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-granules in the endocytic and biosynthetic pathways in megakaryocytes and platelets. Morphologically, we distinguished three types of granules; so-called multivesicular bodies type I (MVB I) with internal vesicles only, granules with internal vesicles and an electron dense matrix (MVB II), and the alpha-granules with mainly a dense content and often internal membrane vesicles at their periphery. The MVBs were prominent in cultured megakaryocytes and the megakaryoblastic cell line CHRF-288, but were less numerous in bone marrow megakaryocytes and platelets, whereas alpha-granules were most prominent in mature bone marrow megakaryocytes and in platelets. The internalization kinetics of bovine serum albumin-gold particles and of fibrinogen positioned the MVB subtypes and alpha-granules sequentially in the endocytic pathway. MVBs contained the secretory proteins von Willebrand factor (vWF) and beta-thromboglobulin (beta-TG), the platelet-specific membrane protein P-selectin, and the lysosomal membrane protein CD63. Within the MVBs, endocytosed fibrinogen and endogenous beta-TG were restricted to the matrix, while vWF was predominantly associated with internal vesicles. CD63 was also observed in association with internal membrane vesicles in the alpha-granules. These observations, and the gradual morphologic transition from granules containing vesicles to granules containing predominantly dense material, suggest that MVBs represent a developmental stage in alpha-granule maturation.
Heijnen
H F
HF
Department of Hematology, University Hospital Utrecht, Utrecht University, Utrecht, The Netherlands.
Debili
N
N
Vainchencker
W
W
Breton-Gorius
J
J
Geuze
H J
HJ
Sixma
J J
JJ
D000818:Animals; D001792:Blood Platelets / Q000378:metabolism / Q000648:ultrastructure*; D002417:Cattle; D002478:Cells, Cultured; D003594:Cytoplasmic Granules / Q000378:metabolism / Q000648:ultrastructure*; D004705:Endocytosis; D006801:Humans; D007150:Immunohistochemistry; D008533:Megakaryocytes / Q000378:metabolism / Q000648:ultrastructure; D008854:Microscopy, Electron; D012710:Serum Albumin, Bovine / Q000378:metabolism
D012710:Serum Albumin, Bovine