The integumentary system includes the largest organ in the human body the skin. The system also includes the projections from the skin, the hair and nails, and the receptors that allow the skin to be a sensory organ. The purpose of the integument is to protect the underlying tissue from foreign particles, damage, and water loss. In humans, the skin is also responsible for producing Vitamin D.
Layers of the Skin
The outer layer of the skin is the epidermis. It is made of epithelial cells, with dead cells on the outermost layer, creating a protective and watertight sheath. Below the epidermis is the dermis. In this second layer are the secretion glands, blood vessels, hair follicles, and most of the receptors. Below this is the subcutaneous tissue, a fatty cushion below the skin separating the dermal layers from the underlying tissues.
Receptors of the Integumentary System
The cutaneous sense, the sense of touch, is “felt” by nerve endings in the skin and provides information about the body’s external environment and the skin itself. There are two types of nerve endings in the skin: free nerve endings and encapsulated nerve endings.
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. A combination of their activation signals intense stimuli, which is why extreme temperatures are both hot/cold and painful.
The encapsulated nerve endings sense touch and pressure and are located in the dermis. The encapsulation is a connective tissue around the nerve ending. 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 and is more in the subcutaneous tissue, deeper than the dermis receptors.
The number of receptors in an area directly correlates to the sensitivity of that area. The highest receptor density occurs in the hands and face.
The hair and nails are formed by keratinizing epithelial cells that release the protein as they die. The hair grows out of follicles that have keratinizing cells at their base. 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. There are also secretion glands alongside the follicles that release oils that keep the skin and hair moist. 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.
Another feature of skin is the ability of melanin, a chemical produced by melanocytes, to convert sunlight to Vitamin D. Melanocytes are also responsible for skin tone and freckles.