Evidence for organ‐specific stem cell microenvironments

The X‐linked Gata1 low mutation in mice induces strain‐restricted myeloproliferative disorders characterized by extramedullary hematopoiesis in spleen (CD1 and DBA/2) and liver (CD1 only). To assess the role of the microenvironment in establishing this myeloproliferative trait, progenitor cell compartments of spleen and marrow from wild‐type and Gata1 low mice were compared. Phenotype and clonal assay of non‐fractionated cells indicated that Gata1 low mice contain progenitor cell numbers 4‐fold lower and 10‐fold higher than normal in marrow and spleen, respectively. However, progenitor cells prospectively isolated from spleen, but not from marrow, of Gata1 low mice expressed colony‐forming function in vitro. Therefore, calculation of cloning activity of purified cells demonstrated that the total number of Gata1 low progenitor cells was 10‐ to 100‐fold lower than normal in marrow and >1,000 times higher than normal in spleen. This observation indicates that Gata1 low hematopoiesis is favored by the spleen and is in agreement with our previous report that removal of this organ induces wild‐type hematopoiesis in heterozygous Gata1 low/+ females (Migliaccio et al., 2009, Blood 114:2107). To clarify if rescue of wild‐type hematopoiesis by splenectomy prevented extramedullary hematopoiesis in liver, marrow cytokine expression profile and liver histopathology of splenectomized Gata1 low/+ females were investigated. After splenectomy, the marrow expression levels of TGF‐β, VEGF, osteocalcin, PDGF‐α, and SDF‐1 remained abnormally high while Gata1 low hematopoiesis was detectable in liver of both CD1 and DBA/2 mutants. Therefore, in the absence of the spleen, Gata1 low hematopoiesis is supported by the liver suggesting that treatment of myelofibrosis in these animals requires the rescue of both stem cell and microenvironmental functions. J. Cell. Physiol. 223: 460–470, 2010. © 2010 Wiley‐Liss, Inc.