Anatomy and Physiology of the Skin
Anatomy and Physiology of the Skin tests the layers, cells, glands and barrier mechanisms that explain why esthetic treatments stay in the epidermis and why certain products sting, peel or irritate.
Layers from outside in
- Epidermis - avascular, keratinized, self-renewing. Strata: basale (mitosis), spinosum, granulosum, corneum; lucidum appears only on palms and soles. Keratinocytes migrate upward in roughly 28 days.
- Dermis - papillary layer (fine capillaries, Meissner corpuscles) and reticular layer (dense collagen and elastin, glands, follicles, nerves). Fibroblasts build collagen and elastin.
- Subcutaneous - adipose tissue for insulation and shock absorption.
Cells and appendages
- Melanocytes in the basal layer produce melanin using tyrosinase and donate it to keratinocytes in melanosomes; this is the skin's main UV defense.
- Langerhans cells are immune sentinels; Merkel cells sense light touch.
- Sebaceous glands empty into follicles, are driven by androgens, and produce sebum that lubricates and waterproofs.
- Eccrine glands cool the body through sweat; apocrine glands in the axillae are scent glands.
Functions and the barrier
The skin provides protection, sensation, thermoregulation, secretion, excretion, vitamin D synthesis and limited absorption. The acid mantle (about pH 4.5–5.5), intercellular lipids and natural moisturizing factor limit water loss (TEWL) and keep irritants out.
Worked example
Why does a 20% glycolic peel sting a client whose cheeks are tight and flaky? Reasoning: flaking signals depleted barrier lipids, so acid penetrates faster and deeper. The correct response is to postpone or reduce the acid, not to add a stronger neutralizing step afterwards.
Traps that cost marks
- Claiming the epidermis contains blood vessels.
- Placing melanocytes in the dermis.
- Confusing sebaceous glands with sweat glands.
- Forgetting that the basal layer, not the corneum, is where cell division happens.