Epithelial cells line the surfaces of organs in the body and function as a protective barrier. Epithelial cells are often associated with the skin (the epidermis). However, the epidermis is only one of the many types of epithelial tissue.
Epithelial cells form the tissue that lines the surfaces of organs and cavities in the body. These cells act as a barrier; anything enters the body must pass through at least one layer of epithelial cells. There are several types of epithelial cells, classified based on their shape and the number of layers that they form.
Where Are Epithelial Cells Found?
Epithelial cells are often associated with the skin. Particularly the outer layer of the skin, which is called the epidermis. However, epithelial cells line many areas of the body, including the cells of the respiratory, reproductive, urinary, circulatory, and gastrointestinal systems. Epithelial cells also form much of the tissue of the glands in the body. Different types of epithelial cells are associated with distinct locations and functions.
General Features of Epithelial Cells
Epithelial cells are usually characterized by the presence of a large nucleus and a polar arrangement. That is, they have a defined structural orientation, with the membranes on either side possessing a distinct set of proteins.
Epithelial cells form epithelial tissues (an epithelium) which line the surfaces of organs. Epithelial cells are usually tightly packed, resulting in ‘cell-rich’ tissue with very little extracellular space between them. They form uniform sheets of cells, often in layers. Despite these similarities, epithelial cells are diverse, and the resulting tissues are variable. This diversity allows them to perform a range of functions.
Types of Epithelial Cells
There are many different types of epithelial cells. They are usually classified in two ways, by their shape and by their formation of layers. In terms of shape, epithelial cells can be categorized as squamous, columnar, or cuboidal. Epithelial tissue can be further divided into ‘simple,’ where there is only one layer of cells, or ‘stratified,’ where there is more than one layer of cells.
Not all the layers of cells have to be of one particular type for the tissue to be considered a stratified epithelium. Some of the deeper layers may contain cells of different shapes. For example, a stratified squamous epithelium could have columnar or cuboidal cells at the deeper layers of the tissue.
Squamous Epithelial Cells
Squamous epithelial cells are characterized by their flat appearance, like tiles on a bathroom floor. They can either form a simple squamous epithelium or a stratified squamous epithelium.
A simple squamous epithelium consists of a single layer of these flat cells. This type of epithelium is leaky and therefore enables materials to pass through it quite easily. As a result, it is most often found in regions of the body where fluid and gas exchange is critical. For example, simple squamous epithelium lines the blood vessels, some cells of the lungs, and the heart.
A stratified squamous epithelium is made of two or more layers of squamous epithelial cells. It is associated with rapid regeneration by cell division; the outer layers can be ‘sloughed’ off and replaced by new cells. As a result, it is particularly suitable for regions that are subject to abrasion, such as the outer layers of the skin, mouth, esophagus, vagina, and anus.
Columnar Epithelial Cells
Columnar epithelial cells are long, vertically arranged cells, with an appearance like bricks standing upright. They can either form a simple columnar epithelium (which can be further subclassified as ciliated or non-ciliated), a stratified columnar epithelium, or a pseudostratified columnar epithelium.
A simple columnar epithelium consists of a single layer of non-ciliated columnar epithelial cells. These cells are most often associated with producing secretions and are therefore found lining the cells of the gastrointestinal tract. There is also a sub-type of columnar epithelial cells that are ciliated. These cells feature an arrangement of hair-like structures on their surface that act to move mucus down a tract. Simple ciliated columnar epithelial tissues are found in the cells of the respiratory system and the female reproductive system.
Stratified columnar epithelia are quite rare, but can be found in part of the eye, and some parts of both the male and female reproductive systems.
There is an additional type of columnar epithelial tissue, a pseudostratified columnar epithelium. This tissue is comprised of only a single layer of cells, but the polar arrangement of these cells (i.e., the positioning of the nuclei) makes the cells look more like those in a stratified epithelium. These cells are involved in secretion and absorption and can be ciliated or non-ciliated. Ciliated pseudostratified columnar epithelium line the respiratory tract. Non-ciliated pseudostratified columnar epithelia line portions of the male reproductive system.
Cuboidal Epithelial Cells
Cuboidal epithelial cells are cuboid in shape, and typically have a central nucleus, distinct from columnar and squamous epithelial cells. They are specialized for secretion and absorption and are frequently found in the cells of glands.
Simple cuboidal epithelia are important for secretion and absorption. The tissue also acts as a protective barrier. Simple cuboidal epithelia are found in regions such as the kidney tubules, the ovaries, and the thyroid gland. Stratified cuboidal epithelial tissues are made up of multiple layers of cuboidal cells. These tissues are most often found in glands. For example, sweat glands and salivary glands.
Functions of Epithelial Cells
Epithelial cells have a variety of functions throughout the body. Epithelial cells are the first cells to detect and respond to stimulation from the environment. They are the first line of defense against physical injury and attack from pathogens such as bacteria and viruses. Therefore, epithelial tissue represents a crucial protective barrier against the outside world.
Epithelial cells are also important in secretion and absorption. They can release specific compounds onto the surface of the epithelium. For example, the cells of the gastrointestinal and respiratory tracts might release mucus, and the cells of glands will secrete specific hormones. Epithelial cells can also absorb nutrients and other compounds that come into contact with the surface.
Epithelial cells are selectively permeable, so they provide a layer of control over what enters the body. This also allows them to regulate homeostasis, where they significantly contribute to the regulation of fluid levels in the body.
Epithelial Cell Abnormalities
Epithelial cell abnormalities have clinical significance in a variety of diseases. As evidence of this, epithelial tumors encompass approximately 90% of all cancers. For example, epithelial cell abnormalities are searched for in pap smears to detect the presence of cells that could develop into cervical cancer.
Epithelial cell abnormalities arise when cells undergo morphological changes that could indicate the presence of cancerous or pre-cancerous cells. Their presence could also identify the existence of a non-cancerous growth such as a cyst, or a human papillomavirus (HPV) infection.
The presence and quantity of epithelial cells are measured in urine analysis. Epithelial cells in urine can indicate the presence of urinary tract infections, kidney disorders, cancers, and many other diseases that epithelial cells.