MAIN FEATURES OF EPITHELIAL CELLS
Epithelial cells are arranged in layers or clusters, with the cells closely packed together. Only a very small amount of extracellular space separates neighboring cells.
The shape of epithelial cells varies considerably. They may be flat, cuboidal, columnar, or pyramidal, among other shapes. The link Cell shapes displays drawings of cell shapes.
Epithelial tissue is usually supported by connective tissue, which contains blood and lymphatic vessels. These vessels supply the epithelial cells with oxygen, nutrients, and other substances. With very few exceptions, epithelial tissue does not contain blood vessels. Therefore, substances needed by epithelial cells must reach them from the underlying connective tissue by diffusion.
Many epithelial cells have a free surface that faces the external environment, a large body cavity such as the abdominal cavity, or the lumen of an organ. This surface is called the apical surface and may have specialized structures such as microvilli, cilia, or stereocilia.
Another important characteristic of epithelial cells is the presence of numerous intercellular junctions between neighboring cells. These junctions connect the cells to one another and help maintain the integrity and organization of the epithelial layer.
At the interface between most epithelial tissues and the underlying connective tissue, there is a very thin layer of macromolecules called basal lamina. It supports the epithelial cells and separates them from the connective tissue. In most epithelia, the basal lamina is so thin that it is difficult to see with a light microscope.
However, in some epithelia — particularly in sections treated with special techniques, such as the Periodic Acid–Schiff (PAS) reaction, a distinct layer can be observed beneath the epithelial cells. This layer is traditionally called basement membrane. It consists of the basal lamina together with molecules and fibrils contributed by the underlying connective tissue.
Many epithelial cells have a characteristic organization of their organelles known as cell polarity. In a polarized cell, different regions have different arrangements and concentrations of organelles and other cellular components. As a result, different regions of the same cell can perform different functions.
In polarized epithelial cells, the region that rests on the basal lamina is called the basal region. The opposite region, which usually faces a cavity, a lumen, or the external environment, is called the apical region. This organization allows epithelial cells to perform different functions at their basal and apical surfaces.