Epithelial tissue covers all internal and external body surfaces, lines cavities and hollow organs, and forms the primary tissue in glands. This tissue acts as the body’s gatekeeper, controlling the selective transfer of materials across a physical barrier. Epithelia are classified based on the number of cell layers and the shape of the cells. The distinction between simple and stratified epithelia, based on layer count, is the most basic structural difference that dictates the tissue’s primary function. Simple epithelia consist of a single layer of cells, while stratified epithelia are composed of two or more stacked layers.
Simple Epithelia: Single Layer Structure and Function
Simple epithelia consist of a single layer of cells, with every cell attached directly to the underlying basement membrane. This thin, single-cell barrier maximizes efficiency for functions involving rapid exchange. Simple epithelia are typically found where absorption, secretion, and filtration are necessary processes.
Simple Squamous Epithelium
Simple squamous epithelium is a single layer of flat, scale-like cells that lines the air sacs of the lungs and capillaries. This minimal barrier allows for the quick passive movement of compounds like oxygen and carbon dioxide through diffusion.
Simple Cuboidal Epithelium
Simple cuboidal epithelium, with its cube-shaped cells, is commonly found in kidney tubules and glands. Here, it facilitates the active secretion and absorption of molecules.
Simple Columnar Epithelium
Simple columnar epithelium consists of a single row of tall, column-shaped cells, often lining the stomach and intestines. These cells are specialized for absorption, frequently possessing microvilli that increase the surface area for nutrient uptake. The single-layer structure allows for swift and effective transport.
Stratified Epithelia: Multi-Layer Structure and Protective Role
Stratified epithelia are composed of multiple stacked layers of cells, with only the deepest layer resting on the basement membrane. This multi-layered arrangement sacrifices transport efficiency for increased durability and resistance. The primary role of this tissue is to provide substantial protection against mechanical abrasion, friction, and chemical stress.
The outer layers are sloughed off and replaced by cells migrating up from the basal layer, making the tissue well-suited for surfaces subjected to wear and tear. The skin’s outermost layer, the epidermis, is stratified squamous epithelium, providing a robust, multi-layered barrier. Apical cells in the skin become filled with keratin, which makes the tissue waterproof and highly resistant to damage.
Non-keratinized stratified squamous epithelium lines moist areas like the mouth and esophagus, protecting them from friction. Stratified cuboidal and columnar epithelia are less common, found in the ducts of some glands, where they offer protection while still allowing for some secretory function.
Classification by Cell Shape: Squamous, Cuboidal, and Columnar Types
Beyond the layer count, all epithelia are classified by the shape of the cells in the single layer (simple epithelia) or the uppermost, or apical, layer (stratified epithelia). There are three fundamental cell shapes used in this naming system: squamous, cuboidal, and columnar.
Squamous Cells
Squamous cells are characteristically flat and thin, resembling scales, with a flattened nucleus. Simple squamous epithelium is ideal for rapid filtration and diffusion due to its minimal thickness. Stratified squamous epithelium utilizes this flat shape in its apical layers to maximize the protective surface area.
Cuboidal Cells
Cuboidal cells are box-like, with their width and height being approximately equal, and their nucleus is typically round and centrally located. Simple cuboidal tissue is highly active in processes like secretion and absorption. Stratified cuboidal epithelium supports a balance between protection and transport in larger ducts.
Columnar Cells
Columnar cells are distinctly taller than they are wide, appearing like columns, often with an elongated nucleus positioned toward the base. Simple columnar epithelium offers space for complex cellular machinery needed for absorption and secretion. Stratified columnar epithelium is the least common type, providing specialized protection in limited locations.
Specialized Epithelia
Two specialized types complicate this classification: pseudostratified columnar and transitional epithelium. Pseudostratified columnar epithelium is categorized as simple because all cells are attached to the basement membrane, despite the illusion of multiple layers created by nuclei at varying heights. This tissue, often ciliated, lines the trachea to secrete mucus and sweep debris out of the airways. Transitional epithelium (urothelium) is a stratified tissue found exclusively in the urinary tract, such as the bladder. Its superficial cells can change shape, transitioning from a rounded appearance when the bladder is empty to a flattened appearance when the organ is stretched and full.