IL-1RI is expressed by cells in the E11 region aortic hematopoietic, endothelial, and mesenchymal cells. Immunostaining performed with (A) a control antibody and (B,C) an IL-1RI–specific antibody on transverse cryosections from E11 embryos. (C) An enlargement of the boxed area in panel B. The dorsal aorta of the AGM region is shown with the ventral side at the bottom of the section. Positive signal is seen as an orange-brown precipitate from the DAB chromogen. The sections were counterstained with hematoxylin. Orange arrowheads indicate single IL-1RI–expressing cells in the circulation and scattered in the tissue. Slides were viewed with an Olympus BX40 research microscope (Olympus Nederland B.V., Zoetewoude, The Netherlands) using an Olympus lens at 20×/0.40 PH. Images were acquired and processed with Adobe Photoshop version 7.0 (Adobe Systems, San Jose, CA).