General characteristics and functions
In order to accurately characterize any tissue, it is necessary to understand the concept of a differon. It is precisely the presence of differons — sequentially differentiating cells of a single type — that distinguishes the tissue level of organization from a simple aggregation of cells or an organ as a whole. Tissues possess specific functions that are realized through the coordinated cooperation of all cellular elements.
Epithelial tissues (Textus epithelialis) are historically and functionally divided into two main groups:
- Surface epithelia. Their main task is boundary and barrier functions. They create a reliable border between various environments (for example, separating the internal environment of the intestinal wall from its lumen), providing protection and regulated metabolism between these environments. They are found on the skin and mucous membranes.
- Glandular epithelia. They specialize in secretory function. They produce and release specific secretions onto the body surface, into the lumens of internal organs, or directly into the blood.
In addition, there are cells with non-standard functions that also have an epithelial nature. These include sensory (receptor) cells responsible for the perception of taste, hearing, and balance, as well as stromal cells of the thymus.
Genetic classification
A unique feature of epithelia is that during embryogenesis they develop from absolutely all germ layers. Depending on the embryonic source, five main types of epithelial tissue are distinguished.
| Embryonic source | Type of epithelium | Examples of localization |
|---|---|---|
| Surface ectoderm | Epidermal | Skin epidermis, sebaceous and sweat glands, salivary glands |
| Neural tube | Ependymoglial | Lining of the brain ventricles and spinal canal |
| Mesoderm | Coelonephrodermal | Mesothelium of serous membranes, epithelium of renal tubules |
| Mesenchyme | Angiodermal | Vascular endothelium |
| Endoderm | Enterodermal | Gastrointestinal epithelium, parenchyma of the liver and pancreas |
The issue of vascular endothelium classification deserves special mention. From the standpoint of histogenesis, the endothelium originates from mesenchyme, which leads some specialists to classify it as connective tissue. However, the modern morphofunctional approach prioritizes structure and function. Because the endothelium forms a continuous cell layer and performs a typical barrier function, it is classified as surface epithelium.
Morphofunctional features
Despite their diversity, surface epithelia share a set of strict morphological criteria. It is important to remember that in glandular epithelia, some of these features may be reduced or absent.
- Cellular structure (sheet). Epitheliocytes lie closely against each other, forming a single sheet. Intercellular substance between them is practically absent. The strength of the sheet is ensured by specialized junctions: desmosomes, interdigitations, and tight junctions (zonula occludens). Exceptions include some endocrine glands (e.g., islets of Langerhans or parathyroid glands), where cells are arranged diffusely.
- Basement membrane (BM). The epithelium is always separated from the underlying loose fibrous connective tissue by a basement membrane. The thickness of the BM ranges from 0.01 to 1.0 µm (it is often not visible under a light microscope). Cells attach to it via hemidesmosomes. The membrane consists of fibrillar structures and amorphous substance (glycoproteins and proteoglycans), with the degree of polymerization of the latter determining the permeability of the barrier. The BM is the result of the combined synthesis of epitheliocytes and connective tissue cells. It is absent in the ependyma and most endocrine glands.
- Polarity. Cells and the sheet as a whole have an asymmetric structure. The basal pole faces the membrane, and the apical pole faces the free surface (lumen). These poles differ critically in their set of organelles and structure. An exception is the epithelia of many endocrine glands.
- Avascularity. There are no blood vessels in epithelial sheets. Nutrition occurs through the diffusion of substances across the basement membrane from the connective tissue side (or from the bathing fluid side). The only exception in the body is the vascular stria (stria vascularis) of the inner ear.
- Specific cytoskeleton. Intermediate filaments of epitheliocytes (both covering and glandular) are made of the protein keratin. Ependymal and endothelial cells have a different type of filaments.
- High regenerative capacity. Epithelia are characterized by both constant physiological and active reparative regeneration after injuries.
Briefly: answers to topical questions
Which types of intercellular junctions ensure the strength of the epithelial sheet?
Epitheliocytes form a sheet and are connected to each other by intercellular junctions:
- Desmosomes — their participation in ensuring the strength of cell connections is directly indicated.
- Interdigitations — finger-like indentations/invaginations of cell membranes.
- Tight junctions (zonulae occludentes, tight junctions) — provide sealing and isolation of the lumen/internal environment.
What structural components and chemical substances does the basement membrane consist of?
The basement membrane consists of fibrillar structures and an amorphous substance, including proteoglycans and glycoproteins.
At the border of the epidermis and dermis in the basement membrane, two layers are distinguished:
- Lamina lucida (lamina lucida) — adheres to basal keratinocytes; contains the proteins laminin and fibronectin.
- Lamina densa (lamina densa) — faces the papillary layer of the dermis; contains heparan sulfate and type IV collagen, which forms a framework in the form of a rhomboid network.
Among the main components of the basement membrane, type IV collagen, laminin, heparan sulfate proteoglycans, and nidogen are also indicated.
Into which types is the covering epithelium divided according to morphological classification?
According to the main morphological criterion (the relationship of cells to the basement membrane), covering epithelia are divided into two main groups:
- Simple epithelia — all cells of the sheet are in contact with the basement membrane. These include simple columnar (prismatic) epithelium, as well as pseudostratified epithelia.
- Stratified epithelia — only the lower (basal) layer of cells is in contact with the basement membrane, while the remaining layers have no connection with it. A representative is stratified non-keratinizing squamous epithelium.
To easily remember histogenesis: Ectoderm is What is outside (skin), Endoderm is What is inside (intestine), and Mesoderm is Between them (serous membranes).
Class and exam questions
Why is vascular endothelium classified as epithelium, given that it develops from mesenchyme?
From the standpoint of histogenesis, it is close to connective tissue, but from a morphofunctional approach, the endothelium forms a continuous sheet and acts as a barrier, which is the main criterion for epithelial tissue.
How do epithelial cells receive nutrients if there are no blood vessels in the epithelial sheet?
Nutrients arrive via diffusion: they pass from the blood vessels of the underlying loose fibrous connective tissue directly through the basement membrane.
Do all epithelia have a basement membrane and polarity?
No, this rule primarily applies to covering epithelia. In the ependymoglial type and most endocrine glands, the basement membrane and pronounced cellular polarity are absent.
What cytoskeletal structure distinguishes an epithelial cell from other cells?
The presence of intermediate filaments consisting of the specific protein keratin (except for ependymal cells and vascular endothelium).
What else to study on this topic
- Morphological classification of covering epithelia (simple, stratified, transitional)
- Molecular structure of intercellular junctions (desmosomes, tight junctions)
- Types and mechanisms of glandular epithelium secretion
- Processes of physiological and reparative regeneration of epithelia
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