Cell Tissue Res (1983) 228:51-59 Cell and Tissue Research 9 Springer-Verlag 1983 Periodic, multimodal distribution of granule volumes in mast cells Ilan Hammel*, David Lagunoff**, Michael Bauza*, and Emil Chi** * Department of Pathology, St. Louis University, School of Medicine, St. Louis, Missouri, USA; ** Department of Pathology, Universityof Washington, School of Medicine, Seattle, Washington, USA Summary. The areas of 2327 mast cell granules in transmission electron micrographs of sections of peritoneal mast cells from adult rats were measured by digitized planimetry. A histogram constructed using equivalent volumes calculated from the measured areas assuming approximation of the granules to spheres showed a periodic multimodal distribution in which the modes fell at volumes that were successively larger integral multiples of the volume at the first mode. Application of a moving-bin technique to the data confirmed the presence of the modes. We propose a mechanism of fusion of unit sized granules to account for the multimodal distribution. The presence of pear- and dumbbell- shaped granules in mast cells is consistent with this mechanism. Key words: Mast cells - Morphometry - Membrane fusion - Cell compartment Secretory granules in a variety of cell types have been examined by morphometric methods. For granules whose electron microscopically visualized cross-sections approach circles, it has been customary to measure diameters and construct frequency distributions in the form of histograms (Underwood 1970). In a few instances the distributions appear consistent with slices of spheres from a population with a narrow range of diameters (Ermak and Rothman 1980); in the preponderance of published studies, there are broad distributions of diameters with multiple maxima and minima (Itoh 1977; Kazawa 1976). Such fine structure has usually been attributed, implicitly or explicitly, to random variation with the consequence that overall distribution has been usually analyzed in terms of goodness of fit to a log-normal frequency distribution. We have made measurements on mast-cell secretory granules and sought to determine if the derived histograms reveal any underlying features of the organization of the granules. When the distributions of equivalent granule volumes, calculated as the 3/2 power of measured cross sectional areas, are plotted, Send offprint requests to: Dr. David Lagunoff, Department of Pathology,St. Louis University,School of Medicine, 1402 S. Grand Blvd., St. Louis, Missouri 63104, USA 0302-766X/83/0228/0051/$01.80