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Skin is the largest organ of the integumentary system made up of multiple layers of ectodermaltissue, guards the underlying muscles, bones, ligaments and internal organs. Because it interfaces with the environment, skin plays a key role in protecting the body against invasion of pathogens and excessive water loss; it acts as a receptor for the sensations of touch, heat, cold, pressure, and pain. Its other functions are insulation, temperature regulation, and sensation.
Epidermis, “epi” coming from the Greek meaning “over” or “upon’, is the outermost layer of the skin. It forms the waterproof, protective wrap over the body's surface and is made up of stratified squamous epithelium with an underlying basal lamina. The epidermis contains no blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main types of cells which make up the epidermis are Merkel cells, keratinocytes, with melanocytes and Langerhans cells also present. The epidermis can be further subdivided into the following strata (beginning with the outermost layer): corneum, lucidum (only in palms of hands and bottoms of feet), granulosum, spinosum, basale. Cells are formed through mitosis at the basale layer. The daughter cells move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein keratin is inserted. They eveantually reach the corneum and slough off (desquamation). This process is called keratinization and takes place within about 27 days. This keratinized layer of skin is responsible for keeping water in the body and eliminating harmful chemicals and pathogens. The epidermis helps the skin to regulate body temperature.
The layers of the epidermis are:
The stratum germinativum(basal layer) – the cells in this innermost layer multiply continuously to compensate for the constant loss of cells from the surface of the epidermis. These new cells push upward into each succeeding layer, eventually die, and are sloughed off. The process is continuous. Many of the cells of the stratum germinativum are melanocytes, which synthesize melanin. Other cells are keratinocytes, the predominant epidermal cells.
The next layer of epidermis toward the body surface is the stratum spinosum. This layer consists of keratinocytes having a spiny appearance. The spines are points where cells adhere to one another as parts of intercellular junctions called desmosomes. Keratinocytes synthesize the protein keratin, which provides a waterproof layer to the epidermis.
The stratum granulosum(granular layer) is so-called because it contains granules visible in the cytoplasm of the cells, which begin to die in this layer. It may not be present in some areas of thin skin. Langerhans cells related to the immune system are located in the stratum granulosum. The layer contains three to five cell layers, and as the cells die, they are replaced by keratin and keratohyalin.
The stratum lucidum (clear layer) is so-called because of its closely packed, clear cells, and found only in the thicker skin of the soles of the feet and the palms of the hands.
The stratum corneum(horny layer) is composed of flat, lifeless, keratinized cells, which appear as overlapping dry scales making up the outer skin layer. If these scales are unbroken, they can prevent the entrance of microorganisms. About 25 layers of dead and dry squamous cells make up this tissue layer. Dead cells are continuously sloughed off this layer and replaced by new one from the stratum germinativum.
Skin colour is determined by the amount of melanin(skin pigment) in the stratum germinativum layer of the epidermis. Melanin serves as protection by screening ultraviolet rays from harming the underlying tissue. All cells of the epidermis receive nourishment from blood vessels in the dermis. Since sensory receptors are found in the dermis, any heat, cold, pressure, or other stimulus must pass through the epidermis to reach the receptors. However, touch and pain receptors extend into the epidermis and are stimulated more easily than other receptors.
The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbours many mechanoreceptors (nerve endings) that provide the sense of touch and heat. It contains the hair follicles, sweat glands, sebaceous glands, apocrine glands, lymphatic vessels and blood vessels. The blood vessels in the dermis provide nourishment and waste removal from its own cells as well as from the Stratum basale of the epidermis. The dermis is structurally divided into two areas: a superficial area adjacent to the epidermis, called the papillary region, and a deep thicker area known as the reticular region.
The papillary region is composed of loose areolar connective tissue. This is named for its fingerlike projections called papillae that extend toward the epidermis. The papillae provide the dermis with a “bumpy” surface that interdigitates with the epidermis, strengthening the connection between the two layers of skin.
The reticular region lies deep in the papillary region and is usually much thicker. It is composed of dense irregular connective tissue, and receives its name from the dense concentration of collagenous, elastic, and reticular fibres that weave throughout it. These protein fibres give the dermis its properties of strength, extensibility, and elasticity. Also located within the reticular region are the roots of the hair, sebaceous glands, sweat glands, receptors, nails, and blood vessels.
The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying bone and muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body. Another name for the hypodermis is the subcutaneous tissue.