- Key People:
- Friedrich Gustav Jacob Henle
- On the Web:
- National Center for Biotechnology Information - Histology, Epithelial Cell (Oct. 17, 2026)
epithelium
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What is epithelium in the human body?
Epithelium is a layer of tightly bound cells that form continuous sheets, covering surfaces that may interact with foreign substances in both plants and animals. In animals, structures like the central nervous system, sensory surfaces of special sense organs, glands, hair, and nails originate from outgrowths or ingrowths of these surfaces.
Epithelium serves protective, absorptive, or secretory functions. It can produce specialized structures like hairs, nails, and horns, and manufacture chemical materials such as keratin. Epithelial cells vary in type, including columnar, cubical, squamous, irregular, or ciliated cells. These cells form membranes that can be a single layer thick, as in the gastrointestinal tract, or multiple layers thick, as in the epidermis of the skin.
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Where can epithelium be found in our bodies?
Epithelium is a layer of cells that forms continuous sheets, covering surfaces that may contact foreign substances. In animals, the central nervous system, special sense organs, glands, hair, and nails originate from the epithelium.
Epithelial cells can be columnar, cubical, squamous, irregular, or ciliated. Columnar epithelium covers the intestinal tract, while cubical epithelium can be found in glands, ducts, the middle ear, and the brain. Squamous epithelial cells cover the alveoli of the lung, glomeruli, and the kidney capsule. Ciliated epithelium lines the trachea, bronchi, nasal cavities, uterus, oviduct, vas deferens, and epididymis.
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What are the main functions of epithelial tissue?
Epithelial tissue forms continuous sheets that cover surfaces that may contact foreign substances. Epithelium can be protective, absorptive, or secretory. It may also produce specialized structures such as hair and nails.
Different types of epithelial cells include columnar, cubical, squamous, irregular, or ciliated. These cells form membranes that can be a single-cell thick, as in the gastrointestinal tract, or consist of multiple layers, as in the epidermis. Stratified epithelium protects surfaces exposed to friction, with surface cells constantly being replaced by new cells from below. Ciliated epithelium lines respiratory passages and parts of the male and female reproductive systems.
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What are the main types of epithelium?
Epithelium is a layer of cells that forms continuous sheets, covering surfaces that may contact foreign substances in plants and animals.
Different types of epithelial cells include:
* Columnar: Covers the intestinal tract and lines the ducts of many glands.
* Cubical: Found in glands and ducts like the kidney, middle ear, and brain.
* Squamous: Covers the alveoli of the lung, glomeruli, and kidney capsule.
* Ciliated: Lines the trachea, bronchi, nasal cavities, uterus, oviduct, vas deferens, and epididymis.When multiple layers are present, epithelium is further classified as stratified, stratified ciliated, or transitional.
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How does the structure of epithelial cells help them perform their jobs?
Epithelial cells form continuous sheets that cover surfaces that may contact foreign substances. Their structure is closely related to their function, whether protective, absorptive, or secretory.
Different types of epithelial cells exist, including columnar, cubical, and squamous, in single or multiple layers. For example, the microvilli on epithelial cells increase the surface area to facilitate absorption in the small intestine. Junctions like adhering junctions (desmosomes) and tight junctions connect epithelial cells, providing mechanical strength and forming leakproof seals.
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Why is epithelium important for protecting the body and allowing substances to move in and out of organs?
Epithelium is a layer of tightly bound cells that covers surfaces that may contact foreign substances. It protects the body and allows for the movement of substances into and out of organs.
Epithelial membranes can be only one cell thick, as in the gastrointestinal tract, or consist of multiple layers, as in the epidermis of the skin. The skin acts as a mechanical barrier against infection and secretes substances that can kill bacteria. Mucous membranes trap particles with mucus and use cilia to expel them, while also containing protective antibodies.
Epithelium may be protective, absorptive, or secretory.
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epithelium, in anatomy, layer of cells closely bound to one another to form continuous sheets covering surfaces that may come into contact with foreign substances. Epithelium occurs in both plants and animals.
In animals, outgrowths or ingrowths from these surfaces form structures consisting largely or entirely of cells derived from the surface epithelium. In this way the central nervous system, the sensitive surfaces of special sense organs, glands, hair, nails, and other structures all originate. The epithelial cells possess typical microscopical characteristics: the cell outline is clearly marked, and the nucleus large and spherical or ellipsoidal. The cytoplasm of the cell is usually large in amount and often contains large numbers of granules.
Epithelium may be protective, absorptive, or secretory. It may produce special outgrowths (hairs, nails, horns on animals), and manufacture chemical material (e.g., keratin), in which case the whole cell becomes modified. In other instances it contains fat droplets, granules of various kinds, protein, mucin, watery granules, or glycogen. In a typical absorbing cell, granules of material are absorbed. A secreting cell forming specific substances stores them until they are utilized—e.g., fat, in sebaceous and mammary glands; enzymes in salivary and gastric glands; and various excretory substances in the renal epithelium of the kidney.
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The cells forming an epithelial membrane are of various types: columnar, cubical, squamous (flattened), irregular, or ciliated (i.e., with hairlike projections). The membranes formed by these cells may be only one cell thick, as in the major part of the gastrointestinal tract, or consist of several layers, as in the epidermis of the skin. Columnar epithelium covers the intestinal tract from the end of the esophagus to the beginning of the rectum. It also lines the ducts of many glands. A typical form covers the villi (nipple-like projections) of the small intestine. Cubical epithelium is found in many glands and ducts (e.g., the kidney), the middle ear, and the brain. Squamous, or flattened, epithelial cells, very thin and irregular in outline, occur as the covering epithelium of the alveoli of the lung and of the glomeruli and capsule of the kidney. Ciliated epithelium lines the trachea, bronchi of the lungs, parts of the nasal cavities, the uterus and oviduct of the female, and the vas deferens and epididymis of the male. A single projection from the exposed surface of a cell, usually large and long, is called a flagellum. Flagellated cells are common on the surface of many simple animals.
When the cells of an epithelial surface are several layers deep, various epithelial types can be distinguished: stratified, stratified ciliated, and transitional epithelium. In stratified epithelium, which is found in the epithelium of the skin and of many mucous membranes (e.g., mouth, esophagus, rectum, conjunctiva, vagina), the surface cells are flattened, those of the middle layer are polyhedral, and those of the lowest layer are cubical or columnar. This type of epithelium covers surfaces exposed to friction. The surface cells are constantly being rubbed off and are replaced by new cells growing up from below. Hence, the deepest layer is formative, and successive stages upward reveal a gradual transformation into scaly cells that no longer show any sign of being alive.
In stratified ciliated epithelium the superficial cells are ciliated and columnar. This epithelium lines parts of the respiratory passages, the vas deferens, and the epididymis. Transitional epithelium lines the urinary bladder; its appearance depends upon whether the bladder is contracted or distended.