Immature Megakaryocytes in the Mouse : Physical Characteristics , Cell Cycle

The heterogeneity among immature megakaryocytes has been examined by physical properties. cell cycle status. and responsiveness to thrombopoietic stimulatory factor. Three types of immature megakaryocytes exist that can be recognized by acetylcholinesterase staining. nuclear shape. high nucleus/cytoplasm ratio, and small size (818 ) with respect to mature megakaryocytes (>18 ii). These three acetylcholinesterase-containing cell types are distinguished by their nuclear configuration: a round, indented, and lobed nucleus. The lobed cell type was found to overlap with and enhance detection of megakaryoblasts (stage I megakaryocytes). These cells had a sedimentation velocity range of 3.5-19.0 mm hr1 and a density range of 1 .072-1 .095 9 cm . Separation of these three classes of immature megakaryocytes was achieved by equilibrium density centrifugation with modal buoyant densities of