Ciliated epithelium is a category of epithelium, a tissue whose cells line the outermost and innermost surfaces of the body. It is named for the presence of cilia, or thin, finger-like hairs, on its surface. These cilia move in one direction in a wavelike pattern, allowing the cells to sweep away debris, direct the flow of particles, and create a current.

Ciliated epithelium can be found in the body’s air passages, including the lungs, trachea, and nose; in the fallopian tubes and uterus; and in the brain. In the airways, ciliated epithelium is necessary to keep dust and debris out of the lungs, because it controls the flow of mucus. Particles in the air are trapped by the mucus in the nose and lungs, and the sweeping motions of the cilia direct the mucus away from the lungs and out of the body. In the fallopian tubes, cilia sweep an ovum down toward the uterus, where uterine cilia position it or sweep it out of the body. Ciliated epithelium in the ventricles of the brain generates circulation of the cerebral fluids.

The cilia on the edges of the cells are comprised of microtubules, which are long protein strands known as filaments that make up a cell’s cytoskeleton and give it structure. These microtubules bind together to form dimers, or pairs, which then associate with each other into a cylindrical shape for more strength. These cylindrical tubes are held together by linking proteins, and they extend up through each of the cilium hairs on the surface. The sweeping movements of the cilia are energy-dependent, and they rely on enzymes that use adenosine triphosphate (ATP) to generate motion. These enzymes connect to the microtubule cylinders and whip them in one direction, allowing for the cilia’s characteristic pulsing waves.

The classification of ciliated epithelium is based on several factors, including location, cell shape, and overall appearance of the tissue as a whole. It is only found on the inner surfaces of the body, so it is part of the endothelium, or internal epithelium. The shape of individual cells is cylindrical, like a column, placing this tissue in the “columnar epithelium” category.

Although the cells exist in a single layer, the cells’ nuclei are positioned unevenly, giving the tissue the appearance of multiple layers, or stratification. This places the tissue in the category of “pseudostratified epithelium,” or epithelium that appears to be stratified but is not. These terms can be combined to form the more specific category of “ciliated pseudostratified columnar epithelium.”