MAIN MORPHOLOGICAL FEATURES OF THE EPITHELIAL TISSUE
The cells of the epithelial tissue maintain very little space between them, therefore, they are juxtaposed, with very little extracellular material between them.
Their cells establish many intercellular junctions with each other.
The shape of its cells is quite diverse, from flat to pyramidal, including , cuboidal and columnar. Recall cell shapes in Chapter 1.
The epithelial tissue does not contain blood vessels, with very few exceptions.
Its cells are always supported by connective tissue containing blood and lymphatic vessels that supply oxygen, nutrients and other molecules to the epithelium and collect carbon dioxide, liquid, metabolites and secretions.
At the interface between the epithelial cells and the connective tissue lays is a complex thin layer of macromolecules called the basal lamina. The basal lamina together with connective tissue fibers adjacent to the basal lamina can form a tihcker layer, visible under a light microscope, and this complex is called the basal membrane. Epithelial cells are, therefore, always almost supported on a basal lamina.
The contact of epithelial cells with the basal lamina induces a specific organization in the vast majority of epithelial cells, called polarization. Polarization means that different regions of epithelial cells can have characteristic organizations and different organelle contents resulting in different functions of different regions of the cells.
As a result of polarization, the portion of the cell that rests on the basal lamina is called the basal region and the opposite portion, often facing a cavity or an opon space, is called the apical region.
Epithelial cells often have specializations of their plasma membrane, such as microvilli, cilia, stereocilia, in addition to intercellular junctions.