Epithelium

Types of epithelium

In biology and medicine, epithelium is a tissue composed of cells that line the cavities and surfaces of structures throughout the body. Many glands are also formed from epithelial tissue. Epithelium lines both the outside (skin) and the inside cavities and lumen of bodies. The outermost layer of our skin is composed of dead stratified squamous, keratinized epithelial cells.

Tissue that line the inside of the mouth, the esophagus, and part of the rectum are composed of nonkeratinized stratified squamous epithelium. Other sufaces that separate body cavities from the outside environment are lined by simple squamous, columnar, or pseudostratified epithelial cells.

Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands. The outer surface of the cornea is covered with fast-growing, easily-regenerated epithelial cells.

Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability.

Endothelium (the inner lining of blood vessels, the heart, and lymphatic vessels) is a specialized form of epithelium. Another type, mesothelium, forms the walls of the pericardium, pleurae, and peritoneum.

In humans, epithelium is classified as a primary body tissue, the other ones being connective tissue, muscle tissue and nervous tissue. Epithelium is often defined by the expression of the adhesion molecule e-cadherin, as opposed to n-cadherin, which is used by cells of the connective tissue.

Classification

Epithelial cells are classified by the following three factors:

Shape (of most superficial cells)

Squamous

Squamous epithelium covers many surfaces in the human body, including the cheeks, inner surfaces of blood vessels, filtration tubules and the major cavities found within the body. It also form the walls of the alveoli in the lungs. The individual cells are smooth, flat and very thin, as well as typically soft. They fit closely together in tissues; providing a smooth, low-friction surface over which fluids can move easily. In the alveoli, the thiness of the cells allows for rapid diffusion of gases between the alveoli and the blood. They are not regarded as being relatively active in terms of metabolism, but are highly key in the diffusion of water and specific substances through certain tissues.

Cuboidal

As the name suggests, these cells have a shape similar to a cube, meaning their width is the same size as their length and height. The Cell nucleus nuclei of these cells are usually located in the center. The cuboidal epithelium forms are the smallest.

Columnar

These cells are taller than they are wide like a cylindrical structure. Simple columnar epithelium is made up of a single layer of cells that are longer than they are wide. The nucleus is also closer to the base of the cell. The small intestine is a tubular organ lined with this type of tissue. Unicellular glands called goblet cells are scattered throughout the simple columnar epithelial cells and secrete mucus. The free surface of the columnar cell has tiny hairlike projections called microvilli. They increase the surface area for absorption.

Transitional

This is a specialized type of epithelium found lining organs that can stretch, such as the urothelium that lines the bladder and ureter of mammals. Since the cells can slide over each other, the appearance of this epithelium depends on whether the organ is distended or contracted: if distended, it appears as if there are only a few layers; when contracted, it appears as if there are several layers.

Stratification

There is clearly more than one layer of cells.

Simple

There is a single layer of cells.

Stratified

More than one layer of cells. The superficial layer is used to classify the layer. Only one layer touches the basal lamina. Stratified cells can usually withstand large amounts of stress.

Pseudostratified

This term is used mainly in one type of classification (pseudostratified columnar epithelium). There is only a single layer of cells, with each cell touching the basal lamina directly. However, without careful observation, the location of the nuclei within the cells may give the impression that the epithelium is stratified. For example, the epithelium lining the bronchi in mammals is pseudostratified epithelium composed of at least three cells types (basal cells with nuclei located close to the basal lamina; ciliated cells with nuclei halfway between the basal lamina and the cellular layer; and mucous cells so full of mucous granules that the nuclei are often difficult to clearly discern but are almost always near the basal lamina).

Specializations

• Keratinized cells contain keratin (a cytoskeletal protein). While keratinized epithelium occurs mainly in the skin, it is also found in the mouth and nose, providing a tough, impermeable barrier.

• Ciliated cells have apical plasma membrane extensions composed of microtubules capable of beating rhythmically to move mucus or other substances through a duct. Cilia are common in the respiratory system and the lining of the oviduct. Additionally, the structure of the cilia found on ciliated epithelial cells is identical to that covering the single-cell organisms collectively known as ciliated prozoans. this cell secretes out of pores and hairs . The have ordor fission. Absorption is also a main key in this.

Examples

Cell junctions

Main article: Cell junction

A cell junction is a structure within a tissue of a multicellular organism. Cell junctions are especially abundant in epithelial tissues. They consist of protein complexes and provide contact between neighbouring cells, between a cell and the extracellular matrix, or they built up the paracellular barrier of epithelia and control the paracellular transport.

Secretory epithelia

As stated above, secretion is one major function of epithelial cells. Glands are formed from the invagination / infolding of epithelial cells and subsequent growth in the underlying connective tissue. There are two major classification of glands: endocrine glands and exocrine glands. Endocrine glands are glands that secrete their product directly onto a surface rather than through a duct. This group contains the glands of the Endocrine system

Embryology

In general, there are epithelial tissues deriving from all of the embryological germ layers:

• from ectoderm (e.g., the epidermis);
• from endoderm (e.g., the lining of the gastrointestinal tract);
• from mesoderm (e.g., the inner linings of body cavities).

However, it is important to note that pathologists do not consider endothelium and mesothelium (both derived from mesoderm) to be true epithelium. This is because such tissues present very different pathology. For that reason, pathologists label cancers in endothelium and mesothelium sarcomas, whereas true epithelial cancers are called carcinomas. Also, the filaments that support these mesoderm-derived tissues are very distinct. Outside of the field of pathology, it is, in general, accepted that the epithelium arises from all three germ layers.

Additional images

References

• Molecular Biology of the Cell, 4th edition, Alberts et al., 2002