Analysis of various platelet proteins by immunofluorescence demonstrated that platelet glycoproteins Ib, IIb, and IIIa, as well as plasma factor VIII antigen (factor VIII:AGN), platelet factor 4, and fibronectin are present in the vast majority of morphologically recognizable megakaryocytes. In addition, a small number of lymphoid-like mononuclear marrow cells, representing approximately 1.4--2.9/10(4) marrow cells, was found to express the same platelet proteins. This population of early marrow megakaryocytes is analogous to small acetylcholinesterase-positive rat and mouse marrow cells. Fc receptors for IgG were expressed in all megakaryocytes and megakaryocyte precursors, whereas the Ia antigen was detected only on a proportion of mature megakaryocytes and not on only early or precursor megakaryocytes. Platelet glycoproteins Ib, IIb, and IIIa, as well as factor VIII:AGN, and platelet factor 4 were established as distinct markers for marrow megakaryocytes and may be helpful for identifying megakaryocytic cells as well as for monitoring events of megakaryocyte differentiation.

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