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Despite its complexity, the human body is composed of only four basic types of tissue: epithelial, connective, muscular, and nervous. These tissues, which are formed by cells and molecules of the extracellular matrix, exist not as isolated units but rather in association with one another and in variable proportions, forming different organs and systems of the body. The main characteristics of these basic types of tissue are shown in Table 4–1. Also of great functional importance are the free cells found in body fluids such as blood and lymph.

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Table Graphic Jump Location
Table 4–1. Main characteristics of the four basic types of tissues. 
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Connective tissue is characterized by the abundance of extracellular material produced by its cells; muscle tissue is composed of elongated cells specialized for contraction and movement; and nerve tissue is composed of cells with elongated processes extending from the cell body that have the specialized functions of receiving, generating, and transmitting nerve impulses. Organs can be divided into parenchyma, which is composed of the cells responsible for the main functions typical of the organ, and stroma, which is the supporting tissue. Except in the brain and spinal cord, the stroma is made of connective tissue.

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Epithelial tissues are composed of closely aggregated polyhedral cells with very little extracellular substance. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities.

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The principal functions of epithelial (Gr. epi, upon, + thele, nipple) tissues are:

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  • Covering, lining, and protecting surfaces (eg, skin)
  • Absorption (eg, the intestines)
  • Secretion (eg, the epithelial cells of glands)
  • Contractility (eg, myoepithelial cells).

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Specific cells of certain epithelia are also highly specialized sensory cells, such as those of taste buds or the olfactory epithelium. Because epithelial cells line all external and internal surfaces of the body, everything that enters or leaves the body must cross an epithelial sheet.

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The forms and dimensions of epithelial cells range from high columnar to cuboidal to low squamous cells. Their common polyhedral form results from their close juxtaposition in cellular layers or masses and is similar to what would be observed if a large number of inflated balloons were compressed into a limited space. Epithelial cell nuclei have a distinctive shape, varying from spherical to elongated or elliptic. The nuclear form often corresponds roughly to the cell shape; thus, cuboidal cells have spherical nuclei, and squamous cells have flattened nuclei. The long axis of the nucleus is always parallel ...

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