MULTIVESICULAR BODIES ARE AN INTERMEDIATE STAGE IN THE FORMATION OF PLATELET ALPHA-GRANULES

We have used ultrathin cryosectioning and immunogold cytochemistry to study the position of alpha-grannies in the endocytic and biosynthetic pathways in megakaryocytes and platelets. Morphologically, we disting uished three types of granules; so-called multivesicular bodies type I (MVB I) with internal vesicles only granules with internal vesicles a nd an electron dense matrix (MVB II), and the alpha-granules with main ly a dense content and often internal membrane vesicles at their perip hery. The MVBs were prominent in cultured megakaryocytes and the megak aryoblastic cell line CHRF-288, but were less numerous in bone marrow megakaryocytes and platelets, whereas alpha-granules were most promine nt in mature bone marrow megakaryocytes and in platelets. The internal ization kinetics of bovine serum albumin-gold particles and of fibrino gen positioned the MVB subtypes and alpha-granules sequentially in the endocytic pathway. MVBs contained the secretory proteins von Willebra nd factor (VWF) and beta-thromboglobulin (beta=TG), the platelet-speci fic membrane protein P-selectin, and the lysosomal membrane protein CD 63. Within the MVBs, endocytosed fibrinogen and endogenous beta-TG wer e restricted to the matrix, while vWF was predominantly associated wit h internal vesicles. CD63 was also observed in association with intern al membrane vesicles in the alpha-granules. These observations, and th e gradual morphologic transition from granules containing vesicles to granules containing predominantly dense material, suggest that MVBs re present a developmental stage in alpha-granule maturation. (C) 1998 by The American Society of Hematology.