Origin and Tissue Localization
Mast cells are cellular elements with a proven hematogenous origin. Initially developing from blood stem cells, they migrate and settle in tissues, where they perform functions fully analogous to those carried out by basophils in the peripheral blood. For this reason, they are frequently called "tissue basophils."
A key topographical feature of labrocytes is their specific distribution. In histological sections, these cells are clearly observed grouping predominantly perivascularly—meaning they align in close proximity along the walls of blood vessels. This localization is not random; it ensures rapid responses to changes in the vascular bed while serving as an indirect morphological confirmation of their hematopoietic origin.
Microscopic Appearance and Tinctorial Properties
For proper visualization and identification of labrocytes in histology, a classic staining technique identical to standard blood smear processing is applied. A mixture of azure II and eosin is used as the stain.
Under light microscopy, a mast cell demonstrates the following morphological characteristics:
- Nucleus: Relatively small, stains basophilically (blue), and is located strictly in the center of the cell.
- Cytoplasm: Its main distinguishing feature is abundant granularity. Like blood basophils, labrocyte cytoplasm contains two types of granules:
- Nonspecific granules: Extremely small, making them generally invisible and unassessable under standard light microscopy.
- Specific granules: Large, strongly basophilic structures. Their presence defines the classic appearance of a mast cell and allows for easy identification in preparations.
Biochemical Composition of Granules and the Phenomenon of Metachromasia
The large specific granules filling the cytoplasm of labrocytes serve as their primary functional apparatus. These granules contain two critically important biologically active compounds:
- Heparin;
- Histamine.
The presence of heparin endows mast cell granules with a unique optical-chemical property known in histology as metachromasia. Metachromasia is the ability of cellular structures to change the initial color of a dye upon binding to it. Through this phenomenon, labrocyte granules acquire a specific hue contrasting with surrounding tissues, greatly facilitating their detection.
Functional Significance and Mechanism of Allergic Reactions
The overarching physiological role of mast cells is the precise regulation of local tissue homeostasis. This process is carried out via degranulation—the rapid release of intracellular specific granule contents into the extracellular microenvironment.
The functions of labrocytes are most prominently and clinically significantly manifested during allergic reactions. This process follows a strict cascade of sequential events:
- Specialized receptors designed to bind immunoglobulin E (IgE) are permanently present on the outer cell membrane of the mast cell.
- When a specific antigen (allergen) enters the tissue, it binds to IgE molecules already fixed on the surface of the labrocyte.
- The formation of the immune complex leads to rapid and powerful cell activation.
- Degranulation is triggered, during which massive amounts of histamine are released into the surrounding tissues.
Biological Effect of Histamine: Released histamine exerts a pronounced effect on the vascular bed near which mast cells reside. It causes sharp vasodilation combined with a significant increase in vascular wall permeability. This leads to the leakage of the fluid portion of blood into the intercellular space $\rightarrow$ development of edema and formation of a marked inflammatory focus.