Skin Anatomy, Physiology, and Disorders
Layers and cells
Skin has three layers: epidermis, dermis, and subcutaneous tissue. The epidermis is avascular and built in strata - corneum, lucidum (palms and soles only), granulosum, spinosum, and basale. Keratinocytes make keratin and migrate upward; melanocytes in the basal layer produce melanin; Langerhans cells are immune sentinels; Merkel cells sense touch. The dermis (papillary and reticular) holds collagen, elastin, blood vessels, nerves, sebaceous glands, eccrine and apocrine sweat glands, and hair follicles with arrector pili muscles.
Barrier and physiology
Healthy skin maintains an acid mantle near pH 4.5–5.5, natural moisturizing factors, and controlled water loss. Barrier damage shows as tightness, flaking, and sensitivity - over-exfoliation is a common cause.
Types and disorders you must recognize
Skin types: normal, dry, oily, combination, sensitive. Conditions and disorders: acne (open/closed comedones, papules, pustules), rosacea, telangiectasia, hyperpigmentation (melasma, post-inflammatory), seborrheic dermatitis, milia, keratosis, and photoaging. Fitzpatrick I–VI predicts sun response and treatment risk.
Contagious - no service
Herpes simplex (cold sores), impetigo, verruca (warts), conjunctivitis, scabies, ringworm/tinea, and pediculosis are contraindications: do not treat; refer.
Traps
- The epidermis has no blood vessels, so it cannot 'bleed' - bleeding means you have gone into the dermis.
- Sebaceous glands are absent on palms and soles.
- Melanocytes sit in the basale layer, not the corneum.
Rule of thumb
Memorize the ABCDE skin-cancer check (Asymmetry, Border, Color, Diameter, Evolving) and refer any suspicious lesion - you analyze, you do not diagnose.