• If an epithelium is one cell thick it is said to be simple (Fig. 2.1)

• If there is more than one layer it is said to be stratified or compound (Fig. 2.1).

• Columnar cells – column-shaped (the height is greater than the width) (Fig. 2.2).

• Simple columnar epithelium – this has tall narrow cells and is one layer thick. Generally, simple columnar epithelium lines organs that have an absorptive function, e.g. the small and large intestines, or a secretory function, e.g. digestive glands (Fig. 2.2).

• Ciliated epithelium – this is a more specialised epithelium consisting of a single layer of column-shaped cells (Figs 2.1, 2.3). The free surface of the cells has tiny hair-like projections called cilia whose function is to ‘waft’ foreign particles along the epithelial surface and out of the body. Ciliated epithelium lines the upper respiratory tract, where it helps to trap solid particles that have been inhaled, preventing them from entering the more distal parts of the respiratory system. The uterine tubes are also lined with ciliated epithelium, which helps to move the fertilised egg along the reproductive tract.

• Stratified epithelium – this is composed of a number of layers of cells and is thicker and tougher than the other types of epithelium. It is found in areas that are subjected to wear and to friction and shearing forces, e.g. the epidermis of the skin (see p. 44). Pseudostratified epithelium (Fig. 2.3) appears to be multilayered because of the irregular positioning of the nuclei but is actually a single layer of cells. This may be found in areas such as the trachea.