Skin, outer body covering of an animal. The term skin is commonly used to describe the body covering of any animal but technically refers only to the body covering of vertebrates (animals that have a backbone). The skin has the same basic structure in all vertebrates, including fish, reptiles, birds, and humans and other mammals. This article focuses primarily on human skin. The skin is essential to a person’s survival. It forms a barrier that helps prevent harmful microorganisms and chemicals from entering the body, and it also prevents the loss of life-sustaining body fluids. It protects the vital structures inside the body from injury and from the potentially damaging ultraviolet rays of the sun. The skin also helps regulate body temperature, excretes some waste products, and is an important sensory organ. It contains various types of specialized nerve cells responsible for the sense of touch. The skin is the body’s largest organ—that of an average adult male weighs 4.5 to 5 kg (10 to 11 lb) and measures about 2 sq m (22 sqft) in area. It covers the surface of the body at a thickness of just 1.4 to 4.0 mm (0.06 to 0.16 in). The skin is thickest on areas of the body that regularly rub against objects, such as the palms of the hands and the soles of the feet. Both delicate and resilient, the skin constantly renews itself and has a remarkable ability to repair itself after injury. II. STRUCTURE OF THE SKIN
The skin consists of an outer, protective layer (epidermis) and an inner, living layer (dermis). The top layer of the epidermis is composed of dead cells containing keratin, the horny protein that also makes up hair and nails. The skin is made up of two layers, the epidermis and the dermis. The epidermis, the upper or outer layer of the skin, is a tough, waterproof, protective layer. The dermis, or inner layer, is thicker than the epidermis and gives the skin its strength and elasticity. The two layers of the skin are anchored to one another by a thin but complex layer of tissue, known as the basement membrane. This tissue is composed of a series of elaborately interconnecting molecules that act as ropes and grappling hooks to hold the skin together. Below the dermis is the subcutaneous layer, a layer of tissue composed of protein fibers and adipose tissue (fat). Although not part of the skin itself, the subcutaneous layer contains glands and other skin structures, as well as sensory receptors involved in the sense of touch. A. Epidermis
About 90 percent of the cells in the epidermis are keratinocytes, named because they produce a tough, fibrous protein called keratin. This protein is the main structural protein of the epidermis, and it provides many of the skin’s protective properties. Keratinocytes in the epidermis are arranged in layers, with the youngest cells in the lower layers and the oldest cells in the upper layers. The old keratinocytes at the surface of the skin constantly slough off. Meanwhile, cells in the lower layers of the epidermis divide continually, producing new keratinocytes to replace those that have sloughed off. As keratinocytes push up through the layers of the epidermis, they age and, in the process, produce keratin. By the time the cells reach the uppermost layer of the epidermis, they are dead and completely filled with the tough protein. Healthy epidermis replaces itself in a neatly orchestrated way every month. Scattered among the keratinocytes in the epidermis are melanocytes, cells that produce a dark pigment called melanin. This pigment gives color to the skin and protects it from the sun’s ultraviolet rays. After being produced in the melanocytes, packets of melanin called melanosomes transfer to the keratinocytes. There they are arranged to protect the deoxyribonucleic acid (DNA), or genetic material, of the keratinocytes. All people have roughly the same number of melanocytes. Differences in skin color, such as that between light-skinned people of European...
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