Epithelial tissue, or epithelium, forms continuous sheets of tightly packed cells that function as selective barriers throughout the body. These tissues cover all exposed body surfaces, like the skin, and line internal cavities, such as the digestive and respiratory tracts. The primary functions of epithelium are to provide protection, control permeability, and facilitate absorption or secretion. Scientists use a precise method to classify them based on their visible structural characteristics, which provides a shorthand for understanding a tissue’s function simply by knowing its name.
The Primary Rule Layer Count
The initial step in classifying epithelial tissue is determining the number of cell layers present. This layer count dictates the tissue’s primary role, distinguishing between tissues built for substance transfer and those designed for durability. Simple Epithelium consists of a single layer of cells where every cell touches the underlying basement membrane.
Simple epithelia are found where rapid material exchange is necessary, such as the air sacs of the lungs or kidney tubules, allowing for quick filtration, diffusion, and absorption. Conversely, Stratified Epithelium is composed of multiple stacked layers of cells, with only the bottom layer touching the basement membrane.
This multi-layered structure is thicker, making it highly effective at resisting physical and chemical stress. Stratified epithelia, such as the epidermis of the skin, are primarily dedicated to protection. When naming a stratified tissue, the classification is based exclusively on the shape of the cells in the most superficial, or apical, layer.
The Secondary Rule Cell Shape
Once the layer count is established, the next step involves observing the shape of the cells in the most apical layer. Three basic cell shapes combine with the layer count to generate the full name of the tissue, such as simple squamous or stratified cuboidal.
The thinnest cell shape is Squamous, describing cells that are flattened and scale-like with a flattened nucleus. Squamous epithelia are ideal for surfaces where substances need to pass through quickly, such as the lining of blood vessels or lung alveoli.
Cuboidal cells are roughly cube-shaped, meaning they are about as wide as they are tall. These cells feature a central, spherical nucleus and are involved in secretion and absorption in structures like kidney tubules and small glandular ducts.
Columnar cells are taller than they are wide, resembling columns. The nucleus is elongated and usually positioned closer to the basal surface of the cell. This type of epithelium is often found lining the digestive tract, where its height accommodates machinery for complex absorption and secretion functions.
Specialized and Modified Epithelia
Some epithelial tissues possess unique structural modifications that alter their appearance and do not fit neatly into the simple two-part naming matrix.
Pseudostratified Columnar Epithelium appears to have multiple layers because the nuclei are positioned at varying heights within the cells. Despite this deceptive appearance, it is a true simple epithelium because every cell maintains contact with the underlying basement membrane. A common example lines the trachea and upper respiratory tract, where it often features hair-like projections called cilia on its apical surface. These cilia move a surface layer of mucus, trapping and eliminating dust and foreign particles from the airways.
Transitional Epithelium, also known as urothelium, is a specialized stratified tissue found exclusively in the urinary system, lining organs like the urinary bladder and ureters. It requires a unique ability to adapt to changes in volume. When the bladder is relaxed, the apical cells appear large and dome-shaped. As the organ fills, the tissue stretches, causing these cells to flatten out and resemble squamous cells. This adaptability accommodates distension while providing an impermeable barrier against the toxic contents of the urine.
Glandular Epithelium
A significant portion of epithelial tissue is classified based on its dedicated function of producing and releasing substances. This Glandular Epithelium is formed from specialized epithelial cells that create organized secretory structures called glands. Glands are broadly categorized into two major functional types based on where they release their product.
Exocrine glands retain a connection to the surface epithelium via a tubular duct. They release their secretions, such as sweat, saliva, or digestive enzymes, onto a body surface or into a body cavity. Examples include the salivary glands and sweat glands.
In contrast, Endocrine glands are ductless, meaning they lose their connection to the surface epithelium during development. These glands secrete their products, known as hormones, directly into the interstitial fluid surrounding the cells. The hormones then diffuse into the bloodstream, traveling to target cells throughout the body.