Epithelial tissue is a layer of densely packed cells that covers the body as well as the internal organs.
Epithelial tissue is one of four basic human tissue types. (The others are muscle, nerve, and connective tissue). It is found in many places in the body. Epithelial tissue consists of specialized cells that cover the exterior of the body (skin) or line internal structures such as blood vessels and the intestine. During development, endothelial tissue arises from all three primary germ layers: ectoderm, mesoderm, and endoderm.
There are many types of epithelial tissue. They are classified by the shape of the cells and whether they are form a single-cell layer or a multiple-cell layer. Simple epithelium forms a sheet only one cell thick. Epithelium lining the blood vessels is an example of simple epithelium. Stratified epithelium forms a layer two or more cells thick. Skin is an example of stratified epithelium.
Epithelial cells come in three shapes: squamous, cuboidal, and columnar. All three shapes can form either simple or stratified epithelium. Squamous epithelium cells are thin and flat with irregular edges and elliptical nuclei. Their width is greater than their height. Simple squamous epithelium lines the lungs and blood vessels and the chest and abdominal cavities. The simple squamous epithelium that lines the blood vessels is given the special name endothelium. It arises from the endoderm during development. The simple squamous epithelium that lines the chest and abdominal cavities is called mesothelium. It develops from the middle germ layer or mesoderm. Stratified squamous epithelium make up the tough, protective barrier of the skin. Skin epithelial cells produce keratin, a protein that stiffens the cells and makes them waterproof and better able to withstand abrasions. More flexible stratified squamous epithelial tissue that does not contain keratin is found in moist surfaces exposed to the environment such as lining the vagina, anus, and mouth.
Cuboidal epithelial cells are shaped like cubes. Their width and height are roughly equal with round nuclei are located at the center of each cell. Simple cuboidal epithelium is found in glands such as the thyroid and the lining of kidney tubules. It also makes up the germinal epithelium that produces eggs in females and sperm in males. Stratified cuboidal epithelium is found in skin glands that secrete oil and sweat and in the ovarian follicles.
Columnar epithelial cells are elongated, roughly rectangular cells that are taller than they are wide. The nuclei in columnar epithelial cells are near the base of the cell. Columnar epithelium lines the stomach, intestine, and gall bladder. Some simple columnar epithelium have fine hair-like outgrowths called cilia. The cilia move in a coordinated way, pushing material such as mucus in a single direction or moving the egg down the Fallopian tubes after ovulation. Stratified columnar epithelium is rare, but is found in the membrane covering the whites of the eyes.
Transitional epithelium is an intermediate type of epithelial tissue found only lining the bladder and the ureter. It is unique in that it is elastic and can stretch as the bladder fills. Glandular epithelium is columnar or cuboidal epithelium that is specialized to synthesize and secrete substances such as wax, hormones, sweat, mucus, milk, and saliva. It is found in a variety of glands.
The type of epithelium (simple or stratified and its shape) is directly related the function it performs. For example, simple epithelium is found in areas where rapid diffusion of nutrients and fluids is required. Stratified epitheliim is found in areas that get greater wear and tear. Epithelial tissue performs many functions in the body. They include:
- Protection. Epithelial tissue acts as a barrier to microorganisms, keeps tissues from drying out, and protects against harmful chemicals and ultraviolet rays of the sun. Protective epithelium has a free surface exposed to the environment (skin, mucous membranes of the throat and vagina). Most, but not all, protective epithelium is stratified squamous epithelium.
- Secretion. Epithelial tissue is specialized to form glands that secrete a variety of hormones, enzymes, oils, and mucus. These glands may secrete their product either into ducts (exocrine glands such as sweat glands) or directly into the bloodstream (endocrine glands that produce hormones such as thyroid hormone).
- Sensation. Many sensory receptors are derived from specialized epithelial cells. These include the rods and cones that respond to light entering the eye, olfactory receptors that detect odors, and taste receptors on the tongue.
- Absorption. Epithelial tissue that lines the small intestine absorbs molecules of digested food.
- Diffusion. Simple epithelial tissue that lines the lungs and blood vessels allows rapid diffusion of gasses, fluid, and nutrients because they form a layer only a single cell thick.
- Movement. Epithelial cells that have cilia move materials in a single direction. For example, cilia move the egg from the ovary through the Fallopian tubes and into the uterus.
Role in human health
Epithelial tissue plays a major role in maintaining health and homeostasis or balance within the body. When protective epithelial tissue breaks down, infections are more likely. When secreting epithelium under produces or overproduces secretions such as hormones, the entire metabolism of the body is affected. Because epithelial tissue is found in so many places in the body, it affects almost every metabolic function, for example, hormone secretion, fluid balance, gas exchange, nutrient absorption, and reproduction.
Common diseases and disorders
Skin diseases are the most obvious type of epithelial tissue disorders. Other epithelial disorders can interfere with the absorption of nutrients, exchange of gasses, and fertility. Common epithelial diseases and disorders include:
- atopic eczema
- atopic dermatitis
- contact dermatitis
- sweating disorders
- yeast infections of the mucous membranes
Germ layer— One of three layers, the endoderm, mesoderm, or ectoderm, that develop in a fertilized egg and give rise to all tissues in the body.
Homeostasis— The chemcial, fluid, and metabolic balance the body must maintain to remain healthy.
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American Academy of Dermatology. P. O. Box 4014, Schaumburg, IL 60166-4014. (847) 330-0230. http://www.aad.org.
Curry, Charles J. "Cellular Organization of the Body." University of Florida Physician Assistant Program. Summer 2005. http://medinfo.ufl.edu/pa/chuck/summer/handouts/epi.htm (October 19, 2005).
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