Overexpression of Irf8 promotes the development of macrophages but suppresses that of neutrophils. (A-B) Anti-Myc antibody staining indicates a significant induction of Irf8^myc protein expression in the heat shock-treated (HS) Tg(hsp70:Irf8^myc) embryo (n = 25/30) compared with the untreated (No-HS) embryo (n = 32/32). (C-F′) WISH shows csf1r RNA expression in 26 hpf untreated (No-HS) WT (n = 22/22) and Tg(hsp70:Irf8^myc) (n = 18/18) and heat shock-treated (HS) WT (n = 16/16) and Tg(hsp70:Irf8^myc) (n = 10/13) embryos. (C′-F′) Higher magnification (×20) views of the boxed regions (blue) in panels C, D, E, and F, respectively. (G) Histogram represents the overall number of csf1r⁺ cells (macrophages) in panels C, D, E, and F (n ≥ 9, mean ± SE). *Statistical differences with corresponding control (48% increase compared with No-HS Tg(hsp70:Irf8^myc), t test, P < .05; 58% increment compared with HS WT t test, P < .05). (H-K′) WISH shows mpx RNA expression in 26 hpf untreated (No-HS) WT (n = 17/17) and Tg(hsp70:Irf8^myc) (n = 20/20) and heat shock-treated (HS) WT (n = 13/15) and Tg(hsp70:Irf8^myc) (n = 11/16) embryos. (H′-K′) Higher magnification (×20) views of the boxed regions (blue) in panels H, I, J, and K, respectively. (L) Histogram represents the overall number of mpx⁺ cells (neutrophils) in panels H, I, J, and K (n ≥ 10, mean ± SE). *Statistical differences with corresponding control (46% decrease compared with No-HS Tg(hsp70:Irf8^myc), t test, P < .05; 38% reduction compared with HS WT, t test, P < .05). (C-F′,H-K′) Blue arrows indicate myeloid cells positive for csf1r and mpx, respectively. (C-F′) Black arrowheads indicate neural crest-derived pigment cells positive for csf1r.