Epithelium is further characterized by several physical features. These include :
- The number of layers of cells: an epithelium with only one layer is referred to as simple. When there are more than one layer, the epithelium is referred to as stratified. A confusing exception to this is pseudostratified epithelium which appears to be more than one cell thick since the nuclei lie at different heights within the cell, but all cells are in contact with the basement membrane.
- Shape of cells at free surface: Shapes of epithelial cells include squamous (flattened), cuboidal and columnar.
- Function of the epithelium
- Surface modifications (if present): Surface modifications include cilia and microvilli. Their presence often depend on the requirements of the tissue location where the epithelial cell resides.