Characterization of mouse embryonic/fetal macrophages. (A) A representative E13.5 Tie2-GFP transgenic embryo analyzed for GFP (green), Cd45 (red), and Cd31 (blue) expression. The dorsal area of the embryo is shown. GFP is uniformly expressed by Cd31⁺ blood vessels and by scattered Tie2-GFP⁺Cd45⁺Cd31⁻ hematopoietic cells (arrows). Note that several Cd45⁺Cd31⁻ hematopoietic cells are GFP⁻ (arrowheads). Scale bar represents 60 μm. Similar results were obtained in E8.5 to E15.5 embryos. Results are representative of n = 8 litters (each including 2 or 3 embryos) analyzed. For each embryo, at least 3 sections were analyzed. (B) A representative E14.5 Tie2-GFP transgenic embryo analyzed for GFP (green), Cd31 (red), and F4/80 (blue) expression. The brain of the embryo is shown. GFP is uniformly expressed by Cd31⁺ blood vessels and by scattered Tie2-GFP⁺F4/80⁺Cd31⁻ macrophages (arrows). Note that several F4/80⁺Cd31⁻ macrophages are GFP⁻ (arrowheads). Scale bar represents 60 μm. Embryos were analyzed as described in panel A. (C) A representative E12.5 Tie2-GFP transgenic embryo analyzed for GFP (green), Mrc1 (red), and F4/80 (blue) expression. The spinal cord of the embryo is shown. GFP is expressed by Mrc1⁺F4/80⁺ macrophages that are intermixed with GFP⁺Mrc1⁻F4/80⁻ blood vessels in the meninges (inset). Scale bar represents 60 μm. The 2 bottom panels show a high-power magnification of the inset shown above. Note the perivascular GFP⁺Mrc1⁺F4/80⁺ macrophages. Embryos were analyzed as described in panel A.