Epithelium

The epithelium [ epite ː l] ( gr ἐπί epi " on, over " " sprout, be abundant " and θάλλω Thallo ) is a bio- medical collective name for deck tissue and glandular tissue. It is a single or multiple cell layers that cover all internal and external body surfaces of multicellular animal organisms (except joint capsules and bursae of the musculoskeletal system ).

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

• 3.1 Protection function
• 3.2 absorption
• 3.3 secretion
• 3.4 sensory function
• 3.5 Transport Function

Construction

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

Another common characteristic of all epithelial cells is their polarity:

• The outer side is the apical exterior (for example to the skin ), or facing the lumen (such as the intestines or glands ).
• The basal side is connected by a basement membrane with the underlying tissue.

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

In addition, epithelial cells possess an adhesion complex ( End slats complex) consisting of zonula occludens ( tight junction ), zonula adherens ( adherens junction) and desmosome ( macula adherens ). The prison complex is on the one hand is a physicochemical barrier and connects to the other adjacent epithelial cells together.

The cells are close together and are rich in cell contacts. Consequently, the tissue has only small intercellular spaces with correspondingly little intercellular substance. With the help of Emperipolesis other cells penetrate the epithelium.

Classification of epithelia

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

• Surface epithelia have mainly protective function (eg, the skin). You can pick up substances (absorption, eg, intestinal mucosa ) and form a barrier that separates the respective organ of the environment (especially by the aforementioned cell contacts, the tight junctions ).
• Glandular epithelia determine the function of all glands ( secretion, excretion). They produce secretions of all kinds ( including in salivary glands and sweat glands or in the intestinal mucosa ).

For the distinction of the numerous Epitheltypen it has proven useful to highlight two features: the number of cell layers and the shape of the cells in the superficial cell layer ( see below).

Single-layered epithelia

Simple epithelia

• Single-layered squamous epithelium: Such epithelia are mainly used to smooth lining interior surfaces. Since they are very thin, single-layered squamous allow an exchange of material ( eg, gas exchange in the alveoli ). Examples: Endothelium ( epithelial lining of blood and lymphatic vessels )
• Mesothelium ( pleural, pericardial, peritoneal epithelium ( serous membranes ) )

• Renal tubules
• Submandibular gland ( salivary gland )
• Bile ducts
• Ovarian epithelium

• Stomach lining
• Intestinal mucosa
• Oviduct
• Gallbladder

Multi-row epithelia

And the multi-row epithelium is still one layer, all the cells are anchored, such as the single-layered epithelium on the basal lamina but not reaching the lumen. High prismatic cells perform the actual function, while ready small basal cells as a reserve for lost cells. The cell nuclei are so different in height, thus forming apparent layers ( rows ).

• Ciliated epithelium in the trachea and other airways up to and including the segmental bronchi.
• Vas deferens
• Epididymis courses
• Eustachian tube

Multi-layered epithelia

In multilayered epithelium are superimposed many ( more than ten) cell layers. It can basically make a threefold division: In the basal layer, which is anchored to the basal lamina, find cell divisions take place. The cells to grow and differentiate in a medium or Intermediärschicht in a specific way. Finally they reach the surface or Superfizialschicht.

• Stratified squamous epithelium: This epithelium is of great importance and can be found wherever the mechanical load is large. Cytoskeleton and cell contacts are matched to this load. In regions that are constantly moistened, the stratified squamous epithelium remains unverhornt where it is exposed to air, it horny. multilayered squamous nonkeratinized: Oral cavity, esophagus, anal canal
• Vagina
• Cornea and conjunctiva of the eye
• In the male urethra shortly before the outer mouth

• In humans, the epidermis is the only keratinizing squamous
• In ruminants it is also found in the reticulum, omasum and rumen

• In the male urethra, in their course from the prostate up to just before the outer mouth
• In the main excretory ducts of the major salivary glands ( two layers )
• Conjunctival fornix, a reserve fold of the conjunctiva

Transitional epithelium ( " urothelium " )

As a transitional epithelium ( " urothelium " ) a specific, depending on bladder filling (respectively elongation of the urothelium ) is multiple rows to multilayered epithelium of the urinary tract called ( renal pelvis, ureter, bladder ). Here, especially the deck / screen / umbrella cells is of great importance. They form the so-called crusta, which have the task of uric acid protection. In contrast to the squamous epithelium, the upper cell layer shows more cubic.

Functions of epithelia

Protective function

The epithelium fulfills basically two different protection features: Firstly, the purely mechanical protection primarily through the stratified epithelia. Thus, the epidermis of the skin must have sufficient tensile strength and must not be detached from the underlying connective tissue. On the other hand, the epithelium has the inner body orifices seal: stomach and intestinal contents must be recovered controls ( columnar epithelium ), the urine must remain in the bladder and ureter ( transitional epithelium ), remain the blood -brain barrier must be maintained ( capillary endothelium ). Of course, here too mechanical loads must be endured, but are crucial for sealing the tight junctions that are more common in those cells.

Absorption

Resorption is defined as the transport of exactly certain substances from apical to basal. The classic example is the absorption of nutrients in the intestinal mucosa. The apical surfaces are often differentiated, then a Epithelienzelle increase their surface area, for example, by the formation of numerous microplicae ( folds ) or microvilli. The exact mechanisms (transport, phagocytosis, pinocytosis, lysosomes ) are covered in other articles.

Secretion

All Sekretionsvorgänge the body happen out of the glandular epithelium. Accordingly, there is a wide variety from which individual goblet cells of the intestinal mucosa via the sweat glands of the skin to whole organs such as the salivary glands or pancreas. Glands are organs of specialized epithelial cells; they serve the secretion. We distinguish:

• Exocrine glands that bring their secretions through a duct to the surface. They retire to the inner or outer surfaces (eg lacrimal gland, salivary gland, sweat gland ), and
• Endocrine glands which secrete their secretions directly into the surrounding extracellular fluid and have no excretory duct. Often the secretions (hormones ) diffuse into the blood vessels and then distributed throughout the organism (eg, thyroid, pituitary ).

Even the secretory pathway can distinguish you, so

• Holocrine ( cell disintegrates for secretion, typical of the sebaceous glands of the skin),
• Apokrin ( Vesikelabschnürung, such as lactating mammary gland ),
• Merokrin ( by exocytosis ) and
• Ekkrin ( by transporter),

Where the last are divided according to the composition of the secretion in

• Serous ( thin, albuminous, sometimes verdauungsenzymhaltig, narrow glandular lumen, such as the parotid gland, pancreas),
• Mucoid ( thick, slimy, serves the formation of mucus transport, wide lumen, such as Brunner's glands in the duodenum ) and
• Seromucinous ( mixed - secretion is both serous and mucous, this case is the most common, such as the submandibular gland ).

In addition, a distinction intraepithelial and extra-epithelial glands:

• Intraepithelial glands are embedded in the surface epithelium of individual cells (eg, slime-forming goblet cells of the intestine ).
• Extra-epithelial glands are multicellular organs, in the epithelium itself therefore have no more room and were moved into the deeper layers of tissue. They consist of Drüsenendstücken forming the discharge. A distinction tubulöse ( tubular ), alveolar ( bubble-like ) and acinar ( bubble form, but thicker " wall " and smaller lumens) and mixed forms of extraepithelialen glands. Switching points take on the secretion of the end pieces and direct it into the strip pieces / secretion tubes ( of epithelium ); many secretions pipes collect into the side ducts, which open into the main excretory duct that eventually the secretion of an epithelial surface, such as the intestinal mucosa, which emits.

Sensory function

Much of the human sensory cells embedded in epithelial cell aggregates. This construction makes sense, because epithelia occupy as superficial cell layers naturally a mediating position between inside and outside. Examples:

• Retina (retina ) of the eye
• Inner and outer hair cells of the inner ear, wherein in the case of the outer hair cells, sensors and change in length of the cell body are directly coupled to
• Olfactory mucosa ( epithelium ) in the nasal mucosa
• Taste cells of the tongue dorsum
• Merkel cells ( mechanoreceptors ), as well as pain and temperature receptors in the epidermis

Transport function

Some epithelia additionally possess cilia on their surface, which have a transport function. You can discharging with her powerful blow debris from the body.