Differences in the regulation of megakaryocytopoiesis in the murine bone marrow and spleen.

Murine splenic megakaryocytopoiesis has been analysed and compared to that of the bone marrow. Quantification of megakaryocytes by acetylcholinesterase staining indicated a reduction in the total numbers of megakaryocytes in the spleen, with the largest decrease being in the total numbers of immature megakaryocytes. On a per organ basis, the spleen also contained a lower number of the megakaryocyte progenitor cells (CFU-Mk) than the bone marrow. The splenic and bone marrow progenitor cells had similar in vitro responses to megakaryocyte colony-stimulating activities. However, the splenic progenitor cells developed a lower number of megakaryocytes per colony, compared to bone marrow. This lower number of cell divisions was not compensated by increased endomitotic activity, since the splenic colony megakaryocytes had a similar distribution of DNA to those derived from marrow megakaryocytes. Cell cycle analysis indicated that, in contrast to marrow cells, splenic megakaryocyte progenitor cells are a rapidly-cycling population. This change in cell cycle status, together with altered proportions of progenitor cells, immature and mature megakaryocytes, suggests that the regulation and kinetics of megakaryocyte development are different in spleen and bone marrow.