Skin Sciences: Anatomy, Physiology, and Histology
Layers, from the outside in
The epidermis is avascular and stratified: stratum corneum (dead keratinocytes, the barrier), stratum lucidum (palms and soles only), stratum granulosum, stratum spinosum, and stratum basale, where keratinocytes divide and melanocytes sit. Langerhans cells (immunity) and Merkel cells (sensation) also live in the epidermis. The dermis holds collagen and elastin from fibroblasts, ground substance with glycosaminoglycans and hyaluronic acid, blood vessels, lymphatics, nerves, sebaceous glands, and hair follicles. The hypodermis is subcutaneous fat for insulation and shock absorption. The epidermis has no blood supply - nutrients diffuse up from the dermis, which is why surface cells are dead and why topical products must penetrate the barrier to act.
Function and physiology
Skin protects, senses, thermoregulates (sweat, vasodilation), secretes sebum, excretes small amounts of waste, absorbs select ingredients, and synthesizes vitamin D. The acid mantle (roughly pH 4.5–5.5) and intercellular lipids maintain barrier integrity; over-cleansing or over-exfoliating disrupts it. Cell turnover slows with age, prolonging desquamation.
Rule of thumb: damage you can see as wrinkling and laxity is largely dermal (collagen and elastin), so superficial products and peels improve texture and pigment, not deep structure.
Traps: melanocytes are in the basale, not the corneum; the stratum lucidum exists only on palms and soles; sebaceous glands are attached to hair follicles, eccrine glands are not; the dermis, not the epidermis, contains blood vessels.