
Skin and hair
- Skin has the epidermis, dermis, and subcutaneous layer; appendages include the follicle, sebaceous gland, sweat gland, and arrector pili.
- The hair follicle contains the bulb, papilla, and matrix; the papilla supplies blood and nutrients, and the matrix builds the hair.
- Hair has three layers: medulla, cortex, and cuticle; melanin in the cortex gives color.
- Sebum lubricates skin and hair; the arrector pili muscle contracts with cold or emotion and causes goose bumps.
Growth and anatomy
- Anagen is the growth phase and lasts about two to six years; catagen is a transition of about two to three weeks; telogen is a rest phase of about three months.
- Shedding about 50 to 100 hairs a day is normal.
- Skull bones include the frontal, parietal, temporal, occipital, zygomatic, maxilla, and mandible.
- Muscles include the frontalis, temporalis, occipitalis, and masseter; nerves include the supraorbital, supratrochlear, and facial.
Worked example
Why does shaving not make hair thicker? The blade cuts hair at the surface, while the follicle and papilla below determine thickness and growth. Stubble feels coarser because the cut end is blunt, not because the follicle changed.
Traps that cost marks
Confusing epidermis with dermis, saying hair grows from the cuticle, thinking shaving changes growth, and mixing up anagen and telogen.