Epithelial Tissue Definition
Epithelial tissues are thin tissues that cover all the exposed surfaces of the body. They form the external skin, the inner lining of the mouth, digestive tract, secretory glands, the lining of hollow parts of every organ such as the heart, lungs, eyes, ears, the urogenital tract, as well as the ventricular system of the brain and central canals of the spinal cord.
The cells making up epithelia are often closely bound to one another through specialized structures called tight junctions. They are also free from blood vessels and nerves and are supported by a connective tissue called the basement membrane. They have polarity with a distinct basal domain facing the basement membrane and the other apical surface facing the lumen of an organ or the external environment.
Functions of Epithelial Tissue
Epithelial tissue has a number of functions, which include protection against abrasion, radiation damage, chemical stress and invasion by pathogens. A single organ can have different types of epithelial tissue based on the substances to which different surfaces are exposed. Protective tissue tends to be thicker, made of multiple layers of cells and often has inclusions such as keratin to provide mechanical strength and resistance. The skin of most mammals contains layers of thick keratinized dead epithelial cells protecting them against water loss and other stresses. Similarly, the esophagus is also exposed to a wide range of different textures, pH levels and chemical compositions from food and drink. Therefore, it also contains protective epithelium. Due to its involvement in the digestive process, however, it remains non-keratinized, and secretes mucus to smoothen the passage of food.
Alternatively, epithelial tissue can be involved in absorption, secretion and movement of substances. These epithelia are usually thin, containing cilia or microvilli and are often made of one layer of cells. With the exception of the mouth and esophagus, the rest of digestive tract consisting of the stomach, small and large intestine is covered by these kinds of thin epithelia. These cells secrete enzymes and play an important role in selectively absorbing digested food. The small intestine is particularly noted for the presence of microvilli on the epithelium that increase the surface area for absorption. Epithelia in bronchioles of the lungs contain cilia that move mucus and improve immune function. Similar ciliated epithelia in the fallopian tubes move the egg from the ovaries towards the uterus.
Some tissues, such as transitional epithelia, have a specialized structure that allows them to stretch and expand the capacity of the organ. Transitional epithelia are found lining the urinary bladder as well as the ureters and urethra. Small numbers of these cells are found in the urine, as part of the normal sloughing of dead cells. However the presence of a large number of cells from the transitional epithelium or from epithelial cells in the kidney indicate urinary tract infection, high cholesterol, diabetes or kidney disease.
Types of Epithelial Tissue
There are different types of epithelial tissue depending on their function in a particular location. The simplest classification of these tissues is based on the number of cell layers.
- Simple epithelia
- Stratified epithelia
When the epithelium is composed of a single layer of cells, it is called simple epithelial tissue and those containing two or more layers of cells are called stratified epithelial tissues. One particular type is called pseudostratified because a single layer of cells having varying heights gives the appearance of being stratified.
Epithelia can also be classified based on the shape of the cells, giving rise to three types:
- Squamous epithelial tissue: consists of extremely thin cells that resemble the scales of a fish
- Cuboidal epithelial tissue: contains cells that appear square in cross-section but are marginally longer than they are wide
- Columnar epithelial tissue: consists of elongated cell involved in absorption of materials
The number of cell layers and cell types together give rise to 6 different types of epithelial tissue.
- Simple squamous epithelia
- Simple cuboidal epithelia
- Simple columnar epithelia
- Stratified squamous epithelia
- Stratified cuboidal epithelia
- Stratified columnar epithelia
Additionally, there are pseudostratified epithelia and transitional epithelia.