Slide #DMS 029. [Trachea, monkey; x-section, PTS]. The lumen of the trachea is lined by pseudostratified columnar epithelium. Note that all the surface cells have cilia at their free (apical) surface. (What function do these cilia serve in the trachea?) Mucus-secreting unicellular gland cells ("goblet cells") are interspersed among the ciliated cells.

The term "pseudostratified" is commonly applied to certain epithelia which, when sectioned, appear to be stratified. However, observation of cells separated by teasing shows that all of the cells are attached to the underlying basement membrane, but only some reach the free surface of the epithelium. This kind of epithelium is most commonly found lining the passages of the respiratory system (e.g., trachea and bronchi).

There are motile apical surface specializations called cillia on the pseudostratified columnar epithelium in the trachea (and in other places such as the oviduct). Scan the slide first at low power. Locate well-stained individual cells or clumps of cells with tufts of cilia. Observe ciliary basal bodies. Review the ultrastructure of cilia (below).

This is a low power view of the trachea. The luminal surface of this organ is covered with a ciliated, pseudostratified columnar epithelium.

This medium power view of the tracheal epithelium reveals its pseudostratified columnar architecture. Nuclei are seen at different levels within the epithelium giving the appearance of a stratified epithelium. If one could tease the cells apart, however, it would be apparent that each cell in this epithelial layer is in contact with the basement membrane. Numerous cilia may be observed extending from the apical surface of these cells.

With oil immersion, one may just resolve individual cilia. Note also the dense eosinophilic line beneath the cilia representing the basal bodies that give rise to each individual cilium.

Study the EM appearance of cilia.

Apical Region of Epithelial cells, showing Cilia cut longitudinally. 1 = Cell Membrane; 2 = Central Microtubules; 3 = Peripheral Microtubules; 4 = Basal Body; 5 = Rootlets; 6 = Microvilli, for comparison of size with cilia. The Densities of some cell junctions can be seen in the upper right quadrant of the photo, along the cell membranes of the two opposing cells. X71,400.

Cross-cuts of cilia, showing the typical 9 + 2 arrangement of microtubules within the cytoplasm: A ring of nine doublets plus two singles in the center. Ordinary cell membrane (arrow) completely covers each cilium. X140,000