Epithelial tissue forms protective coverings and linings throughout the body, serving as a boundary between different environments. This versatile tissue covers external surfaces like the skin and lines internal cavities, such as the digestive and respiratory tracts. Beyond its role as a barrier, epithelial tissue also forms glands that produce various secretions. Its functions encompass protection from physical damage and pathogens, absorption of nutrients, secretion of substances, and even sensory reception.
Understanding Epithelial Cell Classifications
Epithelial cells are categorized based on two main features: the number of cell layers and the shape of the individual cells. These classifications help define the specific roles and locations of different epithelial tissues within the body.
When classifying by the number of layers, epithelial tissues can be simple, consisting of a single layer of cells, or stratified, meaning they have multiple layers. Pseudostratified epithelium appears to have multiple layers due to varying cell heights but is actually a single layer with all cells touching the basement membrane. Transitional epithelium is a specialized type that can change shape depending on the degree of stretching.
Cell shape is the second classification criterion, describing the appearance of the cells in the outermost layer for stratified tissues or the single layer for simple tissues. Squamous cells are flat and scale-like, resembling thin tiles. Cuboidal cells are cube-shaped, with approximately equal height and width. Columnar cells are tall and column-shaped, being significantly taller than they are wide.
Simple Epithelial Tissues and Their Locations
Simple squamous epithelium’s thinness allows for efficient passage of substances by diffusion and filtration. It forms the endothelium, lining blood and lymphatic vessels, facilitating gas and nutrient exchange. It also forms the thin walls of lung alveoli, where oxygen and carbon dioxide diffuse between air and blood. Additionally, it constitutes serous membranes (e.g., peritoneum, pleura, pericardium), lining body cavities and covering organs to reduce friction.
Simple cuboidal epithelium is well-suited for secretion and absorption. It lines kidney tubules, filtering waste and reabsorbing substances. This tissue also forms the ducts of many small glands, transporting and modifying glandular secretions. Additionally, simple cuboidal epithelium covers the surface of the ovary.
Simple columnar epithelium is specialized for absorption and the secretion of mucus and enzymes. This tissue lines most of the gastrointestinal tract, absorbing digested nutrients. Goblet cells, which produce mucus, are often interspersed among the columnar cells to protect the lining and facilitate movement of contents. Some simple columnar epithelia are ciliated, with hair-like projections that move substances, such as propelling eggs in the uterine tubes. Non-ciliated forms are present in areas like the gallbladder.
Pseudostratified columnar epithelium often contains goblet cells and cilia. It lines the trachea and most of the upper respiratory tract, where its cilia work to propel mucus, trapping dust and pathogens, away from the lungs. This type of epithelium also occurs in parts of the male urethra.
Stratified and Specialized Epithelial Tissues and Their Locations
Stratified squamous epithelium provides robust protection against abrasion and physical stress. The epidermis, the outermost layer of the skin, is a keratinized stratified squamous epithelium, meaning its surface cells are filled with the tough protein keratin, making it waterproof and highly protective. Non-keratinized stratified squamous epithelium lines moist internal surfaces like the mouth, esophagus, and vagina, offering protection against friction without the added waterproofing of keratin.
Stratified cuboidal epithelium is a relatively rare tissue type, typically composed of two or more layers of cuboidal cells. Its primary function is protection, though it can also be involved in some limited secretion and absorption. This tissue is found lining the ducts of larger glands, such as sweat glands, mammary glands, and salivary glands, providing a more substantial protective lining for these passageways.
Stratified columnar epithelium is also uncommon, featuring multiple layers where the superficial cells are columnar in shape. Similar to stratified cuboidal, its main role is protection, alongside some secretory capabilities. Small amounts of this tissue are located in specific parts of the male urethra and in the large ducts of some glands, where a more robust lining than simple columnar is required, but extensive absorption or secretion is not the primary function.
Transitional epithelium is a specialized stratified tissue that can stretch and flatten, allowing organs to distend without tearing the lining. Its cells appear cuboidal or columnar when the tissue is relaxed but flatten out significantly when stretched. This unique flexibility is particularly suited for organs that undergo significant volume changes. It lines the urinary bladder, ureters, and parts of the urethra, accommodating the varying amounts of urine these organs hold.