Physiology and Histology of the Skin
Epidermis
The epidermis is avascular and stratified. From the deepest layer up: stratum basale (cell division, melanocytes), stratum spinosum, stratum granulosum, stratum lucidum (palms and soles only), and stratum corneum (dead keratinized cells that desquamate). As keratinocytes migrate upward they flatten and fill with keratin - this is keratinization, and it takes roughly a month. Langerhans cells handle immune surveillance; Merkel cells are touch receptors.
Dermis and subcutaneous layer
The papillary layer holds capillaries and provides nutrition by diffusion. The reticular layer contains collagen (strength), elastin (recoil), glycosaminoglycans (hydration), fibroblasts, sebaceous glands, eccrine and apocrine sweat glands, hair follicles, and nerves. The subcutaneous layer of fat cushions and insulates.
Functions and the acid mantle
The skin protects, senses, excretes and secretes, absorbs selectively, regulates temperature, and helps synthesize vitamin D. The acid mantle sits around pH 4.5–5.5, which controls bacteria and keeps barrier enzymes working.
Traps
Remember the epidermis has no blood vessels - that is why superficial peels do not bleed. Stratum lucidum exists only on palms and soles. Barrier damage shows as tightness, stinging, and flaking rather than "pores opening."
Rule of thumb: anything that strips the acid mantle (harsh scrubs, over-exfoliation) increases sensitivity and infection risk.