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.

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Abstract
Transmission electron micrographs of human lung mast cells were analyzed by digitized planimetry and point counting to determine the cross-sectional areas of 2 distinct cytoplasmic organelles: specific, granules and lipid bodies. Specific granules have a limiting membrane and often contain 1 or more cylindrical scroll-like inclusions. Lipid bodies are on average much larger than granules and lack both limiting membranes and inclusions. The measured cross-sectional area 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 periodic, multimodal distribution of granule equivalent volumes in which the modes fell at volumes that were integral multiples of the volume defined by the 1st mode (the unit volume), and a modal granule equivalent volume frequency that occurred at a magnitude equal to 4 unit volumes. Specific granules appear to be composed of units of a narrowly fixed volume. The mean volume of intragranule inclusions was 0.0061 .mu.3, a value very similar to that calculated for the unit volume (0.0071 .mu.3). Each unit volume comprising the individual scroll-type granules may contain (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. Apparently the volumes of human lung mast cell granules and lipid bodies are regulated by distinct mechanisms.