The integumentary system is more than just the skin - it also includes its accessory features, including the hair, nails, and sweat glands. The integument is the protective outer layer of an organism. In the case of humans and other vertebrates, the integument is the skin. The purpose of the outer layer is to protect the underlying tissue from foreign particles, damage, and water loss. Human skin is also responsible for producing Vitamin D and acts as a sensory organ because of the presence of receptors and nerve endings within its layers.
The skin is made of epithelial cells. The outer, thinner layer is the epidermis, which is itself made up of four to five layers. The outermost layer is the stratus corneum, which is made of dead cells that absorb water for moisture and are shed in a constant turnover process. The epithelial cells rise to the outer layer, starting in the stratus basale, as they mature and die. The cells leave behind protein - keratin or melanin depending on the exact type of epithelial cell. The proteins add to the protective features of the skin
Melanin is formed by melanocytes. Melanin converts sunlight (ultraviolet light specifically) into Vitamin D. Melanocytes are also responsible for skin tone and freckles. The pigmentation somewhat protects against damage from the sun's UV rays.
The hair and nails are formed by keratinizing epithelial cells found at the base of hair follicles and in nail beds. A small muscle attached to the base of the hair follicle is responsible for the hair "standing on end" in cases of fright or cold. Hair is thought to have originally been a protective feature for temperature regulation. There are also secretion glands alongside the follicles that release oils, keeping the skin and hair moist. Nails are a stronger appendage that protect the ends of the sensitive fingers and toes from damage.
Below the epidermis is the thicker dermis layer. The skin cells and epithelium receive nutrients and oxygen from blood vessels and capillaries in the dermis. The layer is made mostly of connective tissue. Also found in this layer are the secretion glands, hair follicles, and most of the receptors.
The cutaneous sense, or sense of touch, is "felt" by the receptors, which are nerve endings that may or may not be in a capsule connective tissues. The receptors provide information about the body's external environment and the skin itself. Free nerve endings sense pain, heat, cold, and intense stimuli - the free nerve endings in the epidermis sense pain; the free nerve endings in the dermis sense temperature. Encapsulated nerve endings sense touch and pressure and are often named after the person who discovered their function. Specifically, Merkel discs and Meissner corpuscles sense touch, the Ruffini corpuscle and Pacinian corpuscle sense pressure, and the Pacinian corpuscle senses vibration. The number of receptors in an area corresponds to the sensitivity of that area, with the highest receptor density being found in the hands and face.
Below the skin is the subcutaneous tissue, a fatty cushion separating the dermal layers from the underlying tissues.
Other types of organisms have similar, yet very different, integuments consisting of a cell membrane and the proteins embedded within it in invertebrates and a high hair follicle density or feathers in other mammals and vertebrates. The common denominator is the protection of the organism by anatomy of the outer layer.