Epithelium

Epithelium, or epithelial tissue, is a vital component of animal life. It forms thin, continuous layers of cells that act as protective barriers.

This tissue is one of the four fundamental types of animal tissue. The other three types are connective, muscle, and nervous tissue. Epithelium covers the outside of the body, such as the epidermis of the skin. It also lines internal organs, body cavities, and the inner surfaces of blood vessels. The specific lining of blood vessels is called endothelium. The lining of body cavities, like the pleurae, is called mesothelium.

Epithelial cells perform many complex roles to keep an organism functioning. They are responsible for diffusion, which is the movement of substances through a barrier. They also handle filtration, secretion, and selective absorption. Some cells are even specialized for transcellular transport or germination. All glands in the body are composed of epithelial cells.

These cells are tightly packed into sheets with almost no space between them. This closeness allows them to form effective barriers against the environment.

Cells are classified by their shape and the number of layers they form. There are three principal shapes: squamous, cuboidal, and columnar. Squamous cells are thin, flat plates that look like scales. Cuboidal cells are cube-shaped and appear square in cross-section. Columnar cells are tall and elongated, often being four times as high as they are wide.

When these cells form a single layer, the tissue is called simple epithelium. When they form multiple layers, it is called stratified or compound epithelium.

Simple epithelium is often found where absorption and filtration must occur. Because these layers are thin, they facilitate the easy movement of substances. For example, simple squamous epithelium lines the alveoli in the lungs to allow passive diffusion. Cuboidal epithelium is common in secretory tissues, such as exocrine glands, or absorptive tissues like the pancreas. Columnar epithelium forms the lining of the stomach and intestines. Some columnar cells even have microvilli to increase surface area for absorption, or cilia to move mucus.

Stratified epithelium provides much stronger protection. It is found in areas that must withstand mechanical or chemical stress. In these tissues, the top layers can be lost or abraded without damaging the underlying layers. Some stratified tissues are keratinized. This means the outermost cells are dead and filled with a tough protein called keratin. This makes the skin water-resistant. Other tissues, like the lining of the esophagus, are non-keratinized and remain moist. There is also transitional epithelium, which is found in the bladder. These cells can change shape from cuboidal to squamous as the organ stretches.

Epithelial tissue is unique because it is avascular. This means it lacks its own blood or lymph supply. To survive, the cells must receive nourishment through diffusion. This nourishment comes from the underlying connective tissue. The cells sit on a structure called the basement membrane. This membrane acts as a scaffold for growth and regeneration after an injury. It also acts as a selectively permeable barrier, controlling which substances enter the epithelium.

To maintain their structure, epithelial cells use specialized cell junctions. These are protein complexes that provide contact between neighboring cells. There are five main types: tight junctions, adherens junctions, desmosomes, hemidesmosomes, and gap junctions. Tight junctions fuse the membranes of two cells together. Adherens junctions attach the internal microfilaments of cells. Desmosomes and hemidesmosomes help anchor cells to each other or to the basement membrane. Finally, gap junctions connect the cytoplasm of adjacent cells, allowing them to communicate.