The literature on mast cells/eosinophilic granule cells (MCs/EGCs) of teleosts is reviewed. By employing special techniques of fixation, embedding and staining, several investigators have been able to demonstrate cells with metachromatically stained cytoplasmic granules characteristic of the mast cell in tissue samples from species belonging to a diverse group of teleostean families, including cyprinids, erythrinids, esocids, salmonids, pleuronectids and molids. Besides showing metachromasia with dyes like toluidine blue and thionin, these cells can also be stained with Alcian blue, retaining the blue colour after sequential staining with safranin. Most of the studies referred to were performed on intestinal tissues, gills or skin, and in the majority of species fixation of tissue samples in water-based fixatives had to be avoided to preserve the basophilic components of the granules. With other preparation and staining procedures the cytoplasmic granules are acidophilic, i.e. they appear red after staining with haematoxylin and eosin, displaying the characteristics of EGCs. Susceptibility to watery fixatives and/or blockade of metachromatic staining with aldehydes is a feature of mammalian mucosal mast cells. Massive recruitment of mucosal mast cells and increased acidophilia of the granules of such cells when they enter epithelia from subepithelial tissue layers and become 'globule leucocytes' (intraepithelial mucosal mast cells) in response to infection with intestinal parasites is well-known in intestinal tissues of mammals. In teleostean fish, similar responses seem to occur in parasitized intestine, gills and skin. By applying the concept of mast cell heterogeneity, as described for mammals, the results from studies hitherto published indicate that MCs/EGCs of teleosts are mast cells of the mucosal mast cell type, which is not surprising since most studies on teleostean MCs/EGCs are focusing on cells in mucosal tissues (C) 1998 Academic Press.
