P4.2 gene expression in E12.5 embryos. Sagittal sections of embryos at E12.5 days were hybridized with the P4.2 antisense riboprobe and photographed under bright-field (A) or dark-field (B) illumination. P4.2 hybridization signal was detected in the heart, liver, and blood vessels. H, heart; L, liver; DA, dorsal aorta. Higher-power views of the heart ([C] bright-field illumination and [D] dark-field illumination) or dorsal aorta ([E] bright-field illumination and [F] dark-field illumination) region are also shown. Arrowhead points to P4.2-specific labeling in erythroid cells in the dorsal aorta (E and F). Micrographs were taken with a 10× (A and B) or 20× (C, D, E, and F) objective.