Epithelia are classified using two primary criteria: the shape of the cells and the number of cell layers. These two features combine to produce a naming system that describes virtually every type of epithelial tissue in the body. A few additional criteria, including keratinization status, apical surface features, and secretory mode, further refine the classification in specific contexts.
Cell Shape: The First Criterion
Every epithelial cell falls into one of three basic shape categories. Squamous cells are flat and sheet-like, much wider than they are tall. Cuboidal cells are roughly cube-shaped, with equal width, height, and depth. Columnar cells are rectangular, taller than they are wide. These shape differences aren’t just cosmetic. They reflect what each tissue is built to do. Flat squamous cells create a thin barrier ideal for rapid diffusion, which is why they line your lung air sacs and blood vessel walls. Cuboidal and columnar cells have more internal volume, making them better suited for absorption and secretion in places like your intestines and kidney tubules.
Number of Cell Layers: The Second Criterion
The second criterion is whether the tissue is one cell thick or several cells thick. A single layer of cells is called simple epithelium. Multiple stacked layers are called stratified epithelium.
Simple epithelia are found where substances need to pass through quickly, such as the lining of blood capillaries or the air sacs of the lungs. Stratified epithelia show up in areas that take a beating, like your skin, mouth, and esophagus, where the extra layers provide a buffer against friction and damage.
There is also a third arrangement called pseudostratified epithelium. “Pseudo” means false. Under a microscope, this tissue looks like it has multiple layers because the cells are irregularly shaped and their nuclei sit at different heights. In reality, every cell touches the basement membrane, making it a single layer. Pseudostratified columnar epithelium lines most of your upper respiratory tract and is usually covered in cilia that sweep mucus and debris out of your airways.
How the Two Criteria Combine Into Names
The naming system pairs layer count with cell shape. Simple squamous epithelium is a single layer of flat cells. Stratified columnar epithelium is multiple layers of tall, rectangular cells. Most combinations you’ll encounter follow this straightforward formula.
One important convention applies to stratified tissues: the name refers to the shape of cells in the outermost layer, not the deeper ones. In stratified squamous epithelium, for example, the surface cells are flat and squamous, but the deeper layers are typically cuboidal. The surface layer determines the name because it reflects the tissue’s primary function at the point of contact.
Keratinization: A Subclassification of Stratified Squamous Epithelium
Stratified squamous epithelium gets one more level of classification: whether it is keratinized or non-keratinized. Keratinized epithelium has a tough outer layer of dead cells filled with high molecular weight keratin proteins. Your skin is the classic example. This keratin layer acts as a waterproof, abrasion-resistant shield.
Non-keratinized stratified squamous epithelium lacks that hardened outer layer and stays moist. It lines your mouth, esophagus, and vagina, places that need protection from friction but also need to stay flexible and lubricated. The distinction between the two types comes down to whether those surface cells have completed the process of filling with keratin and losing their nuclei.
Transitional Epithelium: A Special Category
Transitional epithelium, also called urothelium, doesn’t fit neatly into the standard shape-and-layer system. It lines the urinary bladder, ureters, and parts of the urethra, and its defining feature is the ability to stretch. When the bladder is relaxed, the tissue is five to seven layers thick, with dome-shaped “umbrella cells” on the surface that are often binucleated. When the bladder fills with urine, the tissue thins to just two or three layers as the umbrella cells flatten out, all without tearing or losing function.
This ability to transition between shapes and apparent layer counts is why it gets its own classification rather than being labeled squamous or cuboidal. Tight junctions between the surface cells and a specialized protein coating called uroplakin keep urine from leaking into underlying tissues, regardless of how stretched the lining becomes.
Apical Surface Features
Some epithelial types are further described by what sits on top of the cells. These apical specializations don’t change the core classification, but they’re often mentioned alongside it because they’re so functionally important.
- Cilia are tiny hair-like projections that beat in coordinated waves. Pseudostratified columnar epithelium in the respiratory tract is heavily ciliated, pushing mucus and trapped particles toward the throat.
- Microvilli are microscopic finger-like extensions that dramatically increase surface area. Simple columnar epithelium in the small intestine is packed with microvilli to maximize nutrient absorption.
- Stereocilia are long, immobile extensions found in specific locations like the lining of the epididymis. Despite the name, they function more like extra-long microvilli than true cilia.
When these features are present, you’ll often see them noted in the full tissue description, such as “ciliated pseudostratified columnar epithelium.”
Glandular Epithelium: A Separate Classification System
Not all epithelial tissue covers surfaces. Glandular epithelium forms the secretory portions of glands, and it uses its own set of classification criteria.
The first division is between endocrine glands, which release their products (hormones) directly into the bloodstream, and exocrine glands, which release their products through ducts onto a surface. Your thyroid is endocrine. Your sweat glands are exocrine.
Exocrine glands are then classified by four additional features:
- Shape of the secretory portion: tubular (tube-shaped) or alveolar (rounded, flask-shaped).
- Duct organization: simple (a single unbranched duct) or compound (a branching duct system).
- Type of secretory product: serous (thin, watery, protein-rich) or mucous (thick, slippery, carbohydrate-rich).
- Mode of secretion: merocrine (product is released without any cell damage), apocrine (the top portion of the cell pinches off with the product), or holocrine (the entire cell disintegrates to release its contents, as in oil-producing sebaceous glands).
These glandular criteria operate independently from the surface epithelium classification system, though glandular tissue originates from surface epithelium during development. Together, the two systems cover essentially every epithelial tissue in the human body.