Skin

About the Skin

The skin is the largest organ, weighing several kilograms and covering roughly 2 square meters in adults. It has three layers: epidermis, dermis, and hypodermis subcutis. The epidermis has five strata in thick skin: corneum, lucidum, granulosum, spinosum, and basale. It contains keratinocytes producing keratin for waterproofing, melanocytes producing melanin for UV protection, Langerhans cells for immune surveillance, and Merkel cells for touch sensation. The epidermis continually renews itself, with new cells taking several weeks to migrate from the basal layer to the surface and shed. The dermis contains collagen and elastin providing strength and elasticity, blood vessels, nerve endings including Meissner corpuscles light touch, Pacinian corpuscles pressure, Ruffini endings stretch, and free nerve endings pain and temperature, hair follicles, sebaceous glands sebum, sweat glands eccrine thermoregulation and apocrine scent, and nails. The hypodermis contains adipose tissue for insulation, energy storage, and shock absorption. Skin prevents dehydration, infection, and UV damage. Vitamin D synthesis starts in the skin with UV B converting 7 dehydrocholesterol to cholecalciferol. Wound healing involves hemostasis, inflammation, proliferation, and remodeling. Burns are classified by depth: first degree epidermis, second degree dermis blisters, third degree full thickness, fourth degree underlying tissue.