Scanning electron microscopy of cultured human megakaryocytes.

Human megakaryocytes (MK) from rib marrows of 16 patients with normal hematological patterns were isolated by Percoll density centrifugation followed by velocity sedimentation. The surface features of freshly isolated MK and the unseparated marrow suspensions were examined by SEM. A wide range of surface morphology for MK was noted depending on their size and maturation stages. Megakaryoblasts had a relatively smooth surface. During maturation, MK became villous and some had small blebs. In late maturation, many MK had complex surface processes including typical discs, slightly sphered projections, tear drop projections, putative sheet-like platelets or combinations of two or more of such surface features. Both unseparated and MK-rich fractions (F); specifically F1 lower, F2 lower and F3 were cultured up to 19 days. MK maintained their spherical shape in culture. At different time intervals (2, 5, 9 and 19 days), some cultures were terminated. MK retained their characteristic surface features. Promegakaryocytes and young adult MK were seen in short term cultures. The majority of cultured MK had platelet-like bodies on their surface in the prolonged cultures. Sometimes a few morphologically recognizable platelets were seen in the culture media. Differential counts of MK were done in all preparations of both unseparated and MK enriched fractions. F3, having no morphologically recognizable MK, had many MK present after culturing in vitro for 19 days. These MK had typical platelets or platelet-like processes on their surface. This suggests some MK progenitors originally present in the isolated F3 fraction became mature in cultures.