Epithelial tissues are widespread throughout the body. They form the covering of all body surfaces, line body cavities and hollow organs, and are the major tissue in glands. They perform a variety of functions that include protection, secretion, absorption, excretion, filtration, diffusion, and sensory reception.
|Cell shape classification||Squamous, cuboidal, columnar|
|Cell layer classification||Simple, stratified, pseudostratified, transitional|
|Main features||Contiguous cells|
Polarity (apical, lateral and basal cell surfaces)
Basement membrane (extracellular matrix)
Supported by connective tissue (lamina propria)
|Specialisations||Apical: microvilli, cilia, stereocilia, modified stereocilia (sensory receptors)|
Secreting: tubular, acinar and tubuloacinar glands
|Functions||Secretion, absorption, transportation, protection, receptor function (sensation)|
Types of epithelial tissue
Epithelial tissue is classified based on the cell shape and the number of cell layers.
- Cell shape: squamous, cuboidal, columnar
- Cell layers: simple (one layer), stratified (multiple layers).
When there are multiple layers, it is the most superficial surface cell layer (apical domain) that defines the shape classification. There are additional subclasses of epithelial classification for cells that have specializations (e.g. ciliated epithelium).
Squamous epithelial cells appear squashed or flattened, like flakes or fish scales. The cells are wider than they are tall, with an oval shaped nucleus. Cuboidal epithelial cells are square shaped cells, they have a similar width to height ratio. The nucleus is large, round and centrally located, and the cytoplasm is rich with organelles.
Columnar epithelial cells have a rectangular or column shape, meaning that they are taller than they are wide. The nucleus is also elongated, having a long oval shape. They have an organelle rich cytoplasm. These cells may have apical projections, such as microvilli or cilia. Columnar epithelial cells can be modified to produce mucous or other secretions, or form specialized sensory receptors.