epithelium

from Wikipedia, the free encyclopedia
This article or the following section is not adequately provided with supporting documents ( e.g. individual evidence ). Information without sufficient evidence could be removed soon. Please help Wikipedia by researching the information and including good evidence.

The epithelium [ epitel ] ( Greek ἐπί epi on ',' on 'and thele "mother's breast," "nipple") is a biological collective term for cover fabric and gland ngewebe. These are single or multi- layer cell layers that cover all inner and outer body surfaces of the multicellular organisms (exception: joint capsules and bursa of the musculoskeletal system).

In addition to muscle , nerve and connective tissue, the epithelium is one of the four basic types of tissue .

construction

Epithelia are clearly separated from the connective tissue by the basement membrane and contain no blood vessels .

Another characteristic common to all epithelial cells is their polarity :

  • The outer, apical side faces the outside (e.g. in the case of the skin) or the lumen (e.g. in the case of the intestine or the glands).
  • The basal side is connected to the underlying tissue by a basal lamina .

The polarity of epithelial cells is also characterized by structural and functional differences between the apical and basal membrane of the epithelial cells. In this context, one speaks of an apical and basolateral domain .

Furthermore, epithelial cells have an adhesive complex ( final ridge complex ) consisting of zonula occludens ( tight junction ), zonula adhaerens ( adherens junction ) and desmosome ( macula adhaerens ). On the one hand, the adhesive complex represents a physico-chemical barrier and, on the other hand, connects adjacent epithelial cells with one another.

The cells are close together and are rich in cell contacts . As a result, the tissue has only small intercellular spaces with correspondingly little intercellular substance . With the help of emperipolesis , other cells penetrate the epithelia.

Classification of the epithelia

The different types of epithelium. In curved single-layer high prismatic and cubic epithelia, a certain percentage of cell shapes appear, which are called scutoid .
Another illustration for the classification of the epithelial tissue (English labeling)

Epithelia are specifically differentiated in many ways and depending on the organ. First of all, one can distinguish between surface epithelia and glandular epithelia:

To distinguish between the numerous epithelial types, it has proven useful to emphasize two features: on the one hand, the number of cell layers and, on the other hand, the shape of the cells in the superficial cell layer (see below).

Monolayer epithelia

Simple epithelia

Multi-row epithelia

The multi-row epithelium is also still single-layered, all cells are anchored to the basal lamina as in the single-layer epithelium , but not all of them reach the lumen . Highly prismatic cells fulfill the actual function, while small basal cells are available as a reserve for submerged cells. The cell nuclei are at different heights and thus form apparent layers (rows).

Layered epithelia

The vertebrate epidermis ( colored bright blue- red ) is the only keratinizing squamous epithelium

In the multilayered epithelium, many (more than ten) layers of cells lie on top of one another. Basically, a three-way division can be made: Cell divisions take place in the basal layer, which is anchored to the basal lamina. The cells ascend and differentiate in a specific way in a middle or intermediate layer. Eventually they reach the surface or superficial layer.

  • multilayered squamous epithelium: This epithelium is of great importance and can be found wherever mechanical stress is high. The cytoskeleton and cell contacts are tailored to this load. In regions that are constantly humidified, the multilayered squamous epithelium remains keratinized , where it is exposed to the air, it becomes cornified .
  • multilayer isoprismatic epithelium: ovarian follicles that have reached the stage of secondary follicle have such an epithelium.
  • two-layer isoprismatic epithelium: This form of epithelium is found in the ducts of the sweat glands . The ciliary body is also covered by such an epithelium, although it is part of the retina .
  • multi-layer high prismatic epithelium: This less common epithelial form is to be distinguished from the much more important multi-row high prismatic epithelium. It only occurs in three places on the human body:
    • in the male urethra in its course from the prostate to just before the outer mouth
    • in the main ducts of the large salivary glands (two-layer)
    • in the conjunctival fornix , a reserve fold of the conjunctiva

Transitional epithelium ("urothelium")

The transitional epithelium ("urothelium") is a special epithelium of the urinary tract ( renal pelvis , ureter , urinary bladder ) that has multiple rows or layers, depending on the bladder filling (or stretching of the urothelium) . The deck / umbrella cells are particularly important here. They form the so-called crusta , which has the task of protecting uric acid. In contrast to the squamous epithelium, the upper cell layer is more cubic.

Functions of the epithelia

Protective function

The epithelium basically fulfills two different protective functions: On the one hand, purely mechanical protection, primarily through the multilayered epithelia. The epidermis of the skin must have sufficient tear resistance and must not detach from the connective tissue underneath. On the other hand, the epithelium must seal the inner body orifices: stomach and intestinal contents must be used in a controlled manner (highly prismatic epithelium), the urine must remain in the bladder and ureter (transitional epithelium ), the blood-brain barrier must be preserved (capillary endothelium). Of course, mechanical loads have to be withstood here as well, but the tight junctions that occur more frequently in such cells are decisive for the seal .

Absorption

Resorption is the transport of precisely defined substances from apical to basal. The classic example is the absorption of nutrients in the intestinal mucosa. The apical surfaces are often differentiated, for example, an epithelia cell can enlarge its surface through the formation of numerous microplicae (folds) or microvilli . The exact mechanisms ( transport , phagocytosis , pinocytosis , lysosomes ) are the subject of other articles.

secretion

Histological image of the thyroid gland of a horse: follicle with colloid (1), follicular epithelial cells (2), endothelial cells of the capillaries (3)

All secretion processes of the body take place from the glandular epithelia. Accordingly, there is a great variety here, from the individual goblet cells of the intestinal mucous membrane to the sweat glands of the skin to entire organs such as the salivary glands or the pancreas . Glands are organs made up of specialized epithelial cells; they are used for secretion. One differentiates:

  • exocrine glands that bring their secretions to the surface through an excretory duct. They excrete on inner or outer surfaces (e.g. lacrimal gland , salivary gland , sweat gland ), and
  • endocrine glands that deliver their secretions directly to the surrounding extracellular fluid and have no duct. Often the secretions (hormones) then diffuse into blood vessels and are distributed throughout the entire organism (e.g. thyroid , pituitary ).

One can also distinguish between the secretion route, that is

the latter being divided into, according to the composition of the secretion

  • serous (thin fluid, protein-rich, sometimes containing digestive enzymes, narrow gland lumen, e.g. parotid gland , pancreas ),
  • mucous (viscous, slimy; serves to form transport mucus, wide lumen, e.g. Brunner's glands in the duodenum ) and
  • Seromucous (mixed - secretion is both serous and mucous; this case is the most common, e.g., mandibular salivary gland ).

In addition, a distinction is made between intraepithelial and extraepithelial glands:

  • Intraepithelial glands are single cells embedded in the cover epithelium (e.g. the mucous goblet cell of the intestine).
  • Extraepithelial glands are multicellular organs that therefore no longer have any space in the epithelium itself and have been shifted into the deeper tissue layers. They consist of glandular end pieces that form the secretion. A distinction is made between tubular (tubular), alveolar (bubble-shaped) and acinous (bubble-shaped; however, thicker "wall" and smaller lumen) and mixed forms of extraepithelial glands. Switching points take the secretion from the end pieces and guide it into the strip pieces / secretion tubes (made of cylindrical epithelium); many secretion tubes collect to the secondary ducts n, which open into the main duct, which finally the secretion on an epithelial surface, z. B. the intestinal mucosa releases.

Sensory function

Most of the human sensory cells are embedded in epithelial cell structures. This construction is recommended because epithelia, as superficial cell layers, naturally assume a mediating position between inside and outside. Examples:

Transport function

Some epithelia also have cilia on their surface, which have a transport function. They can smuggle foreign bodies out of the organism with their powerful blow.

See also

Individual evidence

  1. ↑ Becher, Lindner, Schulze (ed.): Latin-Greek vocabulary in medicine . 3. Edition. Verlag Gesundheit GmbH, Berlin 1991, ISBN 3-333-00627-8 .