The dense bodies of human platelets: inherent electron opacity of the serotonin storage particles.

SEROTONIN (5-hydroxytryptamine, 5-HT) in blood is transported almost exclusively by circulating platelets.1 Early biochemical and pharmacologic studies demonstrated that most of the 5-HT was found in granules present in platelet hyaloplasm.2” Recently, several laboratories have suggested that platelet serotonin is concentrated within a specific type of platelet organelle, the dense body58 ( Fig. 1 ) . The relationship between serotonin and dense bodies has been confirmed by ultrastructural autoradiography,9 and by chemical determinations on pure fractions of dense bodies obtained from rabbit platelets.5’1#{176} The cytochemical basis for localizing 5-HT to specific platelet organelles at the ultrastructural level has been related to a reaction between scrotonin and metal containing oxidizing agents used to fix the cells for study in the electron microscope.68 Platelets exposed initially to osmic acid rarely contained dense bodies. However, if the cells were fixed in glutaraldehyde before exposure to osmic acid, a small population of electron dense particles remained in the hyaloplasm. The opacity of the dense bodies was significantly greater than other granules in the hyaloplasm, suggesting an affinity for osmium. In vitro experiments corroborated the reaction between serotonin and the metallic fixative. Aqueous solutions of 5-HT yielded a dense precipitate in the test tube when combined with glutaraldehyde and osmic acid. A similar pattern was observed with potassium dichromate was used at low pH to fix platelets. Dense bodies were not preserved when the agent was used alone, but remained if the cells were fixed in glutaraldehyde before exposure to dichromate at pH 4.1 ( Fig. 2) . Aqueous solutions of 5-HT, glutaraldehyde and potassium dichromate also resulted in dark precipitates when mixed together in vitro. The method for cytochemical demonstration of 5-HT in platelets evolved from technics used initially to demonstrate chromaffin granules, and to distinguish between epinephrine and norepinephrine containing particles.12’13 Glutaraldehyde is believed to form an insoluble complex with unsubstituted