8.5:
Classification of Epithelial Tissues: Overview
Epithelial tissues are classified by cell shape and the number of cell layers, both of which determine their function in the body.
Epithelial cells can have three shapes: squamous, cuboidal, and columnar. Squamous cells look like flat, irregular pieces of a jigsaw puzzle and have disc-shaped nuclei.
Cuboidal cells appear square-like with centrally located spherical nuclei. Columnar cells are tall rectangles containing elongated nuclei located near the basement membrane.
All epithelial cell types are arranged either as a monolayer or in multiple layers.
A simple epithelium is a thin monolayer of cells covering the basement membrane. They line the internal compartments of various organs like lungs, kidneys, and intestines, facilitating the rapid exchange of gases and nutrients.
The stratified epithelia consists of several layers of cells on a basement membrane. They are present in the skin surface, esophagus, mouth lining, or anus lining, where they overcome extreme mechanical abrasions and chemical stresses of the environment without losing their integrity.
8.5:
Classification of Epithelial Tissues: Overview
Epithelial tissues are classified according to the shape of the cells and the number of cell layers formed. Cell shapes can be squamous (flattened and thin), cuboidal (square-like, as wide as it is tall), or columnar (rectangular, taller than it is wide). Additionally, the nucleus shape helps identify the type of epithelial cells. Squamous cells have flattened disc-shaped nuclei, cuboidal cells have spherical nuclei, and columnar cells have elongated nuclei.
Based on the number of cell layers, epithelial tissues can be further classified as simple or stratified. Simple epithelium consists of a single layer of cells resting on a basement membrane. If more than one layer is present, it is called stratified epithelium and only the bottom layer of cells rests on the basal membrane.
Simple epithelium contains cells of the same shape in a single layer. This thin and fragile epithelium lines the internal compartments, body cavities, and blood vessels. The thinness of simple epithelia facilitates rapid absorption and secretion of substances – such as nutrient absorption in the intestines or the exchange of gases in alveoli.
In contrast, the stratified epithelium can have different cell shapes in each layer. Hence, it is named after the shape of the cells in the top layer. It covers exposed body areas such as the skin surface or mouth lining and protects against mechanical abrasions, chemical attacks, and invading pathogens.
This text is partially adapted from Openstax, Anatomy and Physiology 2e, Section 4.2: Epithelial Tissues.