Localization of c-fms mRNA in PU.1 knockout animals. (A and B) Comparison of c-fms mRNA localization in wild-type (A) or PU.1(−/−) (B) 11.5 dpc littermates. Note the speckled pattern throughout the embryo representing individualc-fms–positive cells, particularly concentrated in the liver (l). The apparent difference in intensity of staining of individual cells is not significant and is due to the focal plane and lighting. (C and D) Saggital sections through a PU.1(−/−) embryo stained as in (B), showing that cells expressing c-fms in the liver and limb buds have similar morphology and location to those in wild-type mice (see Fig 1). Bar in (A) and (B) represents 250 μm and in (C) and (D), 20 μm.