Mast cells

Origin and tissue localization

Mast cells are cellular elements with a proven hematogenous origin. Initially originating from blood stem cells, they migrate and settle in tissues, where they perform functions completely analogous to those carried out by basophils in peripheral blood. This is why they are often referred to as "tissue basophils."

A key topographic feature of labrocytes is their specific arrangement. In histological preparations, it can be clearly observed that these cells are grouped predominantly perivascularly — that is, they line up in close proximity along the walls of blood vessels. Such localization is not accidental; it not only ensures a rapid response to changes in the vascular bed, but also serves as an indirect morphological confirmation of their origin from the hematopoietic system.

Microscopic appearance and tinctorial properties

For proper visualization and identification of labrocytes in histological practice, a classical staining technique is used that is identical to the processing method for standard blood smears. A mixture of azure II and eosin is used as the stain.

When examining the preparation under a light microscope, a mast cell demonstrates the following morphological characteristics:

• Nucleus: relatively small in size, stains basophilically (blue), and is located strictly in the central part of the cell.
• Cytoplasm: its main distinguishing feature is the presence of abundant granularity. Just like blood basophils, the cytoplasm of labrocytes contains two types of granules:
• Nonspecific granules: extremely small in size, which is why they usually remain invisible and cannot be assessed under standard light microscopy.
• Specific granules: large, markedly basophilic structures. Their presence determines the classic appearance of a mast cell and allows it to be easily identified in the preparation.

Biochemical composition of granules and the phenomenon of metachromasia

The specific large granules filling the cytoplasm of labrocytes are their main functional apparatus. These granules contain two critically important biologically active compounds:

• Heparin;
• Histamine.

The presence of heparin imparts a unique optical and chemical property to mast cell granules, known in histology as metachromasia. Metachromasia is the ability of cellular structures to change the initial color of the dye used upon binding to it. Due to this phenomenon, labrocyte granules acquire a specific hue that contrasts with the surrounding tissues, greatly facilitating their detection.

Functional significance and mechanism of allergic reaction

The overarching physiological role of mast cells is the fine regulation of local tissue homeostasis. This process is carried out through the mechanism of degranulation — the rapid release of the contents of intracellular specific granules into the extracellular microenvironment.

The functions of labrocytes manifest most vividly and clinically significantly during the development of an allergic reaction. This process represents a strict cascade of sequential events:

1. Specialized receptors designed to bind class E immunoglobulins (IgE) are permanently present on the outer cell membrane of the mast cell.
2. When a specific antigen (allergen) enters the tissues, it binds to IgE molecules that are already fixed on the surface of the labrocyte.
3. The formation of the immune complex leads to instant and potent cell activation.
4. The degranulation process is triggered, during which histamine is massively released into the surrounding tissues.

Biological effect of histamine: Released histamine exerts a pronounced effect on the vascular bed near which mast cells are localized. It causes a sharp dilation of blood vessels combined with a significant increase in the permeability of their walls. This leads to the escape of the liquid part of the blood into the intercellular space $\rightarrow$ the development of edema and the formation of a pronounced focus of inflammation.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Mechanisms of increased vascular permeability under the action of histamine
• Structure of the mast cell receptor apparatus and IgE binding
• Differences between specific and nonspecific granules at the ultrastructural level
• Interaction of labrocytes with other connective tissue cells in maintaining homeostasis

Other topics in the section "Proper connective tissues. Connective tissues with special properties"

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