Extramedullary haemopoiesis after bone marrow transplantation.

44 patients underwent bone marrow transplantation (BMT) for treatment of severe aplastic anaemia or haematological malignancies. During their post-transplant phase all patients had erythroblasts and granulocytic precursors in their peripheral blood. 15 patients died between day +6 and +346 after BMT and autopsies were performed. The sections of all 15 patients revealed extramedullary haemopoiesis in the spleen. Extramedullary haemopoiesis in the liver was found only in those patients who died early (between d +6 and d +21 after BMT). Medullary haemopoiesis, normally only occurring in the vertebral body, was also observed in the shaft of the femur. The present data show that after BMT all tissues with a haemopoietic matrix in ontogenesis can be repopulated with haemopoiesis in the early phase of reconstitution, possibly to compensate for the haemopoietic insufficiency after conditioning therapy. The expansion of haemopoiesis in the later period of up to 1 year after BMT, remains to be explained.