Science, Tech, Math › Science The Structure of the Integumentary System Share Flipboard Email Print STEVE GSCHMEISSNER / Getty Images Science Biology Anatomy Basics Cell Biology Genetics Organisms Physiology Botany Ecology Chemistry Physics Geology Astronomy Weather & Climate By Regina Bailey Biology Expert B.A., Biology, Emory University A.S., Nursing, Chattahoochee Technical College Regina Bailey is a board-certified registered nurse, science writer and educator. Her work has been featured in "Kaplan AP Biology" and "The Internet for Cellular and Molecular Biologists." our editorial process Regina Bailey Updated August 08, 2019 The integumentary system consists of the largest organ in the body: the skin. This extraordinary organ system protects the internal structures of the body from damage, prevents dehydration, stores fat, and produces vitamins and hormones. It also helps maintain homeostasis within the body by assisting with the regulation of body temperature and water balance. The integumentary system is the body's first line of defense against bacteria, viruses, and other pathogens. It also helps provide protection from harmful ultraviolet radiation. The skin is a sensory organ, too, with receptors for detecting heat and cold, touch, pressure, and pain. Components of the skin include hair, nails, sweat glands, oil glands, blood vessels, lymph vessels, nerves, and muscles. The skin is composed of three layers: Epidermis: The outermost layer of the skin, which is composed of squamous cells. This layer includes two distinct types: thick skin and thin skin.Dermis: The thickest layer of the skin, which lies beneath and supports the epidermis.Hypodermis (subcutis): The innermost layer of the skin, which helps insulate the body and cushion internal organs. Epidermis Don Bliss / National Cancer Institute The outermost layer of the skin, composed of epithelial tissue, is known as the epidermis. It contains squamous cells, or keratinocytes, which synthesize a tough protein called keratin. Keratin is a major component of skin, hair, and nails. Keratinocytes on the surface of the epidermis are dead and are continually shed and replaced by cells from beneath. This layer also contains specialized cells called Langerhans cells that signal to the immune system when there is an infection. This aids in the development of antigen immunity. The innermost layer of the epidermis contains keratinocytes called basal cells. These cells constantly divide to produce new cells that are pushed upward to the layers above. Basal cells become new keratinocytes, which replace the older ones that die and are shed. Within the basal layer are melanin-producing cells known as melanocytes. Melanin is a pigment that helps protect the skin from harmful ultraviolet solar radiation by giving it a brown hue. Also found in the basal layer of the skin are touch receptor cells called Merkel cells. The epidermis is composed of five sublayers: Stratum corneum: The top layer of dead, extremely flat cells. Cell nuclei are not visible.Stratum lucidum: A thin, flattened layer of dead cells. Not visible in thin skin.Stratum granulosum: A layer of rectangular cells that become increasingly flattened as they move to the surface of the epidermis.Stratum spinosum: A layer of polyhedral-shaped cells that flatten as they get closer to the stratum granulosum.Stratum basale: The innermost layer of elongated column-shaped cells. It consists of basal cells that produce new skin cells. The epidermis includes two distinct types of skin: thick skin and thin skin. Thick skin is about 1.5 mm thick and is found only on the palms of the hands and the soles of the feet. The rest of the body is covered by thin skin, the thinnest of which covers the eyelids. Dermis Kilbad/Wikimedia Commons /P ublic Domain The layer beneath the epidermis is the dermis, the thickest layer of the skin. The main cells in the dermis are fibroblasts, which generate connective tissue as well as the extracellular matrix that exists between the epidermis and the dermis. The dermis also contains specialized cells that help regulate temperature, fight infection, store water, and supply blood and nutrients to the skin. Other specialized cells of the dermis help in the detection of sensations and give strength and flexibility to the skin. Components of the dermis include: Blood vessels: Transport oxygen and nutrients to the skin and remove waste products. These vessels also transport vitamin D from the skin to the body.Lymph vessels: Supply lymph (milky fluid containing white blood cells of the immune system) to skin tissue to fight microbes.Sweat glands: Regulate body temperature by transporting water to the skin's surface where it can evaporate to cool down the skin.Sebaceous (oil) glands: Secrete oil that helps waterproof the skin and protect against microbe build-up. These glands are attached to hair follicles.Hair follicles: Tube-shaped cavities that enclose the hair root and provide nourishment to the hair.Sensory receptors: Nerve endings that transmit sensations such as touch, pain, and heat intensity to the brain.Collagen: Generated from dermal fibroblasts, this tough structural protein holds muscles and organs in place and gives strength and form to body tissues.Elastin: Generated from dermal fibroblasts, this rubbery protein provides elasticity and helps make the skin stretchable. It is also found in ligaments, organs, muscles, and artery walls. Hypodermis OpenStax, Anatomy & Physiology/Wikimedia Commons / CC BY Attribution 3.0 The innermost layer of the skin is the hypodermis or subcutis. Composed of fat and loose connective tissue, this layer of the skin insulates the body and cushions and protects internal organs and bones from injury. The hypodermis also connects the skin to underlying tissues through collagen, elastin, and reticular fibers that extend from the dermis. A major component of the hypodermis is a type of specialized connective tissue called adipose tissue that stores excess energy as fat. Adipose tissue consists primarily of cells called adipocytes that are capable of storing fat droplets. Adipocytes swell when fat is being stored and shrink when fat is being used. The storage of fat helps insulate the body and the burning of fat helps generate heat. Areas of the body in which the hypodermis is thick include the buttocks, palms, and soles of the feet. Other components of the hypodermis include blood vessels, lymph vessels, nerves, hair follicles, and white blood cells known as mast cells. Mast cells protect the body against pathogens, heal wounds, and aid in blood vessel formation.