Differences in the volume distributions of human lung mast cell granules and lipid bodies: evidence that the size of these organelles is regulated by distinct mechanisms

We analyzed transmission electron micrographs of human lung mast cells by digitized planimetry and point counting to determine the cross-sectional areas of two distinct cytoplasmic organelles: specific granules and lipid bodies. Specific granules have a limiting membrane and often contain one or more cylindrical scroll-like inclusions. By contrast, lipid bodies are on average much larger than granules and lack both limiting membranes and inclusions. The measured cross-sectional areas of lipid bodies and scroll-containing granules were converted to equivalent volumes, and the noise in the frequency distribution of these volumes was smoothed using a moving bin technique. This analysis revealed (a) a periodic, multimodal distribution of granule equivalent volumes in which the modes fell at volumes that were integral multiples of the volume defined by the first mode (the "unit volume"), and (b) a modal granule equivalent volume frequency that occurred at a magnitude equal to four "unit volumes." Thus, specific granules appear to be composed of units of a narrowly fixed volume. Furthermore, the mean volume of intragranule inclusions was 0.0061 mu3, a value very similar to that calculated for the "unit volume" (0.0071 mu3). This result suggests that each "unit volume" comprising the individual scroll-type granules contains (or is capable of generating or accommodating) a single scroll-like inclusion. In contrast to the specific granules, mast cell lipid bodies lack a periodic, multimodal volume distribution. Taken together, these findings suggest that the volumes of human lung mast cell granules and lipid bodies are regulated by distinct mechanisms.