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Despite its complexity, the human body is made of only four types of tissue: epithelial, connective (CT), muscle, and nerve. The cells of these tissues associate with one another to form organs. The study of tissues is called histology.

Epithelial tissues are avascular and they are supported by a structure known as the basement_membrane. The basement membrane is amorphous, and consists of a combination of secretions from epithelial cells as well as CT cells. The portion of the membrane that is composed of epithelial secretions is termed the basal_lamina. The portion of the membrane composed of CT secretions is called the reticular_lamina. Epithelial tissues have multiple functions, including covering and lining, secretion, absorption, protection, filtration, excretion and sensory reception.

We classify the covering and lining epithelia based on (1) cell shape and (2) the number of layers. There are three basic epithelial shapes, plus several specializations. The three basic shapes of epithelial cells are squamous, cuboidal, and columnar. Epithelial tissues are composed of closely associated polyhedral cells with little intercellular substance. If you view epithelial cells from above, they all appear polyhedral. If you view epithelial cells from the side, they vary in their shape. Therefore, in order to distinguish among the three cell types you must view them in cross-section or longitudinal section. Epithelium that forms a single layer is called simple_epithelium. If the epithelium exists in multiple layers, it is termed stratified_epithelium. If the epithelium appears to be multiple layers when, in fact, it is only a single layer, it is termed pseudostratified_epithelium.

There are two important types of epithelia that are less easily categorized. They are (1) pseudostratified and (2) transitional. Pseudostratified is a special form of columnar epithelium in which the cells vary in height as well as the placement of their nuclei. Because their heights are asymmetrical and their nuclei are located at various levels, they appear to be "stacked". In fact, each cell in the so-called "stack" does touch the basement membrane and therefore there is no stack, just an irregular single (simple epithelium) layer of cells. Pseudostratified epithelium tends to be ciliated.

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