In all classification schemes, there are exceptions that fail to fit the common criteria. In naming epithelia, there are one or two special types to remember:
Pseudostratified Epithelia
As the name implies, cells of this type of epithelium appear stratified but in reality they are not. Each cell is in contact with the basement membrane but cells are long and intertwine. Nuclei at varying levels suggests a pseudostratified type. Try to remember that pseudostratified epithelia are typically found with cilia. If you see cilia and the epithelium looks stratified, it probably is pseudostratified!
Transitional Epithelia
In the bladder, urethra, and ureters of the urinary tract a special need exists for an epithelial lining layer adapted for stretch. Transitional epithelium gets its name for its ability to be in transition or change as these structures stretch. In a full bladder, transitional epithelium flattens out and appears to consist of squamous cells. Although mistaken for stratified squamous epithelium, a clear identification of this type is made by noting the large, ovoid surface cells. Transitional and stratified squamous epithelia are the only two epithelia that can possess more than three layers of cells. All other stratified types typically contain 2 to 3 layers of cells.
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