Epithelial tissue, formed by epithelial cells, is characterized by the scarce amount of extracellular matrix it contains. As a result, the intercellular spaces are very narrow, epithelial cells stapled together, and they are joined by various types of intercellular junctions.
Epithelial cells exhibit different functional and structural adaptations, resulting in a high diversity of epithelial varieties that can be broadly categorized into two main types:
A typical characteristic of epithelial cells is to show ae well-developed cytoskeleton, mainly composed of intermediate filaments, usually called tonofilaments in these cells, formed by proteins from the cytokeratin family. These proteins serve as a distinctive immunohistochemical marker for epithelial cells.
Another general feature of both lining epithelial tissue and exocrine glands is that they are not vascularized. In other words, they lack blood vessels from the circulatory system within their structure. However, there are exceptions to this rule, such as the vascular stria of the organ of Corti of the inner ear of mammals.
On the other hand, epithelial tissues tend to be richly innervated by nerve endings (neuronal branches), both sensory and effector. Some epithelia also contain neuroepithelial cells with axonal extensions that exists the epithelium and transmit sensory information to the nervous centers, as is the case in the nasal and ofatory epithelia of vertebrates or in the epidermis of many invertebrates.
Due to the lack of intraepithelial vascularization, lining epithelial cells and exocrine gland cells receive nutrients and respire by diffusion from the circulating internal environment, which is contained in vessels or hemocoelomic cavities. Therefore, these epithelia rest (i.e. are supported) on a layer of underlying or surrounding connective tissue, which has different names depending on the organ (dermis in the skin, lamina propia in internal organs, stroma in glands). This connective tissue houses the vessels or cavities containing the internal environment and also provides structural support to the epithelium.
By light microscopy, the epithelial tissue is separated from the connective tissue by a basal membrane, which is more or less noticeable. The basal membrane is formed by a specialized region of the intercellular matrix located between both tissues, which contains abundant glycoproteins. Therefore, this specialized region of the matrix can be visualized with techniques such as the PAS reaction. Additionally, the basal membrane contains particular types of collagen and of adhesion proteins.
Under transmission electron microscopy, the basal membrane consists of at least two layers of extracellular matrix: the lucid (or rare) lamina, which has low electron density and is close to the epithelial base, and the lamina densa towards the connective tissue. The basement membrane serves a mechanical function by facilitating adhesion between the epithelial tissue and the connective tissue matrix, and also exhibits selective permeability for metabolites exchanged between the epithelium and the internal environment.
Epithelial cells display a wide range of functional specializations, such as mechanical protection (cornification, cuticles), separation between distinct internal environments, absorption, transport, and secretion of various products, or uptake of sensory stimuli. Generally, these functions are carried out in specific areas of epithelial cells, resulting in functional and structural polarization of these cells.
Hence, epithelia exhibit distinct regions: a basal pole, adjacent to the basement membrane, an apical pole, opposite to the basal pole, and lateral poles between adjacent epithelial cells. This polarization is evident in the distribution and regionalization of various cellular specializations within epithelial cells, including the plasma membrane and cytoskeleton, as well as the nucleus and cellular organelles.
In adult animals, a wide variety of histological types of lining epithelial and glandular tissues can be found. In these tissues, epithelial cells present diverse specializations that reflect the functional adaptations of these epithelia. Additionally, epithelia generally include other non-epithelial cell types. All of these subjects are described in the following sections.