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| Etiology • Not applicable. |
| Pathogenesis • Not applicable., |
| Epidemiology • Not applicable. |
| General Gross Description •Grossly, the skin is a membranous structure that covers the body and forms an interface between the external environment and the internal structures. •The level of pigmentation varies among ethnic groups. •It varies in thickness from being extremely thick on the palm and soles to being exceedingly thin, such as over the upper and lower eye lids. •The skin is covered with fine to coarse hairs over the entire body, except for a few areas, including the palms and soles, the skin covering the penis in males and clitoris in females, the inner margins of the labia and the lateral margin of the foot. |
| General Microscopic Description •Histologically, the skin is composed of two distinct regions, the epidermis and the dermis or corium. •The epidermis is a classical stratified squamous epithelium. •The precise structure of the stratified squamous epithelium differs somewhat from region to region. •In extremely thick skin, such as the sole of the foot and the palm of the hand, there are five layers, whereas in other parts of skin, there might only be four. •The constant feature of stratified squamous epithelium is a basal layer or stratum germinativum. •It is a single layer of cuboidal cells with relatively large nuclei and a basophilic cytoplasm and is the only layer where cell division normally occurs in stratified squamous epithelium. •Interspersed among these cells are other cells, such as the neural crest derived melanocytes. •As basal cells divide, they do so asymmetrically, with one daughter cell remaining attached to the basement membrane and the other proceeding up towards the external surface. •The layer immediately above the basement layer is the stratum spinosum, so called because in routine histological sections, the cells appear to be connected to each other by spinous processes. •These processes actually represent desmosomes between adjacent cells. •Shrinkage of cells during the fixation process results in retraction of the cells from each other except at the desmosomes, where because of the tight junctions, the plasma membranes cannot separate. •This gives rise to the appearance of a spinous connection between the two adjacent cells; also termed "intercellular bridges". •The stratum spinosum is normally three to four cell layers thick. •Above the stratum spinosum is the granular cell layer characterized by the accumulation of strongly basophilic staining granules. •In addition, the cytoplasm becomes significantly more prominent and the nucleus-cytoplasm ratio decreases significantly. •The nucleus also is elongated and lies with the long axis parallel to the basement membrane. •The granular cell layer is three to four layers thick in thick skin but may be significantly less in thin skin. •Above the granular cell layer, particularly in thick skin, is a stratum lucidum with further accumulation of cytoplasm and granules and markedly condensed nuclei that lie with their long axes parallel to the basement membrane. •Lying above the stratum lucidum is a stratum corneum, where the cells have essentially lost their nuclei. •The layer is composed of dead, scaly cells, significantly extended in the axis parallel to the basement membrane. •The cells desquamate into the external surface. •The skin is also characterized by the presence of adnexae, including hair follicles, sweat glands of two types (eccrine and apocrine) and sebaceous glands. •The dermis is composed of two areas, the reticular dermis and the papillary dermis. •The papillary dermis projects as round pegs into the overlying epidermis. •On either side of the papillary dermal peg is a projection of epidermis called the rete peg. •While cross-sections give this the appearance of this being a projection into the dermis, it is the cut columns that give rise to this appearance. •The papillary dermis also contains numerous blood vessels and muscles that are attached to the epidermis. •It is composed of Type III collagen. •The reticular dermis is composed primarily of Type I collagen with numerous bundles that lie essentially along the long axis of the basement membrane, but may have other bundles running in other directions. •The reticular dermis also contains numerous sensory structures that can detect pain, pressure and temperature change. |
| Clinical Correlation • Not applicable. |
| References Fawcett DW. Bloom and Fawcett a textbook of histology. 12th ed. New York: Chapman & Hall, 1994, pp Gray H. Gray's Anatomy, 15th Edition. New York: Barnes & Noble, Books, 1995, pp [65] et seq. |
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| Synopsis by: T.V.Rajan, M.D., Ph.D. (T01000M00100)[563] |
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