Anatomy and Physiology of the Skin
The skin is the largest organ. The exam tests layers, cells, glands, and how they explain what you see and feel in the treatment room - usually by asking which layer a structure lives in, or which function a product supports.
Layers and what lives there
- Epidermis - avascular, stratified epithelium. Deep to surface: basale (mitosis, melanocytes, Merkel cells), spinosum (Langerhans cells, immunity), granulosum (keratohyalin, water barrier begins), lucidum (palms and soles only), corneum (dead keratinized cells - the barrier you treat).
- Dermis - papillary layer (capillaries, lymph, nerve endings, touch receptors), reticular layer (collagen, elastin, sebaceous glands, hair follicles, sweat glands). Wrinkles, scarring, and elasticity are decided here; esthetic products do not reach it.
- Subcutaneous - fat for insulation and shock absorption.
Functions and barrier chemistry
Protection, limited absorption, secretion (sebum), excretion (sweat), heat regulation, sensation. The acid mantle (roughly pH 4.5–5.5) plus natural moisturizing factor and intercellular lipids hold water in; when the barrier fails, transepidermal water loss rises and skin stings.
Pigment and photoaging
Melanocytes in the basal layer make melanin; more melanin means more protection and a higher Fitzpatrick type. UV thickens the corneum short-term, but long-term it degrades collagen and elastin.
Worked example
A client scrubs daily and reports tightness and redness. Over-exfoliation has stripped corneum lipids and raised water loss. Treatment: gentle cleanser, no scrubs, a ceramide or occlusive moisturizer, and daily SPF.
Traps
Saying the epidermis has blood vessels; placing melanocytes in the dermis; calling the dermis dead tissue; and mixing up sebaceous glands (attached to follicles, sebum) with eccrine glands (everywhere, sweat).