Localization of cells expressing MITF and PU.1 mRNAs in the embryo. (A) Localization of cells expressing PU.1 mRNA in the 11.5-dpc embryo. PU.1-positive cells are particularly concentrated in the liver (l). They can be detected as a fine speckled pattern throughout the embryo, especially flanking the neural tube at the bottom of the field. (B) Localization of PU.1-positive cells in a sagittal section of 12.5 dpc embryo confirming that each focus of stain represents a single labeled cell, in this case located at intervals along the dorsal midline in mesenchyme surrounding the spinal cord. (C) A section through the same embryo as in (B) at higher magnification. Each of the cells expressing PU.1 mRNA in this region contains multiple foci of neutral red staining indicating phagocytosis of pyknotic cells. (D) Localization of cells expressing PU.1 mRNA in an embryo at 12.5 dpc showing accumulation of the positive cells at the limb margins. Note the distinction from the pattern obtained with c-fms in Fig 1D. (E) Localization of MITF mRNA in the embryo. This panel shows an embryo still surrounded by the yolk sac at 9.5 dpc. The yolk sac contains numerous scattered positive cells. The band of positive staining flanking the yolk sac represents expression of MITF in trophoblasts. At higher magnification, the pattern is indistinguishable from MMR (3A) orc-fms.25 (F) At 10 dpc, mi-positive cells spread from the brain throughout the body. Bar in panels A, D, and F represents 250 μm, and in panels B and C, 20 μm.