MAIN FEATURES OF EPITHELIAL CELLS
Epithelial cells are disposed in layers or clusters of closely juxtaposed cells. A very narrow extracellular space exists between them.
The shape of epithelial cells is quite diverse, from flat to pyramidal, including, cuboidal and columnar. The link Cell shapes displays drawings of cell shapes.
The epithelial cells are always supported by a connective tissue that contains blood and lymphatic vessels and supplies oxygen, nutrients and other molecules to the epithelial cells since With very few exceptions, the epithelial tissue does not contain blood vessels. The cell surface that covers the apical region of polarized is called free surface and often has microvilli, or cilia or stereocilia.
Another important feature of epithelial cells is the presence of many intercellular junctions in the cell membrane that face neighbor cells.
In most interfaces between epithelial cells and its underlying connective tissue exists a complex and very thin layer of macromolecules called the basal lamina that supports the epithelial cells. In most epithelia it is very thin and difficult to be observed with an optical microscope. Nevertheless, in many epithelia especially in sections treated by some procedures such as the PAS (Periodic Acid-Schiff) technique, it is possible to observe with an optical microscope a layer under the epithelial cells. It is called basal membrane and results from the association of the basal lamina with components (molecules and fibrils) of the underlying connective tissue.
Many types of epithelial cells have a specific organization of their organelles called cell polarity. One of the most significant features of a polarized cell is the following: different regions of the cell have characteristic contents and arrangements of organelles. This results in different functions performed by these cell regions.
In polarized cells, its area that rests on the basal lamina is called the basal region and the opposite portion, often facing a small cavity or an open internal space of the body, is called the apical region.