Mechanism of extramedullary haematopoiesis in rabbits with saponin‐induced myelofibrosis and myeloid metaplasia

Summary. The saponin-induced myelofibrosis and myeloid metaplasia model in the rabbit was used to study mechanisms of extramedullary haematopoiesis. Haematopoietic progenitor cells, erythroid colony forming units (CFU-E) and burst forming units (BFU-E) were assayed serially in the peripheral blood, spleen and bone marrow after saponin administration, employing the in vitro, methylcellulose culture technique. Animals that had undergone splenectomy prior to saponin administration were also studied. The results demonstrated increases of progenitor cells in the blood and spleen and a simultaneous depletion of such cells in marrow after saponin treatment. The results in splenectomized animals were similar to those observed in non-splenectomized animals after saponin administration. The findings indicate that following saponin administration there is a release of CFU-E and BFU-E from bone marrow into periphery and probably deposition in the spleen, and suggest that myeloid metaplasia in myelofibrosis may result from colonization of extramedullary sites originating from the bone marrow.