Megakaryocyte dense granule components are sorted in multivesicular bodies

Recent studies suggest that multivesicular bodies are an intermediate stage in the formation of -granules. In contrast, the kinetics and mode of appearance of dense granules during megakaryocytic maturation has remained poorly understood. Immunoelectron microscopy was used to monitor the appearance of dense granular markers (granulophysin and serotonin) on cryosections of human megakaryocytes (MKs) cultured from CD34+ precursors. The monitoring was done on days 8 and 13 of culture. The data suggest that dense granules appear in immature MKs early during their maturation, concomitantly with -granule formation. In MKs of intermediary maturation stage, granulophysin was mainly localized within dense granules and multivesicular bodies (MVBs), which were also labeled for serotonin. This study provides evidence that granulophysin is a dense granule marker in human MKs and that MVBs are an intermediary stage of dense granule maturation and probably constitute a sorting compartment between -granules and dense granules.