Istologia - Tessuto Epiteliale (INGLESE)

Histology - Epithelial tissue

The epithelial tissue is made almost only by cells, since they’re very close one to another with almost no extracellular matrix. There are two kinds of epithelial tissues: the lining epithelia, in which cells are tightly apposed forming sheets; the glandular or secreting epithelia, where they form aggregates with different morphology. All epithelial cells lay on a basement membrane, under which we can find a connective tissue. This connective tissue provides nourishment to epithelial cells through diffusion of substances in the extracellular matrix. The part of the tissue laying on the membrane is the basal domain, while the free surface is the apical domain.

In epithelial cells, the cytoskeleton has a very important role, since actin filaments are distributed in a very specific way and give the morphology of cells. These cells are “morpho-functionally polarised”, since the different parts (apical, lateral, and basal) have different functions and morphologies. They are also specialised, since the apical domain has specific functions and structures (microvilli, cilia) depending on the organ in which they are. Also, the lateral and basal domains are specialised, showing cell junctions.

Cytoskeleton and cell junctions

The cytoskeleton is made up of a terminal web (in the villi) of actin and filamin and a tonofilaments part, which are the intermediate filaments arranged in the cytoplasm. Tonofilaments are connected to proteins that create cell junctions.

Specialisations of the apical surface

The apical surface can be specialised in different ways: we can have microvilli, with the function of increasing cell surface, cilia, that move waste and dust or other molecules or organic objects, and so on. Microvilli are specific of cells that absorb substances, since they increase the available plasma membrane surface and therefore allow the cell to absorb more. They are particular of the small intestine. Microvilli are covered by a glycocalyx, which acts as a filter and, in the intestine, also contain enzymes. Microvilli can only be singularly seen with the TEM.

Cilia are longer than microvilli and can be therefore seen at the light microscope too. Cilia are present, as an example, in the airways of the human body, where they block and send away dust, small organisms, and other things that could block the way.

A third possible apical specialisation is stereocilia, which can be very long and are structurally similar to microvilli. Actually, since stereocilia are long microvilli, they should be considered “stereovilli”. They have no axoneme but actin filaments (which means they’re not long cilia but long microvilli). They are present in the male reproduction system and participate in the secretion of factors needed for spermatozoa maturation.

Basal and lateral domains

On the basal surface, a possible specialisation is the basal labyrinth. It’s in cells involved in reabsorption of substances, since there are enfoldments of the basal membrane. Close to them, many mitochondria are present to provide energy for the mechanism of substances exchange.

On the lateral domain, there might be three cell junction specialisations:

• Occluding (tight) junctions, with a very tight connection between the two cells;
• Anchoring (adherens and desmosome) junctions, since two cells are anchored thanks to proteins and better resist to stress;
• Communicating (gap) junctions, where two adjacent cells can communicate with the passage of small molecules from one cell to the adjacent one.

Junctions are made by different components: a transmembrane protein in each cell membrane, connected to the nearby protein, an intracellular link protein, that connects the transmembrane protein to the cytoskeleton.

Going from the free surface to the membrane of an epithelia tissue, we find different junctions between cells. The one which is nearer to the free surface is the occluding (tight) junction. This is a point-to-point fusion of the plasma membrane, which actually creates a very tight connection with almost no extracellular matrix. The proteins that keep the tight junctions together are some transmembrane proteins (occluding, claudin and JAM). In the cytoplasm, they are connected to cytoplasmic proteins such as ZO1, ZO2, and ZO3.

Going on from the free surface to the basal one, we find the adherens junctions. They are connected to actin filaments and are made up of e-cadherin proteins. These proteins are transmembrane proteins. Inside the cytoplasm, a cadherin protein connects with a beta-catenin, which is connected to an alpha-catenin, then a vinculin protein, and finally the actin filament. This is a kind of anchoring junction.

Another kind of anchoring junction, which is just under the adherens junction, is the desmosomes junction. They are similar to previous junctions, but in this case, the desmosomes are attached to a kind of plaque in the cytoplasm to which the intermediate filaments, and not actin, are attached.

Another kind of junction is the gap junction, which is a communicating junction. It allows the passage of material from a cell to the other, but it’s very selective. These junctions are hydrophilic channels that allow the passage of small molecules and ions. They are made up of connexin subunits which form a connexon (1 connexon = 6 connexin subunits). These channels can also be regulated by a mechanism that uses phosphorylation of the connexons. Gap junctions also maintain the correct rate of cell growth/death by influencing apoptosis and other actions, keeping a good homeostasis.

A final kind of junction, which is not between epithelial tissue cells but between the cell and the basal membrane, is the hemidesmosome. It’s similar to desmosomes only for morphology, since different proteins are involved. These proteins are of the integrins family.

Histology - Lining epithelia

Epithelial tissues originate from all the three embryonic germs (ectoderm, endoderm, and mesoderm), depending on their position and function in the human body.

In lining epithelia, cells are closely apposed with tight junctions and there’s almost no extracellular matrix between them. They are avascular, which means there’s no blood in this tissue, and they obtain nourishment from the connective tissue below them; they have different specialisations on the free (apical) edge and have many different intercellular junctions.

The functions of epithelial tissue are to cover and protect the body surface (skin epithelium is a barrier that separates the inner and the outer environments), but also the internal parts of the body from the internal cavities. This separation is not absolute, since many times metabolic exchanges (secretion, absorption) are required. Therefore, these barriers are specialised to allow these exchanges. There is therefore a strict relationship between cells’ function and shape.

Some epithelia are specialized to receive sensitive stimulations. In the epithelium, although there’s not a vascular network, there is a very developed nerve system to allow the sense of tact in the skin.

Epithelia can be divided into three groups, according to cells’ structure: it can be squamous (flat cells), cuboidal (cuboidal cells), or columnar (tall and th...)