RNA is closely associated with human mast cell secretory granules, suggesting a role(s) for granules in synthetic processes

The distribution of ribosomes in mature human mast cells, a major granulate d secretory cell, does not resemble that in other secretory cells, such as pancreatic acinar cells and plasma cells. By routine ultrastructural analys is, ribosomes in human mast cells are often close to, attached to, or even appear to be within secretory granules. To document better these relationsh ips, we used multiple electron microscopic imaging methods, based on differ ent principles, to define RNA, ribosome, and granule relationships in matur e human mast cells. These methods included EDTA regressive staining, RNase digestion, immunogold labeling of ribonucleoproteins or uridine, direct bin ding or binding after ultrastructural in situ hybridization of various poly uridine probes to polyadenine in mRNA, and ultrastructural autoradiographic localization of [H-3]-uridine incorporated into cultured human mast cells. These different labeling methods demonstrated ribosomes, RNA, U1SnRNP (a s mall nuclear RNP specific for alternative splicing of mRNA), mRNA, and urid ine to be associated with secretory granules in human mast cells, implicati ng granules in a larger synthetic role in mast cell biology.