Mast cells in rat dermis and jejunal lamina propria show a five-fold difference in unit granule volume

The cytoplasmic granules of mast cells have a periodic multimodal size distribution in which the volumes of individual granules are integral multiples of the intermodal distance, a volume defined as the "unit granule" or v1. In this study, we used two 3-month-old male rats to analyze two classical mast cell subpopulations, dermal "connective tissue-type mast cells" and jejunal lamina propria "mucosal mast cells", for the morphometric characteristics of their cytoplasmic granules. Both v1 and the mean volume of individual cytoplasmic granules were much smaller in dermal than in jejunal mast cells (ratios of 1:5.5 and 1:4.2, respectively), but dermal mast cells contained 150% more granules per cell than did jejunal mast cells. The two types of mast cells did not differ significantly in total cell volume, nucleus volume, aggregate volume of cytoplasmic granules per cell or numbers of unit granules comprising a granule of mean volume. These findings add unit granule volume to the list of phenotypic characteristics which express significant variation in anatomically distinct populations of mast cells.