TPO treatment (100 μg/kg per day for 5 days) increased the frequency of Mks in hematopoietic tissues from both wild-type and GATA-1^low littermates, but induced fibrosis only in those from wild-type animals. (A) Hematoxylin-eosin staining of spleen sections from untreated and TPO-treated (day 7) wild-type and GATA-1^low littermates, as indicated. Massive hyperplasia of Mks is observed in the spleen from both wild-type and GATA-1^low mice 7 days after TPO treatment. Sections from marrow and spleen of the same animals were processed in parallel and the results are not shown. Original magnification: × 20 in the top panels and × 40 in all the others; numerical aperture of the objectives, 0.55 and 0.70, respectively. (B) Gomori-silver staining of spleen sections from untreated and TPO-treated (day 7) wild-type and GATA-1^low littermates (alternate sections with respect to those presented in the top panels). Note the presence of extensive fiber deposition in TPO-treated wild-type animals while there is no appreciable change in the fibers of the GATA-1^low mice after TPO. Similar results were observed on sections from bone marrow and liver (not shown). Original magnification × 200; numerical aperture, 0.55.