Human skin is man’s first line of defense between the internal structures of the body and the external environment. It’s resilient, tough, and virtually impenetrable to toxic compounds, bacteria, and liquid solutions. In addition, it performs numerous vital functions. The human skin synthesizes vitamin D in the presence of ultraviolet light, serves as an organ of sensation and excretion, regulates body temperature, and provides trauma protection. The human skin differs in thickness and other characteristics from one part of the body to another, which usually accounts for skin disease distribution.
The human skin has three primary layers: The subcutaneous tissue, the dermis, and the epidermis. The subcutaneous tissue consists mainly of fat which consists mainly of triglycerides, provides a reserve of calories, shock absorption, insulation, and heat. Both motor and sensory nerves can be found in the subcutaneous tissue and the dermis.
The dermis consists of fibroblasts, two fibrous proteins, and an intervening ground substance. The proteins are elastin, which give it resilience and collagen, which strengthens the skin to prevent it from tearing. The ground substance, which makes the skin compressible and soft, consists primarily of a jellylike substance called mucopolysaccharides. There are two layers that constitute the dermis: The reticular dermis which is the bottom layer, and the papillary dermis, which is the top layer.
The primary function of the epidermis is to produce keratin. This layer is usually thin; however, it is thicker in areas that are subject to constant friction or pressure, such as the palms of the hands and the soles of the feet. The epidermis is comprised of two sub layers: the cellular stratum, where keratin cells are synthesized and the stratum corneum which is an outer layer of keratin.
The deepest layer of the epidermis is the cellular stratum which consists of the stratum granulosum which is made up of cells containing deeply stained granules of keratohyalin, which are usually thought to become the keratin that forms the stratum corneum of the basal layer, which is where mitosis occurs, the fibrils, which is a precursor of keratin, and the stratum spinosum, which is where the cells start to flatten. A human skin cell moves from the basal layer of the cellular stratum to the stratum corneum in about 14 days. Following another 14 days, normal wear and tear on the skin causes it to slough off. The epidermis also contains Langerhans cells, which are involved in a variety of immunologic reactions and melanocytes, which produce the melanin that gives the skin its color.
The human skin is served by a vast arteriovenous network, extending from the dermis and the subcutaneous tissue. These blood vessels provide nutrients and oxygen to sensory nerves, which control pain, temperature, touch, and motor nerves, which control the smooth muscles under the skin, the arterioles, the activities of sweat glands, and appendages of the skin.