Anatomy and Physiology of the Skin and Body Systems
Layers and what lives where
The epidermis is avascular and keratinized. From the bottom up: stratum basale (mitosis, melanocytes, Merkel cells), stratum spinosum (Langerhans cells), stratum granulosum, and stratum corneum (dead, flattened keratinocytes plus intercellular lipids that form the barrier). The dermis holds collagen and elastin made by fibroblasts, plus blood vessels, nerves, sebaceous glands attached to hair follicles, and eccrine/apocrine sweat glands. Below that is subcutaneous tissue (fat, insulation, shock absorption).
Function and chemistry
Skin protects, senses, regulates temperature, excretes, absorbs minimally, and helps synthesize vitamin D. The acid mantle - a slightly acidic film of sebum, sweat, and lipids around pH 4.5–5.5 - plus normal flora resists pathogens. Barrier damage shows as tightness, flaking, stinging, and increased sensitivity.
Systems that matter to your work
The circulatory and lymphatic systems move fluid and waste (lymph has no pump, so it depends on muscle movement and massage direction); the endocrine system influences sebum and breakouts; the immune system drives allergic reactions and patch-test results.
Traps: melanocytes sit in the basal layer, not the dermis; sebaceous glands are associated with follicles; the stratum corneum is dead cells, not living tissue; collagen is produced by fibroblasts, not by the product you apply.
Rule of thumb: anything a product can realistically change lives at or above the stratum corneum; everything deeper is anatomy you need to know but cannot treat.