Aging-associated alterations in cell-type morphologies. (A) Pairwise comparisons between age groups for specific cell types shows increasing sizes of HSPCs (area 52.04 ± 3.12 μm² vs area 59.65 ± 3.78 μm²; P < .005), myeloblasts (area 59.43 ± 2.98 μm² vs area 69.27 ± 3.32 μm²; P < .001), and promyelocytes (57.96 ± 4.51 vs 62.27 ± 6.13; P = .0185) with increasing age. (B) Conversely, megakaryocytes were significantly larger in ≤20-year-old marrows (270.42 ± 54.17 μm² vs 178.06 ± 58.57 μm²; P < .001), and megakaryocyte size showed a borderline-significant positive correlation with platelet counts (Spearman R = 0.357; P = .06). (C) Representative MxIF images of large mature megakaryocytes in a ≤20-year-old BM (left) and a small megakaryocyte in a >60 years BM (top right). By visual assessment, a subset of small megakaryocytes exhibited cytoplasmic CD34 expression (bottom right). (D) No significant difference between age groups in the observed proportion of CD61⁺ megakaryocytes that also have detectable CD34 expression (top: P > .05 between groups); however, CD61⁺ megakaryocytes that also have detectable CD34 expression are significantly smaller in size (bottom: 230.7 μm² vs 120.7 μm²; P < .001). Plts, platelets.