Bovine mast cells: distribution, density, heterogeneity, and influence of fixation techniques

Mast cells can be distinguished according to various characteristics: rodent mast cells have been subtyped by histochemical criteria, whereas canine and human mast cells have been classified according to their proteases. Comparisons of mast cells from different species have therefore resulted in contradictory and confusing opinions on mast cell heterogeneity. Thus, it is essential to obtain species-specific data on mast cell density and heterogeneity. The present study was carried out to determine the physiological distribution of mast cell numbers and types in bovines according to tissue location, staining, and fixation methods. Samples were fixed in formalin or Carnoy's fluid. The average number of mast cells was determined by using a metachromatic staining method. Protease content of mast cells was examined with a double-enzyme-immunohistochemical staining technique. Three mast cell subtypes were distinguished: T-, TC-, and C-mast cells. The T-mast cell was the predominant subtype in nearly all investigated organs and tissue locations. Only tryptase-positive mast cells could be demonstrated in bovine skin and uterus. No chymase activity was found in these organs, regardless of the fixation type. A larger number of mast cells was observed after fixation in Carnoy's fluid. The three different mast cell subtypes were only demonstrated in formalin-fixed tissue; chymase-positive mast cells were not found after fixation in Carnoy's fluid. Increasing experimental data suggest that mast cell subtypes have different functions in promoting and modulating inflammation and in remodeling the extracellular matrix. Since mast cell tryptase and chymase have different functional properties, these results may clarify the different reaction patterns observed in various organs and species.