Granulation in amine-storage organelles of mouse megakaryocytes: X-ray microprobe analysis and radioautography

The mechanisms and the processes of the storage of bivalent cations, ATP and 5-hydroxytryptamine (5HT) in the precursors of the amine-storage organelles of megakaryocytes were studied at the electron microscopic level. Although the precursors of the amine-storage organelles in the megakaryocytes fixed with glutaraldehyde and osmium tetroxide were empty, the electron opaque granules were observed in these organelles of the freeze-substituted megakaryocytes cut onto ethylene glycol. X-ray microprobe analysis demonstrated that they contained P, Mg and Ca. Quantitative differences in bivalent cations in the granules were not observed between megakaryocytes and blood platelets. Electron opaque uranaffin-reaction products were observed in the precursors of the amine-storage organelles of the megakaryocytes after treatment with the uranaffin reaction for ATP. However, few chromaffin positive granules were observed in the precursors of the amine-storage organelles after the chromaffin reaction for monoamines. Radioautographic analysis demonstrated that blood platelets avidly took up 3H-5HT but megakaryocytes were not able to accumulate 3H-5HT in vivo. These results indicate that megakaryocytes do not yet acquire the well developed uptake system of 5HT in vivo, while they readily accumulate cations and ATP in the precursors of the amine-storage organelles.