Epithelial tissues provide the body’s first line of protection from physical, chemical, and biological wear and tear. The cells of an epithelium act as gatekeepers of the body controlling permeability and allowing selective transfer of materials across a physical barrier. All substances that enter the body must cross an epithelium. Some epithelia often include structural features that allow the selective transport of molecules and ions across their cell membranes. Many epithelial cells are also capable of secretion. Here we’ll look at some of the different types of epithelial tissue, what they look like and where they can be found in the body.
- There are 4 sub-classifications of epithelial tissue: squamous, cuboidal, columnar and transitional .
- Epithelial tissue line, cover, and protect organs and tissues, but also play a role in secretion and diffusion.
- Single layered epithelial tissue is referred to as simple, while multi-layered tissues are referred to as stratified.
Different Types of Epithelial Tissue
The types of epithelia are classified by two features: the shape of the cell and the number of layers. Epithelia composed of a single layer of cells is called simple where multiple lays of epithelial tissue are called stratified. The table below summarizes the different types of epithelial tissues.
Squamous Epithelial Tissue
Squamous epithelial cells are generally round, flat, and have a small, centrally located nuclei. If you looks at the illustration and microscope image, you’ll notice these cell’s outlines are slightly irregular. Collectively, these cells will fit together to form a covering or lining. Squamous epithelia with cells arranged in a single layer are referred to as simple; epithelia with multiple layers are called stratified. These simple tissues line aveoli in the lungs and blood capillaries. Their single layer allow them to aid in diffusion of gases. Stratified tissues can be found in places like skin or mouth and serve a more protective function.
Cuboidal Epithelial Cells
Cuboidal epithelial cells are cube-shaped cells with a central nucleus. They are most commonly found in a single layer representing the simple epithelia in glandular tissues throughout the body. They aid in the preparation and secretion of substances. However, they are also found in the walls of tubules and ducts of the kidney and liver. The image below just happens to show the cuboidal epithelium of tubules in the kidney.
Columnar Epithelial Cells
Columnar epithelial cells are named by their resemblance to a row of columns. They are taller than they are wide and are most commonly found in a single-layer arrangement. These cells are found lining the organs of the digestive tract. Therefore, a big part of their functionality is absorption of nutrients from the lumen of the intestines. Simple columnar epithelial cells come either ciliated or non-ciliated. Cilia are small hair-like structures and aid epithelial tissues in moving mucus. They’re therefore found in places that contain mucus, like the respiratory tract and fallopian tubes. linuclei of columnar epithelial cells line up at the base of each cell.
Transitional Epithelial Cells
Transitional epithelial cells appear only in the urinary system, primarily in the bladder and ureter. These cells are arranged in a stratified layer. They have the ability of ‘piling up’ on top of each other in a relaxed, empty bladder. As the urinary bladder fills with urine, the epithelial layer unfolds and expands to hold the volume. As a result, the epithelial lining becomes thinner and the tissue transitions from thick to thin.
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- Reynolds FA. Micrograph Images. Berkshire Community College Bioscience Image Library.