Origin and Functional Role
Plasma cells do not exist in tissues in their mature form initially; their generation is inextricably linked to the immune response. The precursor cell for Plasmocytus is always a B-lymphocyte.
To initiate transformation, the B-lymphocyte must be stimulated by a specific antigen. Upon receiving the appropriate signal, the lymphocyte drastically alters its metabolism and structure, transforming into a highly specialized factory. The sole and primary function of this factory is the continuous, mass production of antibodies (immunoglobulins) necessary to neutralize foreign agents.
Cytoplasm and Brachet's Stain
Because antibodies are proteins by chemical nature, plasma cells require a robust and extensive apparatus for their synthesis. This explains why the cytoplasm of plasmocytes is extremely rich in rough endoplasmic reticulum (rER) and a massive number of ribosomes.
In histological practice, Brachet's stain, a specialized histochemical reaction for RNA detection, is used to accurately identify plasma cells. This stain utilizes a combination of two dyes:
- Methyl green;
- Pyronin.
Due to the tremendous concentration of ribosomal RNA (required for active protein synthesis), the cytoplasm of the plasmocyte avidly binds pyronin. This phenomenon is known as pyroninophilia. As a result, under the light microscope, the cytoplasm acquires a bright, saturated crimson color.
Characteristic Morphology: Nucleus and the "Hof"
When examining histological slides, plasma cells are easily recognized by two specific morphological features concerning the arrangement of the nucleus and organelles:
- Eccentric nucleus. Unlike many other cells, the nucleus in a plasmocyte is never located strictly in the center. It is always displaced toward one pole of the cell (eccentric position).
- Pale "hof" (perinuclear halo). Against the background of the intensely crimson-stained cytoplasm, a non-stained, visually pale area is clearly visible in the perinuclear zone. This structure is called the "hof".
The pale appearance of the hof is due to the practical absence of ribosomes and RNA in this zone, meaning it does not bind pyronin. This area houses other vital organelles: centrioles and an extensive Golgi apparatus. The role of the Golgi apparatus here is critical—it is the site where newly synthesized antibodies are packaged before being secreted outside the cell.