The TGF-β1 content of MKs from PMF patients and Gata1^low mice is similarly increased. (A-B) TGF-β1–specific immunohistochemistry of representative BM (A) and spleen (B) sections of one nondiseased donor and one PMF patient, untreated wild-type, or Gata1^low mice, and Gata1^low mice treated with either SB431245 or vehicle, as indicated. Arrows indicate representative MKs. Human MKs are on average twice as large as murine MKs. Original magnification ×20 (A) and ×40 (B). Quantifications of MK frequency and intensity of TGF-β1 immunostaining for the different groups are presented in Table 1. (C) TGF-β1 immunogold-staining of one representative MK (C1-4) from untreated wild-type and Gata1^low mice and from Gata1^low mice treated with either SB431245 or vehicle. The selected areas of the MK cytoplasm indicated by rectangles are shown at a higher magnification in C5-8. Arrowheads indicate TGF-β1–specific gold particles. The mean (±SD) number of immunogold-particles/14 μm² of MK cytoplasm obtained in 5 replicate measurements is indicated below each panel. Values statistically different (P < .05) between untreated wild-type and Gata1^low mice and vehicle- and SB431542-treated Gata1^low mice are indicated by ^& and *, respectively. Magnification ×4400 in C1-4 and 30 000× in C5-8.