 The Integumentary System is one of the largest systems of the body consisting of the skin, hair, nails, and exocrine glands. The skin is only a few millimeters thick, but is the largest organ in the body. The skin allows the body to sense its external environment by picking up signals for touch, pressure, vibration, temperature, and pain. This versatile covering retains body fluids and prevents harmful substances and microorganisms from entering the body. The Integumentary System also synthesizes vitamin D in the presence of sunlight while protecting against harmful UV radiation. The epidermis is the outermost layer of the skin and is composed of stratified, squamous epithelial tissue. The top cell layers of this tissue are composed of dead, keratinized cells which form the stratum corneum. The deepest layer of cells in the epidermis is the stratum basali where mitosis occurs. Stratum spinosum, stratum granulosum, and stratum lucidum may all be found between the stratum basali layer and the stratum corneum layer of the skin. The dermis is composed of dense, fibrous, irregular connective tissue and is highly vascular. Many appendages of the skin are found in the dermis. There are two kinds of sweat glands. The eccrine sweat glands excrete waste products and help control body temperature through sweat. The opocrine sweat glands are larger and mature at puberty. The sebaceous glands are usually associated with hair follicles and produce oil or sebum to lubricate hair and skin surfaces. The erector pili muscle is composed of a bundle of smooth muscle fibers. This microscopic muscle attaches from the skin surface to the hair follicle at an angle. Pulling on the hair follicle helps to distribute sebum and opocrine sweat onto the skin surface. The erector pili can form goosebumps when contracting to lift the hair shaft upright from the surface of the skin. This movement results in more air being trapped under the hairs to insulate the surface of the body. Hair follicles are epithelial tubes in the skin. Epidermal cells in the base of the follicle are nourished from dermal tissue. Hair cells are keratinized and get their color from melanocytes. About every 10th cell in the basali layer is a melanocyte cell that produces melanin pigment. Melanin is the pigment that gives human skin, hair, and eyes their color and protects skin cells from damage caused by UV radiation. Many sensory receptors receive messages such as pain, touch, pressure, and temperature for protection and to help maintain homeostasis. Under the dermis is the hypodermis consisting of adipose and loose connective tissue. The hypodermis serves as the flexible connection between the skin and the underlying muscle and bones as well as a fat storage area. You have completed this activity, Skin and the Integumentary System.