Localization of cells expressing lysozyme mRNA in the embryo. (A) Panel shows a 10.5-dpc embryo in which lysozyme mRNA has been detected by whole mount in situ hybridization. The majority of labeled cells at this stage of development can be seen in the liver (l); the band observed in the heart is due to nonspecific trapping. A small number of cells can be seen in this focal plane in the forelimb bud, but few cells are detected elsewhere in the embryo in any focal plane. (B) A section through the liver of one of the embryos stained in (A) demonstrates that the majority of labeled cells have inclusions that stain with neutral red (arrow) and are evidently involved in phagocytosis of dying cells. Bar in (A) represents 250 μm and in (B), 20 μm.