Proliferation, migration and platelet release by megakaryocytes in long-term bone marrow culture in collagen gel

The proliferation of megakaryocytes, their migration, and their platelet release processes were observed in long-term bone marrow culture in collagen gel. Megakaryocytes proliferated for more than 6 months, not only with myeloid cells but also with sinusoid-like capillaries. The megakaryocyte count decreased at 2 weeks of culture, increased to more than 400 at 6 weeks, and then decreased to about 100. Megakaryocyte colonies appeared after 2 weeks of culture; the number increased to more than 10 at 4 weeks of culture, and was maintained at that level. Morphologically, most fully mature megakaryocytes extended several long antennae-like processes, with periodic constrictions, demarcation membranes (DM), and platelet fields. Pro-platelets were released from these processes primarily by a "pinching off" mechanism, but some megakaryocytes released pro-platelets by the dissociation of DM, without extending long processes. Mature megakaryocytes migrated like ameba in collagen gel, and a few migrated to the abluminal side of the capillary or into the capillary; in both cases releasing pro-platelets into the capillary lumina. These observations were very similar to those noted in vivo.