RNA Is Closely Associated with Human Mast Cell Secretory Granules, Suggesting a Role(s) for Granules in Synthetic ProcessesAnn M. Dvoraka, Ellen S. Morgana, Lawrence M. Lichtensteinc, Peter F. Wellerb, and Robert P. Schleimerca Departments of Pathology, Harvard Medical School and Beth Israel Deaconess Medical Center, Boston, Massachusetts b Medicine, Harvard Medical School and Beth Israel Deaconess Medical Center, Boston, Massachusetts c Johns Hopkins Asthma and Allergy Center, Allergy and Clinical Immunology Division, Baltimore, Maryland Correspondence to: Ann M. Dvorak, Dept. of Pathology, East Campus, Beth Israel Deaconess Medical Center, 330 Brookline Avenue, Boston, MA 02215. The distribution of ribosomes in mature human mast cells, a major granulated secretory cell, does not resemble that in other secretory cells, such as pancreatic acinar cells and plasma cells. By routine ultrastructural analysis, ribosomes in human mast cells are often close to, attached to, or even appear to be within secretory granules. To document better these relationships, we used multiple electron microscopic imaging methods, based on different principles, to define RNA, ribosome, and granule relationships in mature human mast cells. These methods included EDTA regressive staining, RNase digestion, immunogold labeling of ribonucleoproteins or uridine, direct binding or binding after ultrastructural in situ hybridization of various polyuridine probes to polyadenine in mRNA, and ultrastructural autoradiographic localization of [3H]-uridine incorporated into cultured human mast cells. These different labeling methods demonstrated ribosomes, RNA, U1SnRNP (a small nuclear RNP specific for alternative splicing of mRNA), mRNA, and uridine to be associated with secretory granules in human mast cells, implicating granules in a larger synthetic role in mast cell biology. (J Histochem Cytochem 48:112, 2000) Key Words: RNA, mast cells, secretory granules, ribosome, EM in situ hybridization, uridine, U1SnRNP, EM autoradiography, EM immunogold immunocytochemistry, EDTA stain
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