Skin Anatomy, Histology, and Physiology
The three layers
- Epidermis - avascular; five strata from deep to superficial: basale, spinosum, granulosum, lucidum (palms and soles only), corneum. Keratinization completes in the corneum; mitosis happens in the basale.
- Dermis - papillary layer (capillary loops, touch receptors) and reticular layer (collagen, elastin, nerves, glands, follicles).
- Hypodermis (subcutaneous) - fat for insulation and shock absorption.
Cells and appendages
Keratinocytes dominate; melanocytes sit in the basal layer and donate melanin; Langerhans cells are immune; Merkel cells sense touch. Sebaceous glands attach to follicles; eccrine glands cover the body and regulate heat; apocrine glands are in the axillae and groin.
Functions and the barrier
The skin protects, senses, regulates heat, excretes, secretes, and absorbs. The acid mantle (about pH 4.5–5.5) plus corneocytes and intercellular lipids form the barrier; hydration depends on controlling water loss and on natural moisturizing factor.
Traps
- "The epidermis contains blood vessels" - false; it is nourished by diffusion from the dermis.
- The stratum lucidum exists only on palms and soles.
- Melanocytes belong in the basal layer, not the dermis; dermal pigment collections are abnormal.
- Thickest skin: palms and soles. Thinnest: eyelids.
Rule of thumb: products act only on the layers they can reach - most cosmetics work in the corneum and epidermis, so promises about "rebuilding dermal collagen" from a cream should raise a flag.