Histamine immunohistochemistry is superior to the conventional heparin-based routine staining methodology for investigations of human skin mast cells

Summary Conventional studies of mast cells are limited by methodological restrictions such as a selective fixative-dependent routine staining blockage. This is thought to depend on the biochemical differences of the mast cell granule contents suggesting a cellular heterogeneity. Investigations of human mast cells, using routine methods, also suffer from the problem of a low signal-to-noise ratio.In the present study, normal human skin was used to compare an immunohistochemical method for histamine with two recommended mast-cell fixatives and a new commercial fixative in combination with three routine stains. Mast cells were found throughout the dermis with all the routine stains used. However, immunohistochemistry gave profoundly better results. Small structures, such as thin cytoplasmatic extensions and single granules, were readily detectable. Double-staining (immunohistochemistry followed by routine staining) revealed differences in staining capacity. All immunoreactive cells were not stained by routine stains and sometimes the opposite was also seen. This supports earlier reported evidence of heterogeneity, not only between skin and intestinal mast cells but also among skin mast cells themselves. Furthermore, by focusing on histamine, instead of heparin, we probably overcame the problems of the selective fixative-dependent routine staining blockage. Finally, the immunofluorescence technique provides a high signal-to-noise ratio and is an excellent method for making high-quality microphotographs of human mast cells.In conclusion, we have found histamine immunohistochemistry (a) to be easy to perform, (b) to show cytoplasmic details better of the, sometimes, dendritic-type mast cells, (c) to result in a higher signal-to-noise ratio, i.e. a better detectability, resulting in a higher number of cells being evident, and (d) to reveal the presence of histamine, instead of heparin, thus being more relevant to all kinds of histamine-related scientific endeavours. However, routine methods occasionally revealed single cells not visualized by the histamine immunohistochemistry.