General Characteristics and Functions
To accurately characterize any tissue, one must understand the concept of the differon. The presence of differons—successively differentiating cells of a single type—distinguishes the tissue level of organization from a simple aggregation of cells or an organ as a whole. Tissues perform specific functions realized through the coordinated cooperation of all cellular elements.
Epithelial tissues (Textus epithelialis) are historically and functionally divided into two main groups:
- Surface (lining) epithelia. Their primary role is border and barrier function. They establish a reliable boundary between different environments (e.g., separating the internal environment of the gut lumen from the tissue), ensuring protection and regulated substance exchange. They are found on the skin and mucous membranes.
- Glandular epithelia. Specialized in secretory function. They synthesize and release specific secretions onto the body surface, into the lumens of internal organs, or directly into the blood.
Additionally, there are cells with specialized functions of epithelial origin. These include sensory (receptor) cells responsible for taste, hearing, and equilibrium, as well as the stromal cells of the thymus.
Embryological Classification
A unique feature of epithelia is that during embryogenesis they develop from absolutely all germ layers. Based on their embryonic origin, five main types of epithelial tissue are distinguished.
| Embryonic Source | Epithelial Type | Localization Examples |
|---|---|---|
| Surface ectoderm | Epidermal | Skin epidermis, sebaceous and sweat glands, salivary glands |
| Neural tube | Ependymoglial | Lining of the brain and spinal cord cavities |
| Mesoderm | Coelonephrodermal | Mesothelium of serous membranes, renal tubule epithelium |
| Mesenchyme | Angiodermal | Endothelium of blood vessels |
| Endoderm | Enterodermal | GI tract epithelium, liver and pancreatic parenchyma |
It is worth noting the classification of vascular endothelium. From a histogenetic standpoint, endothelium originates from mesenchyme, leading some specialists to classify it as connective tissue. However, the modern morphofunctional approach prioritizes structure and function. Because endothelium forms a continuous cellular sheet and performs a classic barrier function, it is classified as surface epithelium.
Morphofunctional Features
Despite their diversity, surface epithelia share a set of strict morphological criteria. Note that in glandular epithelia, some of these features may be reduced or absent.
- Cellular organization (sheet). Epitheliocytes lie tightly packed against one another, forming a continuous sheet. Intercellular matrix is virtually absent. Sheet integrity is maintained by specialized junctions: desmosomes, interdigitations, and tight junctions (zonula occludens). Exceptions include certain endocrine glands (e.g., islets of Langerhans or parathyroid glands), where cells are arranged diffusely.
- Basement membrane (BM). Epithelium is always separated from the underlying loose fibrous connective tissue by a basement membrane. Its thickness ranges from 0.01 to 1.0 µm (often invisible under light microscopy). Cells anchor to it via hemidesmosomes. The BM consists of fibrillar structures and amorphous ground substance (glycoproteins and proteoglycans), with the degree of polymerization of the latter determining barrier permeability. The BM is synthesized cooperatively by epithelial and connective tissue cells. It is absent in the ependyma and most endocrine glands.
- Polarity. Cells and the epithelial sheet as a whole exhibit an asymmetric structure. The basal pole faces the basement membrane, while the apical pole faces the free surface (lumen). These poles differ critically in organelle distribution and structure. Exception: epithelia of many endocrine glands.
- Avascularity. Epithelial sheets contain no blood vessels. Nutrition occurs via diffusion of substances through the basement membrane from the connective tissue (or surrounding fluid). The sole exception in the body is the vascular stria (stria vascularis) of the inner ear.
- Specific cytoskeleton. Intermediate filaments of epitheliocytes (both surface and glandular) are composed of the protein keratin. Ependymal and endothelial cells possess different filament types.
- High regenerative capacity. Epithelia exhibit both continuous physiological regeneration and active reparative regeneration following injury.