Pseudostratified

In some columnar epithelia, the cells become so crowded that some of the nuclei become displaced and several rows of nuclei appear in sections of the cell sheet, even though each cell is still attached to the basement membrane. In these cases, it is difficult to trace the continuity of different cells from base to apex and the epithelium may appear stratified even though it contains just one layer of cells. Such epithelia are called pseudostratified columnar epithelia. Pseudostratified columnar epithelia are most commonly found along the respiratory airways. These cells contain cilia on their apical surface. Cilia are motile, beating in a synchronous rhythm to move fluid in a constant direction. In the trachea, cilia propel mucous and particles toward the pharynx and out of the airways. Cilia are much longer than microvilli.