SPARC stromal deficiency induces features of a myeloproliferative disorder in the presence of Apc^min mutant hematopoietic cells. BM chimeras were obtained by transplanting BM hematopoietic cells from Apc^min donors into either WT or Sparc^−/− recipients to obtain Apc^min > WT and Apc^min > Sparc^−/− BM chimeras (n = 5 mice per group, per experiment). The data shown represent 1 experiment of 2 performed. (A) Histomorphologic and immunohistochemical analysis performed on BM sections from Apc^min > WT and Apc^min > Sparc^−/− BM chimeras highlights that the BM of Apc^min > Sparc^−/− (top right and middle right panels) chimeras is hypercellular compared with that of Apc^min > WT chimeras (top left and middle left panels) because of the marked expansion of mature granulocytes (marked by Gr1 in bottom panels, DAB, brown signal) and morphologically immature myeloid precursors. Four representative sections (1 hematoxylin and eosin-stained and 1 Gr1-immunostained, per group) of the 20 evaluated are shown. Original magnifications: top panels ×200, middle panels ×400, bottom panels ×200. Scale bars: top panels 100 μm, middle panels 50 μm, bottom panels 100 μm. (B) Differential hematopoietic cell counts (mean ± SD) performed on BM sections of Apc^min > WT and Apc^min > Sparc^−/− BM chimeras show the prominent increase in the granulocyte myeloid cell fraction of Apc^min > Sparc^−/− mice. In each BM sample, the number of neutrophils, eosinophils, morphologically immature myeloid cells, erythroid precursors, MKs, and lymphoid cells was counted of 10 HPFs. **P < .01. ***P < .001. ****P < .0001. Data are relative to counts performed on 2 different BM sections, per mice, per group. (C-D) Fractions (C; percentage of total BM cells, mean ± SD) and absolute numbers (D; mean ± SD) of myeloid progenitor populations [Lin⁻c-kit⁺ (LK), CMP, GMP, and MEP] showing the significant enrichment of GMP precursors in the BM of Apc^min > Sparc^−/− chimeras compared with Apc^min > WT chimeras. *P < .05. (E) Flow cytometric analysis of Gr1⁺CD11b⁺ circulating myeloid cells performed on the PB mononuclear cells of Apc^min > WT and Apc^min > Sparc^−/− chimeras showing the significant expansion of circulating granulocytes characterizing Apc^min > Sparc^−/− mice. *P < .05. (F) Hematoxylin and eosin-stained PB smears preparations of Apc^min > WT and Apc^min > Sparc^−/− highlighting the granulocytosis associated with BM stroma SPARC deficiency. Representative areas from 2 smears (of 5 evaluated) per condition are shown. Original magnifications: top panels ×200, bottom panels ×400. Scale bars: top panels 100 μm, bottom panels 50 μm. (G) Morphologic analysis of hematoxylin and eosin-stained PB smears from BM chimeras show that circulating granulocytes from Apc^min > Sparc^−/− are enriched in immature and blastlike forms compared with circulating granulocytes from chimeras with WT stroma. Representative examples of circulating granulocytes from 2 smears per condition (of the 5 evaluated per experiment) are shown. Original magnifications ×1000. (H) Histopathologic analysis of the spleen of Apc^min > WT and Apc^min > Sparc^−/− chimeras showing that the splenic parenchyma of Apc^min > Sparc^−/− mice is characterized by the expansion of the red pulp (RP) resulting from increased extramedullary myelopoiesis, and by the effacement of the white pulp (WP). Original magnifications: top panels ×100, bottom panels ×200. Scale bars: top panels 200 μm, bottom panels 100 μm.