Cells of covering epithelia are organized into one or more layers that cover the surface or line the cavities of an organ. As summarized in Table 1, such epithelia are classified according to the number of cell layers and the cell morphology in the outer layer. Simple epithelia contain one cell layer and stratified epithelia contain two or more layers.
Table1. Common types of covering epithelia.
Based on cell shape, simple epithelia are further classified as squamous (thin cells), cuboidal (cell width and thickness roughly similar), or columnar (cells taller than they are wide). Examples of these epithelial types are shown in Figures 1 through 3.
Fig1. Simple squamous epithelium.
Fig2. Simple cuboidal epithelium.
Fig3. Simple columnar epithelium.
Most stratified epithelia (Figure 4) are classified according to the cell shape of the superficial outer layer(s): squamous, cuboidal, or columnar.
Fig4. Stratified epithelium.
The very thin surface cells of stratified squamous epithelia can be “keratinized” (packed with keratin filaments) or “nonkeratinized” (with relatively sparse keratin). Stratified squamous keratinized epithelium is found mainly in the epidermis of skin, where it helps prevent dehydration from the tissue (Figure 4a). Its cells form many layers, with the less differentiated cuboidal cells near the basement membrane. These cells have many desmosomes and become more irregular in shape and then flatten as they accumulate keratin in the process of keratinization and are moved progressively toward the skin surface, where they become thin, metabolically inactive packets (squames) of keratin lacking nuclei. As discussed with skin, this surface layer of cells helps protect against water loss across this epithelium. Stratified squamous nonkeratinized epithelium (Figures 4b and c) lines moist internal cavities (eg, mouth, esophagus, and vagina) where water loss is not a problem. Here the flattened cells of the surface layer retain their nuclei and most metabolic functions.
Stratified cuboidal and stratified columnar epithelia are both relatively rare. Stratified cuboidal epithelium occurs in the excretory ducts of salivary and sweat glands (Figure 4d). Stratified columnar epithelium is seen in the conjunctiva lining the eyelids, where it is both protective and mucus secreting.
Unique transitional epithelium or urothelium lines much of the urinary tract, extending from the kidneys to the proximal part of the urethra, and is characterized by a superficial layer of large, dome-like cells sometimes called umbrella cells (Figure 5). As discussed further with the urinary system, these cells are specialized to protect underlying tissues from the hypertonic and potentially cytotoxic effects of urine. Importantly, unique morphological features of the cells allow distension of transitional epithelium as the urinary bladder fills.
Fig5. Transitional epithelium or urothelium.
A final morphological type of epithelium is called pseudostratified columnar epithelium (Figure 6). Here tall, irregular cells all are attached to the basement membrane but their nuclei are at different levels and not all cells extend to the free surface, giving a stratified appearance. A good example of pseudostratified columnar epithelium is that lining the upper respiratory tract, where the cells are also heavily ciliated.
Fig6. Pseudostratified epithelium.
