The erythroid progenitors and the MKs present in the marrow of splenectomized Gata1^low/+ mice are predominantly derived from the stem cell population expressing the wild-type Gata1 allele. (A) Photograph of a representative erythroid colony generated from the marrow of untreated and splenectomized 18-month-old Gata^low/+ female littermates. Magnification: ×10. (B) Statistical analyses of the levels of β-globin expressed by single erythroid colonies generated in marrow cultures of untreated wild-type, Gata1^low/0, and Gata1^low/+ mice (top panel) or from untreated and splenectomized Gata1^low/+ littermates (middle panel) or from wild-type and splenectomized Gata1^low/+ mice (bottom panel). Results are expressed as median (the line across the boxes), 25% to 75% interquartile range (the boxes), and maximal and minimal value (the end of the vertical line across the boxes). Differences in expression levels were compared by Kruskal-Wallis and Bonferroni-Dunn posthoc tests (top panel) or by Mann Whitney test (middle and bottom panels). * and & indicate levels of β-globin significantly lower (P < .001) than those expressed by cells from wild-type and untreated Gata1^low/+ females, respectively (see supplemental Table 1 for further details). (C) Gata1 immunostaining of bone sections of untreated Gata1^low/+ mice (top panels) and mice 3 months after splenectomy (bottom panels). All the mice were 9 to 10 months old. Because, to avoid interference with the immunostaining (in brown), the slides were not counterstained with hematoxylin-eosin, MKs are indicated by arrows for clarity. Magnification: ×20. Representative areas (indicated by rectangles) corresponding to the medulla and the bone trabeculae are also represented at ×100 magnification. Similar results were observed with 3 additional untreated and splenectomized Gata1^low/+ mice. (D) Frequency of Gata1^pos MKs (top panel) and level of Gata1 staining in single Gata1^pos MKs (bottom panel) from untreated and splenectomized Gata1^low/+ females. Results are presented as mean (± SD) determinations with 3 mice per experimental group. The frequency of Gata1^pos cells was determined by analyzing at ×40 magnification 100 MKs for each mouse. The level of Gata1 immunostaining in Gata1^pos MKs was determined on 5 MKs randomly chosen in the medullar or in the trabecular area of the bone for each animal with the MetaMorph 6.1 program. Values statistically different (P < .05 or P < .001) from those observed in untreated mice are indicated by * and **, respectively.