Epithelial tissue, often referred to as epithelium, is one of the four fundamental tissue types found in animals. These tissues are composed of densely packed cells that form continuous sheets, creating coverings and linings throughout the body. Epithelium can be found on the body’s exterior surface, such as the outer layer of the skin, and also lines various internal structures. This includes the lining of hollow organs, internal body cavities, and the inner surfaces of blood vessels. Epithelial tissue serves as a boundary between different environments within the body or between the body and its external surroundings.
Classification of Epithelial Tissues
Epithelial tissues are categorized based on two primary characteristics: the number of cell layers and the shape of the cells. Understanding these classifications helps in recognizing their diverse roles in the body.
One method of classification considers the number of cell layers present. Simple epithelium consists of a single layer of cells, where every cell directly touches the basement membrane, a thin layer separating the epithelium from underlying connective tissue. This single-layer arrangement is commonly found in areas where substances need to pass through easily, such as in absorption or filtration processes. Stratified epithelium is composed of two or more stacked layers of cells, providing enhanced durability. Pseudostratified epithelium appears to have multiple layers due to varying cell heights and nuclei positions, but all cells actually rest on the basement membrane, making it a single layer.
The second classification method is based on the shape of the individual cells. Squamous cells are flat and thin, resembling scales or tiles when viewed from above, with flattened nuclei. Cuboidal cells are roughly cube-shaped, as wide as tall, often with spherical nuclei. Columnar cells are taller than they are wide, resembling columns, with elongated nuclei located near their base. Combinations of these classifications describe specific tissues; for instance, simple squamous epithelium, a single layer of thin cells, is found in lung alveoli, facilitating gas exchange. Stratified squamous epithelium, with multiple protective layers, forms the epidermis of the skin.
The Essential Functions of Epithelia
Epithelial tissues perform specialized functions, each tailored to their specific location and cellular structure. These functions include protection, absorption, secretion, and filtration, all contributing to maintaining the body’s internal environment.
Protection is a primary function, as seen in the skin’s stratified squamous epithelium, which acts as a barrier against physical damage, dehydration, and the entry of pathogens. This multi-layered structure withstands abrasion and environmental stressors, shielding underlying tissues. In contrast, simple epithelia are often involved in absorption, such as the simple columnar epithelium lining the small intestine, which absorbs nutrients from digested food. These cells often feature microvilli, tiny finger-like projections that increase surface area for efficient nutrient uptake.
Secretion is another significant role, where epithelial cells produce and release various substances. For example, the epithelial lining of the respiratory tract secretes mucus, which traps inhaled dust particles and microorganisms, preventing them from reaching the lungs. Epithelial tissues are also involved in filtration, particularly in organs like the kidneys. The simple squamous epithelium in the kidney’s glomeruli filters blood, separating waste products while retaining essential components. The thinness of these single-layered epithelia facilitates the rapid exchange of molecules for both absorption and filtration processes.
How Epithelia Form Glands
Some epithelial cells are modified to produce and release substances, forming glands. Glands develop from epithelial tissue that grows into the underlying connective tissue, and they are broadly categorized based on where they release their secretions.
Exocrine glands retain their connection to the epithelial surface through a duct, which acts as a passageway for their secretions. These glands release substances onto an external surface or into a body cavity that opens to the outside. Examples include sweat glands, which secrete perspiration onto the skin, and salivary glands, which release saliva into the mouth. Mucus, digestive enzymes, and breast milk are other examples of products released by exocrine glands.
Endocrine glands, conversely, lose their ductal connection to the surface during development. They release hormones directly into the surrounding interstitial fluid, from where they diffuse into the bloodstream. This allows hormones to travel throughout the body and affect distant target cells. The thyroid gland, which secretes thyroid hormones regulating metabolism, and the adrenal glands, producing hormones like cortisol, are examples of endocrine glands.
Epithelia in Health and Disease
Epithelial tissues, due to their widespread distribution and continuous cell division, are frequently involved in health conditions, including cancer. Carcinoma is a type of cancer that originates in epithelial cells. It accounts for a substantial majority, approximately 80% to 90%, of all diagnosed cancers in adults. This high incidence is attributed to the constant regeneration and exposure of epithelial cells to environmental factors and mutagens.
Common examples of carcinomas include lung cancer, breast cancer, colon cancer, and prostate cancer, all of which arise from the epithelial linings of these organs or associated glands. Adenocarcinomas develop from glandular epithelial cells that produce fluids or mucus, commonly found in the breast or colon. Squamous cell carcinomas originate from squamous epithelial cells, such as those in the skin or the lining of the respiratory tract.
Beyond cancer, epithelial tissue dysfunction is implicated in other diseases. Celiac disease, an autoimmune disorder triggered by gluten, involves significant damage to the simple columnar epithelium lining the small intestine. Gluten consumption leads to an immune response that damages the intestinal villi, the finger-like projections responsible for nutrient absorption. This epithelial damage results in villous atrophy and compromised barrier integrity, leading to malabsorption and gastrointestinal symptoms.