The integumentary system consists of the skin and all of its accessories such as sweat glands, oil glands, hair, and nails. Although relatively simple when compared to other organs, our skin is one of the largest organs in surface area and weight. It makes up 7% to 11% of our total body weight and can cover an area of 20 square feet. No other organ is more easily inspected, exposed to infection, disease, or injury than the skin. It covers our entire body and provides many functions, most of which are defensive. It can help regulate body temperature through the use of perspiration and modifying blood flow, contains a complex system of sensory receptors which transmit information regarding touch, pressure, pain, and temperature stimulation to the nervous system, excretes water, salts, and other organic compounds in the form of perspiration, uses UV rays for the synthesis of vitamin D, and the adipose tissue of the hypodermis stores large amounts of lipids. The most significant aspect of the skin, however, is that it forms a physical barrier between our fragile body and our harsh environment and offers protection from UV radiation, microorganisms, dehydration, and mechanical trauma. The skin has two distinct layers. The thin, superficial layer is the epidermis, a thick epithelium. The thicker, deep layer is the dermis, a fibrous connective tissue. Deep to the skin lies a third, fatty layer called the hypodermis. Although the hypodermis is not part of the integumentary system, it shares some of the skin’s functions and is worth noting.

The epidermis is a stratified squamous epithelium and contains four types of cells: keratinocytes, melanocytes, Merkel cells, and Langerhans cells. The most abundant of these four cells are the keratinocytes, whose main role is to produce a tough fibrous protein called keratin, which helps to waterproof the epidermis and gives it its protective properties. Keratinocytes are tightly connected to one another and are created in the deepest part of the epithelium from cells that continuously undergo mitosis. As new cells being produced push cells toward the surface they manufacture the keratin that eventually fills their cytoplasm. These keratinocytes will become dead, flat sacs filled with keratin by the time they reach the surface of the skin. These dead surface cells eventually wear away, only to be replaced by the deeper cells. This constant renewal of the epidermis occurs every 35 to 45 days. Merkel cells serve as receptors for touch. These hemisphere shaped cells are directly connected to a disk-like circular sensory nerve ending and are found in the deepest layer of the epidermis. Melanocytes are spider-shaped cells that produce the dark skin pigment known as melanin. This pigment is produced in membrane-lined granules and is transported through long processes that give the melanocytes their spidery appearance. These processes carry melanin to the keratinocytes where it settles on the superficial side of the keratinocytes. This creates a shield of pigment over the nucleus to protect against harmful UV rays. This form of protection is better known as tanning Langerhans cells are star-shaped macrophages that arise from bone marrow and migrate to the epidermis and help activate the immune system. These cells are easily damaged by exposure to UV radiation.

The epidermis is divided into 5 strata (layers) in thick skin that covers the palms and soles, and 4 strata in the rest of the body. The stratum basale or stratum germinativum (basal layer) is the deepest layer of the epidermis and is tightly attached to the dermis in a wavy borderline. This layer consists of a row of cells that are mainly stem cells, which give rise to keratinocytes. Merkel cells are scattered among the keratinocytes, and are connected to a sensory nerve. Melanocytes make up from 10% to 25% of the basal layer. These cells have many branching processes that are connected to all the keratinocytes. The stratum spinosum (spiny layer) contains several layers of cells, with less frequent cell division than in the basal layer. This layer contains mostly keratinocytes with spiny extensions, but also contain thick bundles of intermediate filaments called tonofilaments. These tonofilaments consist of a protein called pre-keratin which helps in resisting tension. Also among the keratinocytes are Langerhans cells that help activate the immune system. The stratum granulosum (granular layer) is a thin layer consisting of three to five layers of flattened keratinocytes. These cells contain a great amount of tonofilaments, keratohyaline granules, and lamellated granules, which contain a waterproofing glycolipid. Since cells above the granular layer are too far from dermal capillaries they die and the plasma membranes of the cell thicken to make the outer strata the toughest skin region. The stratum corneum (horny layer) is the most external part of the epidermis. This layer is many cells thick and contains dead cells resembling flat sacs filled with keratin. Upon their death lysosome enzymes digested the organelles and nucleus of the keratinocytes. This dry layer is continually shedding off but is immediately replaced. The average person sheds 40 pounds of skin flakes in a lifetime.

The dermis is the second major layer of skin, and consists primarily of connective tissue, which contains a complicated network of sensory nerves and blood vessels and is extremely vulnerable to UV rays. The dermis has two layers: papillary and reticular. The papillary layer is the superficial 20% of the dermis, and is areolar connective tissue. The surface area is increased by projections called dermal papillae. These ridges are responsible for fingerprints. The papillary layer contains loosely organized fibers of collagen. Elastin fibers in the papillary layer help to anchor the epidermis to the dermis. The deep reticular layer beneath the papillary layer accounts for 80% of the dermis and is dense irregular connective tissue. The collagen fibers here are denser and more branched. The reticular layer is responsible for the skin's elasticity, shape, and strength. Fibers of elastin help the skin return to its original form after it has been stretched and collagen gives skin its strength, keeping it from stretching or sagging too much. Mast cells are located near blood vessels and release substances that cause responses such as fever, edema, and pain. Flexure lines are found around joints and places where skin folds often. Small tears in this layer result in the formation of striae, better known as stretch marks. Lines of cleavage (tension lines) indicate the predominant direction of underlying collagen fibers.

Some similarities between the epidermis and the dermis are that both are part of the integumentary system, both are rather simple in structure compared to other anatomical structures, and both are thicker on the palms of the hands and the soles of the feet. Aside from that the two layers are vastly different.