Skin: The body's outer covering, which protects against heat and light, injury, and infection. Skin regulates body temperature and stores water, fat, and vitamin D. The skin, which weighs about 6 pounds, is the body's largest organ. It is made up of two main layers: the epidermis and the dermis. The outer layer of the skin (epidermis) is mostly made up of flat, scale-like cells called squamous cells. Under the squamous cells are round cells called basal cells. The deepest part of the epidermis also contains melanocytes, cells that produce melanin, which gives the skin its color. The inner layer of skin (dermis) contains blood and lymph vessels, hair follicles, and glands that produce sweat, which helps regulate body temperature, and sebum, an oily substance that helps keep the skin from drying out. Sweat and sebum reach the skin's surface through tiny openings called pores.