Morphologic aspects of bone marrow transplantation in patients with aplastic anemia.

Pre- and post-transplant bone marrow samples from 20 patients with aplastic anemia were studied. Morphologic evidence of marrow reconstitution was noted in 18 patients one to three weeks following transplantation. In most instances the engrafted marrow elements in early weeks appeared as small clusters of erythroid or myeloid precursors. Bone marrow biopsy or clot sections obtained four to eight weeks after transplantation were more cellular with larger clusters of hematopoietic cells, which were most often composed of mixed cellular elements, including megakaryocytes. Two patients with morphologic evidence of engraftment died shortly after transplantation and were excluded from further analysis. In four of the remaining 16 patients grafts were "rejected" three to eight weeks after transplant. A fifth patient who received a marrow graft from his identical twin showed a transient increase in marrow cellularity without clinical improvement. However, the second marrow transplantation in this patient using the same donor after conditioning with cyclophosphamide resulted in moderate clinical improvement. In four of the five patients developing graft "rejection" or failure there was an increase in marrow mast cells in pre- and post-transplant marrow samples. In contrast, only two of 11 patients with successful engraftment had an increase in mast cells. Although the pathophysiologic role of mast cells in marrow transplantation is unclear, the present study suggests a possible inverse correlation between the numbers of marrow mast cells and the likelihood of successful engraftment. Bone marrow samples in patients with graft versus host disease displayed a slight increase in the number of eosinophils, lymphocytes, and plasma cells.