Skin Anatomy, Physiology, and Histology
What the exam tests
This domain asks you to connect structure to function: which layer contains melanocytes, where blood vessels sit, how the barrier works, and why product penetration and sensitivity differ across clients.
Layers and cells
- Epidermis: avascular stratified epithelium. From deep to surface: stratum basale, spinosum, granulosum, lucidum (thick skin only), and corneum.
- Keratinocytes make keratin and form the barrier. Melanocytes in the basale produce melanin. Langerhans cells are immune sentinels. Merkel cells assist touch sensation.
- Dermis: papillary and reticular layers; contains fibroblasts, collagen, elastin, blood vessels, nerves, glands, and hair follicles.
- Hypodermis: adipose and connective tissue for insulation and cushioning.
Physiology
Skin protects, senses, thermoregulates, excretes, secretes, absorbs minimally, and synthesizes vitamin D. The acid mantle is roughly pH 4.5 to 5.5, supporting barrier lipids and limiting pathogens. Sebaceous glands attach to hair follicles; eccrine glands open directly to the surface; apocrine glands open into follicles in axillae and groin.
Worked example
A client asks why an exfoliating acid tingles more on the cheeks than the forehead. Reasoning: the stratum corneum is thinner on the cheeks, so the acid reaches living epidermal cells faster and barrier disruption is more likely. The esthetician should reduce contact time or strength on thinner, drier areas rather than treating the whole face uniformly.
Traps that cost marks
The epidermis has no blood vessels; the dermis does. Melanocytes are in the basale, not the corneum. The hypodermis is not part of the dermis. The acid mantle is not a physical layer, and the stratum lucidum is not present in thin facial skin.