Irradiation and marrow distribution of host surviving megakaryocytes. (A) Bone (Bo) and BM section of a metaphysis taken at 48 hours after irradiation. Among the residual stromal cells and osteoblasts are large cells () with large multilobulated nuclei that appear as megakaryocytes (inset). (B) Immunohistochemical staining with anti-CD9 (red) and anti-BrdU (black) antibodies confirms the presence of physiologically active megakaryocytes after TBI. The inset represents an enlarged image of the results of CD9 and BrdU double staining. (C) Bone section stained with isotype control primary antibody. (D) Specific pattern of megakaryocyte distribution in response to TBI as revealed by staining for CD9 (red). After radioablation, megakaryocytes () can be found in close contact with the bone-lining cells, whereas in the control they are located primarily in the central areas of the BM, often in close proximity to the vessels (V). (A-D) Scale bar represents 50 μm. (E) Total CD9⁺ multilobulated megakaryocyte counts in the epiphysis, metaphysis, diaphysis, and total marrow space of control versus irradiated mice (n = 4). Mean (± SD) values for randomly selected 200× fields (288 sections scored for each group) are shown. Differences between the control and irradiated groups are not significant (P > .05). (F) Mean percentages (± SD) of CD9⁺ multilobulated megakaryocytes in contact with the cells lining the bone in control versus irradiated mice (n = 4). Percentages of bone-associated megakaryocytes were significantly higher in irradiated mice in all 3 major bone regions (P ≤ .01), with the epiphysis and metaphysis both showing greater accumulations of megakaryocytes at the bone surface than was apparent in the diaphysis (P < .001).