The animals were killed by transcardial perfusion with 4% paraformaldehyde in PBS. The eyes were removed and embedded in paraffin after removal of the lens. Rehydrated sections (7 μm) were incubated in 10% horse serum for 20 minutes to block nonspecific binding, washed three times in PBS, and incubated for 1 hour with anti-EPO-R antibody (rabbit polyclonal IgG, clone M-20, 1:100 dilution), anti-VEGF-R1 antibody (rabbit polyclonal IgG, clone c-17, 1:80 dilution), or anti VEGF-R2 antibody (mouse monoclonal IgG, clone A-3, 1:80 dilution). After a wash in PBS, sections were incubated with goat anti-rabbit IgG (1:1000, 1 hour; Vector Laboratories, Burlingame, CA). Immunohistochemical staining was performed with the avidin-biotin complex technique according the manufacturer’s protocol (Vector). Negative controls were performed by omitting the primary antibodies and in parallel (for EPO-R) by preabsorbing the primary antibodies with a 10-fold (by mass) excess of a specific blocking peptide (Santa Cruz Biotechnology, Inc., Heidelberg, Germany). Sections were photographed under a microscope (Axioplan; Carl Zeiss Inc., Thornwood, NY) on TMX400 film (Eastman Kodak, Rochester, NY).