Anatomy and Physiology of the Skin
Structure, outside in
Epidermis - avascular, roughly 0.05–0.1 mm thick, built of keratinocytes. From deep to superficial: basale (mitosis, melanocytes at about 1 in 10 basal cells, Merkel cells for light touch), spinosum (desmosomes, Langerhans immune cells), granulosum (keratohyalin, the water barrier forms here), lucidum (palms and soles only), and corneum (dead, flattened corneocytes surrounded by lipid). Cell turnover averages about 28 days and slows with age.
Dermis - 1–2 mm, about 90% of skin thickness. The papillary layer carries capillaries and Meissner corpuscles; the reticular layer holds dense collagen (roughly 70–80% of dermal dry weight), elastin, glycosaminoglycans that bind water, fibroblasts, sebaceous glands, sweat glands, hair follicles, and nerve endings.
Hypodermis - adipose and connective tissue for insulation, cushioning, and energy storage.
Functions to know
Barrier protection, sensation, thermoregulation (vasodilation and vasoconstriction, sweat), vitamin D synthesis, excretion, and immune surveillance.
Glands and the acid mantle
- Eccrine glands cover the body and produce sweat; apocrine glands sit in axillae and groin.
- Sebaceous glands are dense on face, scalp, chest, and back, and are absent from palms and soles.
- The acid mantle averages about pH 4.5–5.5 and depends on sebum, sweat, and corneocyte lipids.
Pigment
Melanin comes in eumelanin (brown-black) and pheomelanin (red-yellow). Melanocytes transfer pigment to keratinocytes through melanosomes; differences in skin color reflect melanosome size, number, and distribution, not melanocyte count.
Worked example
A client asks why an exfoliating acid stings more on the cheeks than the forehead. Reasoning: the cheeks have a thinner corneum and fewer sebaceous glands, so the barrier is more permeable and the acid penetrates faster - reduce contact time there. Thicker, oilier areas resist penetration and need longer exposure. This is a barrier and thickness question, not a product-strength question.
Traps
Saying the epidermis contains blood vessels (it does not); placing melanocytes in the corneum; claiming sebaceous glands exist on the palms; calling the acid mantle alkaline; confusing dermal collagen with epidermal keratin.