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Columnar Epithelium

The cells of this form of epithelium are of a general columnar or prismatic shape and possess a single nucleus and a cytoplasm that is usually distinctly granular. They are arranged with their long axes parallel to each other, so that their free ends form the surface of the epithelium, while their deeper ends either rest upon the tissues beneath the epithelium or upon other epithelial cells of different shape which form one or more layers between the columnar cells and the underlying tissues. When they rest directly upon the tissues beneath there are usually other epithelial cells of a pyramidal or oval shape which may be regarded as immature cells ready to take the place of such fully developed cells as may become detached or destroyed. The presence of these cells occasions a narrowing of the deep ends of the columnar cells, so that they are not strictly prismatic in form. In cross-section, or when viewed in a direction parallel to their long axes, the cells have a polygonal form due to the lateral pressure they exert upon each other.
The nuclei of the columnar cells are oval, situated nearer the base of the cell than its superficial end with their long axes parallel to those of the cells themselves, and are vesicular in structure with a distinctly reticular arrangement of the chromatin filaments.
Columnar epithelium is found chiefly upon the free surfaces of mucous membranes, but also occurs in some of the secreting glands.
The minute structure of the cells varies somewhat in different situations, but the consideration of these minutiae must be deferred until a description of the structure of the different organs is undertaken in a subsequent chapter.
Ciliated Epithelium
Ciliated epithelium is merely a variety of either columnar or cubical epithelium in which the free ends of the cells are beset with delicate hair-like processes, which execute lashing movements in some one direction.
It is found lining the trachea and bronchi, the cilia here serving to propel toward the larynx such particles of dust as are brought into the respiratory passages by the currents of air during respiration.
Ciliated epithelium also occurs on the lining membranes of the nose and the adjoining bony cavities, the mucous membrane of the uterus and the Fallopian tubes, the vasa efferentia of the testis and a part of the epididymus, the ventricles of the brain (except the fifth), the central canal of the spinal cord, and the ducts of some glands.
The possession of cilia, which are very motile organs, presents a marked departure in specialization from the usual metabolic functions of epithelium. Ciliated epithelium rarely exercises a secretory function, its stock of energy being utilized to produce motion instead of chemical change. But there are secreting varieties of epithelium possessing a "cuticle" which appears to be morphologically analogous to the cilia, but in which the fibrils are less highly developed, probably not motile, and, therefore, functionally not the equivalents of cilia. This cuticle is highly developed in the cells covering the mucous membrane of the intestine.

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