Localization of cells expressing mRNAs encoding endocytic receptors in the embryo. (A) Localization of MMR mRNA at 9.5 dpc. Individual labeled cells appear in a diffuse speckled pattern (arrows) throughout the yolk sac. (B) Localization of MMR mRNA in 11.5 dpc embryo. MMR-positive cells are distributed very similarly toc-fms–positive cells (Fig 1A) at the same time point as diffuse speckled pattern throughout the embryo. In this focal plane, the particular concentration of MMR-positive cells along the dorsal midline flanking the neural tube is evident. The darkened appearance of the liver, immediately below the forelimb bud (fl) is due to numerous MMR-positive cells, which are out of focus in this plane (hl, hindlimb bud). (C) Localization of the MSR mRNA at 10.5 dpc. The first labeled cells can be detected in the liver (l) at this time, immediately below the heart (*). At this time point, few MSR-positive cells could be detected elsewhere in the embryo. (D) The section through the liver of an 11.5-dpc embryo stained for MSR mRNA. Some positive cells are associated with aggregates of hematopoietic cells (to the left of field), but many are clearly filled with engulfed cells (arrow). Most MSR-positive cells elsewhere in this embryo are large and contain neutral red-positive inclusions (not shown). Bar in panels A through C represents 250 μm and in panel D, 20 μm.