The skin is the largest organ in the human body and is part of the integumentary system. The skin is a sensory organ containing many receptors, but is also a protective covering for the body.
Overlaying the tissues of the body is a subcutaneous fatty layer. The skin rests on top of this. The blood vessels and nerve endings extend from the subcutaneous layer into the dermis, a thick inner layer of the skin. The blood vessels terminate in the dermis, as do encapsulated and some free nerve endings, and the sweat glands and hair follicles originate in the dermis.
Between the dermis and the outer layer of the skin, the epidermis, is a basal epithelial cell layer, basal cells, that give rise to the epithelial cells creating the epidermis. In the epidermis are melanocytes, free nerve endings, and immune cells. The melanocytes are responsible for skin pigmentation, producing the dark compound melanin. Vitamin D is also produced in the skin as a chemical reaction response to sunlight.
The skin is a watertight barrier, protecting the tissues from damage, infection, and water loss. The epithelial cells rise to the outer layer as they die, creating a shedding sheath that takes bacteria and foreign particles with it. Sweat and oil production from the glands in the dermis keep the skin moist and release excess minerals and toxins.
The sensory roles of the skin, the cutaneous sense or sense of touch, is “felt” by the nerve endings, providing information about the body’s external environment as well as the skin itself. The free nerve endings sense pain, heat, cold, and intense stimuli depending on their location in the skin. In the epidermis, the nerves sense pain; in the dermis, the nerves sense temperature.
The encapsulated nerve endings sense touch and pressure and are free nerve endings encapsulated by connective tissue. There are different types of encapsulated nerves: Merkel disc and Meissner corpuscles that sense touch, and the Ruffini corpuscle and Pacinian corpuscle that sense pressure. The Pacinian corpuscle also senses vibration. The number of receptors in an area corresponds to the sensitivity of that area. The highest receptor density is found in the hands and face.
Another function of the skin, particularly of keratinizing epithelial cells, is to produce hair and nails, two more protective aspects of the integumentary system, and extensions of the skin itself. Keratin is a tough protein released and deposited as the epithelial cells die. The hair grows out of follicles that have keratinizing cells at their base. Different parts of the body have different densities of follicles with different activities of growth, which differs also between men and women. Hair is thought to have originally been a protective feature for temperature regulation. Similarly, nails occur at the end of the fingers and toes to protect them against damage. Nails are grown from a nailbed as keratin is deposited.
