The Origin of Dense Bodies in the Surface Coat of Negatively Stained Platelets

The relationship between the dense bodies found in the exterior coat of negatively stained platelets, and the dense bodies apparent in the hyaloplasm of sectioned cells has been examined. Dense bodies (S.D.B.) similar to those reported by Bull were readily found in whole mount preparations. Some extended beyond the platelet surface membrane and possessed long, whip‐like tails. Examination of thin sections of glutaraldehyde‐osmium fixed, plastic embedded platelets revealed dense bodies (H.D.B.) in the hyaloplasm, but not exterior to the cell surfaces. The H.D.B. occasionally had long, whip‐like tails similar to S.D.B. in the negatively stained preparations. H.D.B. with narrow extensions were encountered frequently in platelets aggregated with ADP or thrombin. While most granules were centrally oriented in altered cells, H.D.B. with long extensions were found in the peripheral hyaloplasm close to the cell surface. Examples were observed in which membranes enclosing H.D.B. appeared continuous with the platelet surface. These results suggest that S.D.B. found in the coat exterior to the plasma membrane of negatively stained platelets, and H.D.B. present inside plastic embedded cells are identical organelles. The S.D.B. may be accumulations of phospholipid destined for release to the plasma as was previously suggested, but they do not appear to be derived from the cell membrane.