Generality
Mast cells, or mast cells, are immune cells of variable shape, in some cases rounded or oval, in others branched. Inside the mast cells, in the cytoplasm, there are granules rich in heparin and histamine.
Thanks to the particular affinity with certain dyes, the content of the granules is exploited for their visualization under the microscope: they appear red-purple. Mast cells are found in connective tissue proper, of the loose fibrillar type.
Origin
Discovered by Paul Ehrlich, mast cells originate in the bone marrow during hematopoiesis. Hemopoiesis (or hematopoiesis) is the process by which all types of cells in the blood are formed and mature. The term derives from the union of the Greek words αίμα, which means blood, and eποιὲω, which means to create.
Because of their similarity, mast cells were confused with basophils for a long time.
Location
The connective tissue is one of the four fundamental tissues of the organism, together with the epithelial, muscular and nervous ones.
It is useful to remember the structure of the connective tissue to better understand some properties and functions of mast cells; this fabric:
- is composed of various cell types: macrophages, fibroblasts, plasma cells, leukocytes, mast cells, undifferentiated cells, adipocytes, chondrocytes, osteocytes, etc.
- it has a particular component, called intercellular material (or matrix): it is made up of insoluble protein fibers (collagen, reticular and elastic) and of a fundamental substance, or amorphous, of the colloidal and mucopolysaccharide type. In it the exchanges of gases and nutritional substances take place between the blood and the connective cells.
- It mainly performs two functions: mechanical and trophic. By mechanics we mean the action of support, scaffolding and connection, which this tissue guarantees in the organism. The trophic function (from the Greek Ïτροϕή, nutrition), on the other hand, results in the presence of blood vessels, capillaries and lymphatic vessels, through which the exchange of nutrients takes place.
Mast cells are predominantly concentrated in the vicinity of the blood and lymphatic vessels of the loose fibrillar connective tissue. Furthermore, a high number of mast cells is also present in the mucous membranes of the respiratory and gastrointestinal tract.
Cytology and function of granules. The inflammation
Mast cells measure approximately 20-30 µm in diameter. Inside them, the mitochondria are scarce in number and small in size. The Golgi apparatus is well differentiated. The granules (0.3-0.8 µm diameter), containing heparin and histamine, originate from the latter. In addition, there are also lipid droplets, or lipid bodies, containing reserves of arachidonic acid.
Delimited by a fine membrane, the granules are very numerous and therefore appear crammed, so much so that in some cases they also cover the nucleus of the mast cell. The content of the granules, in particular heparin, has an affinity for particular basic dyes, such as toluidine blue, which allows the visualization of mast cells under a microscope.
The content of mast cell granules is released, after precise signals, outside the cells. This process is called mast cell degranulation.
- Heparin is a sulfuric acid mucopolysaccharide with anticoagulant properties. Mast cells, in the vicinity of the blood vessels of the loose connective tissue, release heparin in order to avoid the coagulation of plasma proteins escaped from the blood capillaries. In other words, they monitor and check that an improper coagulation process does not take place.
- Histamine, on the other hand, is a vasoactive, or vasodilator. Therefore, the degranulation of histamine determines, in the nearby blood vessels, an increased vascular permeability.
The release of histamine is linked to the role that mast cells play in the inflammatory process: in fact, they carry out the degranulation of histamine as soon as an inflammatory situation occurs. Increased vascular permeability is intended to encourage the influx of other immune cells (eosinophils, neutrophils, monocytes, T lymphocytes) and platelets to attack the pathogen (in an infection) or an antigen.
However, it may happen that in more predisposed subjects the massive degranulation of mast cells triggers an exaggerated allergic reaction, called anaphylactic reaction. In this case we speak of anaphylactic degranulation. The affected person has different symptoms, such as:
- Itching
- Dyspnea
- Urticaria
- Sense of suffocation
- Hypotension
- Fainting
- Dizziness
- Polyuria
- Heartbeat
This situation, considered pathological, occurs because the mast cells have, on their membrane, IgE immunoglobulins (or reagine), which, coming into contact with the antigen (in this case it is an allergen), trigger a release uncontrolled histamine.
The "anomalous" presence of IgE on the mast cell membrane is not accidental: they are present on the membrane only after a first exposure, by the predisposed organism, to the allergen. In this case, we speak of sensitization of mast cells to the antigen. In other words, the following situation occurs: when an individual, more receptive than normal, comes into contact, for the first time, with a given allergen, the response immune system consists in the over-production of specific IgE. Once the first exposure to the allergen is exhausted, the IgE sensitive to the latter are fixed on the plasma membrane of mast cells. At the second exposure to the same antigen, the IgE, already ready, trigger the Uncontrolled degranulation of histamine This process is defined with the term anaphylactic hypersensitivity and is one of the inflammatory / allergenic reactions.
This explains why, in cases of anaphylactic reactions, antihistamine drugs are administered.
Mast cells and inflammation: the complete picture
To complete this overview on the role of mast cells during the inflammatory process, it must be said that, on the scene, other protagonists intervene:
- The lipid bodies, containing arachidonic acid.
- Interleukins.
- Chemotactic factors.
- The nitric oxide.
Arachidonic acid, contained in the lipid bodies of mast cells, is a precursor of numerous substances involved in inflammatory processes, such as prostaglandins, thromboxanes and leukotrienes. In mast cells, when the immune response to the antigen is triggered, in addition to degranulation, they are also produced leukotrienes, the effects of which are as follows:
- Increased vascular permeability.
- Smooth muscle contraction.
Leukotrienes, therefore, act as chemical mediators and support the action performed by histamine in counteracting antigens.
Interleukins and chemotactic factors regulate the activity of other cells that participate in the regulation of the inflammatory process. In particular, chemotaxis means a process in which an attraction of mobile cells occurs (such as neutrophils, basophils, eosinophils and lymphocytes) towards chemicals. Hence, a release of chemotactic factors by mast cells calls up other immune cells.
Finally, nitric oxide is another endogenous mediator produced by the mast cell by means of an enzymatic system called NOS, nitric oxide synthetase. Released externally, this gas has a vasodilating action.
As with histamine, however, these other elements of mast cell origin can also determine, in certain individuals, an abnormal response to the antigen. In asthma attacks, for example, it is the massive contraction of smooth muscle, induced by some leukotrienes contained in mast cells, that induces bronchoconstriction, triggering the typical symptoms.
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