Esthetics Sciences: Skin Anatomy, Physiology, and Chemistry
Layers and cells
The exam connects structure to service. The epidermis has the stratum corneum (dead, keratinized barrier), lucidum (palms and soles), granulosum, spinosum, and basale, where keratinocytes divide and melanocytes reside. The dermis is papillary and reticular, holding collagen, elastin, vessels, nerves, glands, and follicles; the subcutaneous layer insulates. Skin functions include protection, sensation, heat regulation, excretion, secretion, limited absorption, and vitamin D synthesis. The acid mantle (about pH 4.5-5.5) and natural moisturizing factor maintain the barrier; transepidermal water loss rises when the barrier fails. Sebaceous glands attach to follicles; eccrine glands cover the body; apocrine glands are in the axillae and groin.
Chemistry that shows up on the exam
pH is logarithmic: pH 3 is ten times more acidic than pH 4, not twice. Acids donate hydrogen ions (AHA: glycolic, lactic, mandelic); bases accept them (BHA: salicylic). Surfactants have a hydrophilic head and lipophilic tail; emulsions pair water and oil via an emulsifier; humectants bind water, emollients soften, occlusives seal. Antioxidants neutralize free radicals; peptides signal repair. The Fitzpatrick scale (I-VI) predicts UV response and burning risk.
Traps and a rule of thumb
Common traps: placing melanocytes in the wrong layer, confusing sebaceous with sudoriferous glands, and treating pH numbers as linear. Rule of thumb: go deeper for structural change and stay superficial for barrier care - melanin, new cells, and collagen all live below the stratum corneum, so surface products cannot reach them.