Epithelia are tissues consisting of closely apposed cells without intervening intercellular substances. Epithelia are avascular, but all epithelia "grow" on an underlying layer of vascular connective tissue. The connective tissue and the epithelium are separated by a basement membrane. Epithelium covers all free surfaces of the body. Epithelium also lines the large internal body cavities, where it is termed mesothelium. Furthermore, the internal surfaces of blood and lymph vessels are lined by epithelium, here called endothelium.
Epithelia are classified on the basis of the number of cell layers and the shape of the cells in the surface layer.
- If there is only one layer of cells in the epithelium, it is designated simple.
- If there are two or more layers of cells, it is termed stratified.
- Cells in the surface layer are, as a rule, described according to their height as squamous (scale- or plate-like), cuboidal or columnar.
Simple Epithelia
Simple squamous epithelium
This type is composed of a single layer of flattened, scale- or plate-like cells. It is quite common in the body. The large body cavities and heart, blood vessels and lymph vessels are typically lined by a simple squamous epithelium. The nuclei of the epithelial cells are often flattened or ovoid, i.e. egg-shaped, and they are located close to the centre of the cells.
Simple cuboidal epithelium
Cells appear cuboidal in sections perpendicular to the surface of the epithelium. Viewed from the surface of the epithelium they look rather like small polygons. Simple cuboidal epithelium occurs in small excretory ducts of many glands, the follicles of the thyroid gland, the tubules of the kidney and on the surface of the ovaries.
Simple columnar epithelium
The cells forming a simple columnar epithelium are taller than they are wide. The nuclei of cells within the epithelium are usually located at the same height within the cells - often close to the base of the cells. An example is the simple columnar epithelium which lines the internal surface of the gastrointestinal tract (GIT) from the cardia of the stomach to the rectum.
simple squamous epithelium: any section containing blood vessels, sections of organs which include an outer lining (or serosa) of the organs, or sections of kidney (parietal blades of Bowman's capsules) - H&E, trichrome
simple cuboidal epithelium: sections of ovaries (epithelium lining the surface), thyroid gland (follicles), kidney (tubules) or large glands (e.g. parotid gland) with well preserved small ducts
simple columnar epithelium: sections of the small intestine (duodenum, jejunum or ileum), uterus (uterine glands), liver (large bile ducts) or gall bladder - H&E, trichrome
simple cuboidal epithelium: sections of ovaries (epithelium lining the surface), thyroid gland (follicles), kidney (tubules) or large glands (e.g. parotid gland) with well preserved small ducts
simple columnar epithelium: sections of the small intestine (duodenum, jejunum or ileum), uterus (uterine glands), liver (large bile ducts) or gall bladder - H&E, trichrome
Epithelia
Epithelia are tissues consisting of closely apposed cells without intervening intercellular substances. Epithelia are avascular, but all epithelia "grow" on an underlying layer of vascular connective tissue. The connective tissue and the epithelium are separated by a basement membrane. Epithelium covers all free surfaces of the body. Epithelium also lines the large internal body cavities, where it is termed mesothelium. Furthermore, the internal surfaces of blood and lymph vessels are lined by epithelium, here called endothelium.
Epithelia are classified on the basis of the number of cell layers and the shape of the cells in the surface layer.
- If there is only one layer of cells in the epithelium, it is designated simple.
- If there are two or more layers of cells, it is termed stratified.
- Cells in the surface layer are, as a rule, described according to their height as squamous (scale- or plate-like), cuboidal or columnar.
Simple Epithelia
Simple squamous epithelium
This type is composed of a single layer of flattened, scale- or plate-like cells. It is quite common in the body. The large body cavities and heart, blood vessels and lymph vessels are typically lined by a simple squamous epithelium. The nuclei of the epithelial cells are often flattened or ovoid, i.e. egg-shaped, and they are located close to the centre of the cells.
Simple cuboidal epithelium
Cells appear cuboidal in sections perpendicular to the surface of the epithelium. Viewed from the surface of the epithelium they look rather like small polygons. Simple cuboidal epithelium occurs in small excretory ducts of many glands, the follicles of the thyroid gland, the tubules of the kidney and on the surface of the ovaries.
Simple columnar epithelium
The cells forming a simple columnar epithelium are taller than they are wide. The nuclei of cells within the epithelium are usually located at the same height within the cells - often close to the base of the cells. An example is the simple columnar epithelium which lines the internal surface of the gastrointestinal tract (GIT) from the cardia of the stomach to the rectum.
simple squamous epithelium: any section containing blood vessels, sections of organs which include an outer lining (or serosa) of the organs, or sections of kidney (parietal blades of Bowman's capsules) - H&E, trichrome
simple cuboidal epithelium: sections of ovaries (epithelium lining the surface), thyroid gland (follicles), kidney (tubules) or large glands (e.g. parotid gland) with well preserved small ducts
simple columnar epithelium: sections of the small intestine (duodenum, jejunum or ileum), uterus (uterine glands), liver (large bile ducts) or gall bladder - H&E, trichrome
simple cuboidal epithelium: sections of ovaries (epithelium lining the surface), thyroid gland (follicles), kidney (tubules) or large glands (e.g. parotid gland) with well preserved small ducts
simple columnar epithelium: sections of the small intestine (duodenum, jejunum or ileum), uterus (uterine glands), liver (large bile ducts) or gall bladder - H&E, trichrome
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