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During embryonic development animal cells differentiate and become integrated into four basic tissue types: epithelial, connective, muscular, and nervous tissues, each an assemblage of similarly specialized cells united in performing specific functions. The basic tissues, each containing extracellular matrix (ECM) as well as cells, associate with one another in the variable proportions and morphologies characteristic of each organ. Table 4–1 summarizes the main features of the basic tissue types.

TABLE 4–1Main characteristics of the four basic types of tissues.

Characteristically, connective tissue contains cells producing very abundant ECM, muscle tissue has elongated cells specialized for contraction and movement, and nervous tissue includes cells with long, fine processes specialized to receive, generate, and transmit nerve impulses. Most organs can be divided into the parenchyma, which is composed of the cells responsible for the organ’s specialized functions, and the stroma, the cells of which play a supporting role in the organ. Except in the brain and spinal cord, the stroma always consists of connective tissue.

Epithelial tissues are composed of closely aggregated polyhedral cells adhering strongly to one another and to a thin layer of ECM, forming cellular sheets that line the cavities of organs and cover the body surface. Epithelia (Gr. epi, upon + thele, nipple) line all external and internal surfaces of the body and all substances that enter or leave an organ must cross this type of tissue.

The principal functions of epithelial tissues include the following:

  • Covering, lining, and protecting surfaces (eg, epidermis)

  • Absorption (eg, the intestinal lining)

  • Secretion (eg, parenchymal cells of glands)

Specific cells of certain epithelia may be contractile (myoepithelial cells) or specialized sensory cells, such as those of taste buds or the olfactory epithelium.


The shapes and dimensions of epithelial cells vary greatly, ranging from tall columnar to cuboidal to low squamous cells. The cells’ major functions generally dictate their size and morphology. Epithelial cell nuclei differ in shape and may be elliptic (oval), spherical, or flattened, with nuclear shape corresponding roughly to cell shape. Columnar cells generally have elongated nuclei, squamous cells have flattened nuclei, and cuboidal or pyramidal cells have more spherical nuclei (Figure 4–1).


Epithelia and adjacent connective tissue.

Cuboidal or pyramidal cells of epithelia generally have spherical nuclei, while ...

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