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It has been assumed that endogenous synthesis by the platelet precursor cell, the bone marrow megakaryocyte, is the major source of platelet a-granule protein. To test this hypothesis, we used mRNA phenotyping to detect in megakaryocytes the presence of mRNA transcripts specific for various proteins. Our results indicate that megakaryocytes synthesize platelet factor 4, a protein relatively specific for platelets, but do not express mRNA transcripts for the fibrinogen, albumin, or IgG found in a-granules. We have previously shown that mega- karyocytes endocytose circulating proteins, including fibrino- gen, albumin, and IgG, and incorporate them into a-granules. Thus, platelets appear to contain a unique type of secretory granule whose contents originate by both endogenous synthesis and endocytosis from plasma. Under basal conditions, the source of a-granule fibrinogen is plasma.


Obstetrics and Gynecology | Pathology


Copyright (1990) The American Society for Clinical Investigation. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Society for Clinical Investigations. This article is the publisher’s version (The American Society for Clinical Investigations), previously appearing in JOURNAL OF CLIN. INVEST. (86(October), 1990, pp. 1364-1368). Also available online at