Skin Histology, Structure, and Disorders
Layers, top to bottom
The epidermis is avascular and renews roughly every 28 days. Know the order: stratum corneum (dead keratin, the barrier), stratum lucidum (palms and soles only), stratum granulosum, stratum spinosum, stratum basale (mitosis, melanocytes, Merkel cells). Langerhans cells, which handle immunity, sit in the spinosum.
The dermis holds collagen and elastin from fibroblasts, hyaluronic acid and other GAGs, capillaries, nerves, sebaceous and sweat glands, and hair follicles. Papillary is the upper, looser layer; reticular is deeper and denser. The hypodermis is subcutaneous fat.
The acid mantle (pH about 4.5–5.5) plus intercellular lipids form the barrier; damage shows as tightness, stinging, and water loss.
Disorders you must sort correctly
- Non-contagious: acne (open and closed comedones, papules, pustules, nodules), rosacea (flushing, telangiectasia, no comedones), seborrheic dermatitis, eczema, psoriasis, melasma, vitiligo, milia (keratin cysts, not acne), keratosis pilaris.
- Contagious - do not treat: herpes simplex, verruca/warts, impetigo, tinea, conjunctivitis, scabies, lice.
- Refer: any mole that is asymmetric, irregular-bordered, multi-colored, larger than 6 mm, or changing (ABCDE); any sore that bleeds or will not heal; crusted or firm lesions (possible basal or squamous cell carcinoma).
Traps: calling milia "acne," treating rosacea as acne, and ignoring a cold sore because the client "always gets them."
Rule of thumb: raised, crusted, bleeding, painful, growing, or asymmetric means stop and refer - estheticians describe, they do not diagnose.