Skin Anatomy, Histology, and Physiology
Layers and what lives in them
The epidermis is avascular and built from keratinocytes that migrate upward: stratum basale (single dividing layer holding melanocytes), stratum spinosum, stratum granulosum, and the dead, flattened stratum corneum. The dermis holds collagen for strength, elastin for recoil, glycosaminoglycans for hydration, plus nerves, blood vessels, and the pilosebaceous unit. The hypodermis is subcutaneous fat that cushions and insulates. A question asking where pigment is produced is asking about melanocytes in the basal layer, not the dermis.
Structures that drive treatment choices
Sebaceous glands attach to hair follicles and are largest on the T-zone; eccrine glands are everywhere except the lips and nail beds and produce sweat; apocrine glands sit in the axillae and groin. The acid mantle (roughly pH 4.5–5.5), natural moisturizing factor, and intercellular lipids form the barrier. Damage that barrier with over-exfoliation and you get tightness, stinging, and reactive oiliness.
Functions and aging
Skin protects, senses, thermoregulates, excretes, absorbs, and synthesizes vitamin D. Wound healing runs hemostasis, inflammation, proliferation, remodeling. Aging brings slower cell turnover, thinner epidermis, fewer melanocytes, declining collagen and elastin, and drier skin from reduced sebum and gland activity.
Rule of thumb: epidermis = protection, dermis = structure and nourishment, hypodermis = cushion - if a treatment reaches the dermis, it is a medical event, not an esthetic one.