The human body is organized from cells, which form tissues, then organs, and finally organ systems. Among the four primary tissue types—connective, muscle, nervous, and epithelial—epithelium plays a fundamental role. It forms widespread layers throughout the body, contributing significantly to its overall function and integrity.
Defining Epithelium
Epithelium, or epithelial tissue, is characterized by densely packed cells with minimal extracellular material. This tissue covers external and internal body surfaces, lines body cavities and hollow organs, and forms the primary tissue in glands. Epithelial cells exhibit polarity, having distinct top (apical), side (lateral), and bottom (basal) surfaces, each with specialized structures and functions. The apical surface often faces an open space or lumen, while the basal surface anchors to underlying connective tissue via the basement membrane.
Epithelial tissues are avascular, meaning they do not contain blood vessels. They receive nutrients and oxygen through diffusion from the blood supply of the underlying connective tissue. Cells within epithelial tissue are connected by specialized cell junctions, including tight junctions, adhering junctions, and gap junctions. These junctions ensure cells are tightly bound, forming an effective barrier and facilitating communication.
Essential Functions of Epithelium
Epithelial tissues perform various functions, with specific roles often depending on their location. A primary function is protection, forming a physical barrier against mechanical damage, chemical exposure, fluid loss, and pathogen invasion. For example, the epidermis, the outermost layer of skin, is epithelial tissue that shields deeper tissues from external threats.
Another function is secretion, where epithelial cells produce and release various substances. Glandular epithelium, for example, is specialized to synthesize and secrete enzymes, hormones, and fluids, such as sweat from sweat glands or digestive enzymes in the small intestine. Epithelium also plays a significant role in absorption, particularly in organs like the small intestine, where it absorbs nutrients and water from digested food into the bloodstream.
Filtration is another function, as seen in the kidneys where epithelial tissue helps filter blood to remove waste products. In the respiratory tract, epithelial cells with cilia filter out dirt and particles from inhaled air. Epithelial tissue also contributes to sensation, with specialized neuroepithelium containing sensory nerve endings that allow for the reception of external stimuli, such as those found in taste buds or the olfactory epithelium.
Diverse Types of Epithelium
Epithelial tissues are classified by the number of cell layers and the shape of the cells in the outermost layer. This classification relates their structure to specific functions and locations. The two main categories are simple epithelia, with a single cell layer, and stratified epithelia, with multiple layers.
Simple epithelia are found where absorption, secretion, and filtration occur, as their single cell layer allows for rapid passage. Simple squamous epithelium, with thin, flattened cells, lines blood vessels and lung air sacs (alveoli), facilitating gas and fluid diffusion. Simple cuboidal epithelium, made of cube-shaped cells with central nuclei, is common in glandular tissues and kidney tubules, functioning in secretion and absorption. Simple columnar epithelium consists of tall, column-shaped cells, often with microvilli or cilia, lining the stomach and intestines for absorption and secretion.
Stratified epithelia, with multiple cell layers, primarily serve protective roles in areas subjected to abrasion or stress. Stratified squamous epithelium, the most common type, has flat surface cells and forms the outer skin layer (epidermis), providing robust protection. Stratified cuboidal epithelium is found in sweat and salivary gland ducts, offering protection. Stratified columnar epithelium, with column-shaped surface cells, is in areas like parts of the male urethra and eye conjunctiva, providing protection and secretion.
A unique type, pseudostratified columnar epithelium, appears to have multiple layers due to varying cell heights and nuclear positions, but all cells contact the basement membrane. It lines the respiratory tract, often with cilia to move mucus. Transitional epithelium, also known as urothelium, is a specialized stratified tissue in the urinary bladder and ureters. Its cells can change shape from cuboidal to flattened, allowing the organ to stretch and recoil without damage.