Epithelioid cells and the phenomenon of endocytobiosis
The basis of a specific granuloma is epithelioid cells. The process of their formation begins when a regular macrophage successfully completes phagocytosis. It destroys the microorganism, identifies its antigenic determinants, and triggers a specific immune response.
After that, the body requires special cells that can capture the pathogen but will not digest it. The macrophage transforms into an epithelioid cell, which has a number of distinct features:
- Practically devoid of lysosomes.
- Lacks a phagocytic vacuole.
- Possesses extremely low bactericidal activity.
As a result, the live microorganism persists inside the cell. This phenomenon is called endocytobiosis. It is vital for maintaining non-sterile immunity: as long as the pathogen is inside, it constantly stimulates the immune system. In addition, epithelioid cells actively secrete interleukin-1 (IL-1), transforming growth factor (TGF), and fibroblast growth factor (FGF), thereby stimulating the development of connective tissue.
Multinucleated giant cells
When epithelioid cells fuse by their cytoplasm or their nuclei divide repeatedly without the division of the cell itself, giant cells are formed. They are also capable of selective phagocytosis and endocytobiosis, as they contain few lysosomes.
Depending on the number and arrangement of nuclei, two main types are distinguished:
- Pirogov–Langhans cells: contain up to 20 nuclei arranged along the periphery of the cytoplasm in the shape of a horseshoe.
- Foreign body cells: can have up to 100 nuclei scattered chaotically in the central part of the cell.
Architectonics of the tuberculous granuloma
The tuberculous granuloma belongs to the epithelioid cell type. It has a clear layered structure, the elements of which are arranged from the center to the periphery in a strict order:
- Central zone: an area of caseous necrosis.
- Inner layer: a palisade of epithelioid cells tightly surrounding the necrotic masses.
- Intermediate zone: isolated (usually 1–3) Pirogov–Langhans giant cells located at the border with the lymphoid rim.
- Outer cellular rim: an accumulation of macrophages and sensitized T-lymphocytes.
- Periphery: fibroblasts demarcating the inflammatory focus.
Important feature: blood vessels are completely absent inside such a granuloma. A fine network of argyrophilic fibers can be detected within it only using special staining — silver impregnation.
Morphology of lepromatous granulomas
In leprosy (Hansen's disease), the microscopic picture directly depends on the form of the disease.
Lepromatous form is characterized by the fact that mycobacteria actively destroy cells and spill into the tissue infiltrate, stimulating immunity. A typical diagnostic sign is the appearance of "leprosy cells" (Virchow cells). These are dead macrophages whose cytoplasm is filled with large fat droplets, remnants of destroyed bacteria, and their metabolic products. Subsequently, this cellular debris is engulfed by giant cells. The lesions most frequently occur in the skin and peripheral nerves, but can be found in almost all internal organs.
Tuberculoid form proceeds with the development of delayed-type hypersensitivity (DTH). It forms classic epithelioid cell granulomas, and the number of detectable mycobacteria is significantly lower than in the lepromatous variant.
How not to confuse giant cells: Pirogov and Langhans neatly "horseshoed" the cell along the edges (nuclei on the periphery in the shape of a horseshoe), while foreign bodies "trashed" the center (nuclei lie randomly in the middle).
Class and exam questions
What is located in the center of a specific granuloma?
As a rule, the zone of primary or secondary tissue necrosis is located there. The exceptions are leprosy and scleroma, in which necrotic changes are replaced by abscess formation.
Why are there no leukocytes inside the tuberculous granuloma?
This is due to the complete absence of blood vessels inside the lesion. Leukocytes simply cannot migrate to the center of the granuloma from the bloodstream.
Why does the body need cells that cannot digest a bacterium?
Epithelioid cells with low bactericidal activity are needed to maintain non-sterile immunity. By keeping a live pathogen inside themselves (endocytobiosis), they ensure constant stimulation of the immune system.
What else to study on this topic
- Secretory profile of epithelioid cells: effect of IL-1, FGF, and TGF on fibroblasts.
- Mechanisms of macrophage transformation into multinucleated giant cells.
- Differences in cellular composition between lepromatous and tuberculoid forms of leprosy.
- Histochemical methods for detecting mycobacteria (Ziehl–Neelsen staining) and argyrophilic fibers (silver impregnation).
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