Skin Anatomy, Physiology, and Histology
Layers and cells
The skin has three main layers. The epidermis is avascular, keratinized, and renews from the basal layer. It includes keratinocytes for barrier function, melanocytes for pigment, Langerhans cells for immunity, and Merkel cells for sensation. The dermis holds collagen and elastin fibers, fibroblasts, blood vessels, lymphatics, nerves, and appendages: sebaceous glands, sweat glands, and hair follicles. The hypodermis stores fat and anchors the skin.
Functions and turnover
The stratum corneum and acid mantle limit water loss and block irritants; sebum and sweat form the hydrolipid film. Keratinocytes move upward and desquamate over roughly a month, slowing with age. Sebaceous glands are hormone-responsive and absent on palms and soles. The dermis does not regenerate scarlessly; collagen loss shows as laxity and wrinkles.
Worked example
A client scrubs daily with a gritty cleanser and complains of tightness, redness, and shine by noon. The reasoning: over-exfoliation strips the stratum corneum, raising transepidermal water loss and triggering inflammation. Oiliness is not cured by stripping. A barrier-repair plan with a gentle cleanser, moisturizer, and SPF is more appropriate than more scrubbing.
Traps that cost marks
- Placing melanocytes in the dermis; they sit in the basal epidermis.
- Saying the epidermis has blood vessels.
- Confusing sebaceous glands, which produce sebum and drive acne, with eccrine glands, which produce sweat for thermoregulation.
- Assuming thick skin means more resistant everywhere; it refers to the stratum corneum.
- Treating the dermis as disposable. Actives and needling depth matter.