Mast cell heterogeneity between two different species of Hoplias sp. (Characiformes: Erythrinidae): response to fixatives, anatomical distribution, histochemical contents and ultrastructural features

Mast cells from two erythrinid species: Hoplias malabaricus and Hoplias lacerdae, were studied in several tissues throughout the body using light and electron microscopy. Mast cells were found in all organs studied, but were especially abundant in the gastrointestinal tract, and were always in association with connective tissue. These cells showed different characteristics between the two species studied, like varied morphology, anatomical distribution, density, basophilic/eosinophilic staining and heparin content. In H. malabaricus, the tissues fixed with Helly's solution contained mast cells that were basophilic, metachromatic and had heparin in their cytoplasmic granules, while the tissues fixed with Karnovsky's solution contained eosinophilic and orthochromatic mast cells in which heparin was not detected. In H. lacerdae, the use of both fixatives resulted in mast cells that were eosinophilic, orthochromatic, with no identifiable heparin content. Exclusively in H. malabaricus oesophagus, the mast cells were additionally seen among the epithelial cells. The ultrastructural studies performed in hindgut fixed with Karnovsky's solution revealed that the cytoplasmic granules seen in H. lacerdae mast cells were better preserved than in H. malabaricus mast cells. The latter had electron-lucent granules that were often merged, forming channels. The present study demonstrated that mast cells from two species belonging to the same genus or even mast cells from the same species but under different fixatives can present heterogeneous characteristics, possibly due to their functional properties or to their sensitivity to fixatives.