This means that any substance absorbed across a layer of epithelial cells has to pass through the cells. This provides a mechanism by which epithelial tissue can control absorption, and keeps tissue compartments with different chemical compositions separate. Gap junctions - These junctional complexes contain channels which permit ion flow and the passage of small molecules between contiguous cells. This allows cells to communicate and coordinate their activity - e.g., muscle and nervous tissue. Glandular epithelial tissues and the intestine frequently have cells with numerous microscopic membrane projections called microvilli which increase their absorptive surface area, and also aids secretion.
Epithelial tissues contain no blood vessels.
Cells receive nourishment by diffusion from a highly vascular area of loose connective tissue just below the basement membrane called the lamina propria . Epithelial cells covering the cornea and lens, as well as sensory receptors of the eyes, ears, and nose, are also of ectodermal origin. The epithelial lining of the respiratory system and digestive tracts - as well as the parenchyma of liver, pancreas, gallbladder, thyroid, and parathyroid, are derived from endoderm. In this way, epithelial cells serve as selective barriers between the environment and the internal structures of the body. They protect underlying tissues from drying, and from mechanical and chemical injury.