Sudan Black (SB)–stained granulocytes coincide with peroxidase-positive leukocytes in zebrafish embryos. (A-D) At 24 hpf, 10 hours before the first detection of SB-stained granules, Cy3-tyramide based detection of endogenous peroxidase activity already reveals numerous peroxidase-positive leukocytes in the yolk sac (A,C), cephalic mesenchyme (B), and tail (A,D); note the unstained nuclei in panel D. Unlike at later stages, the staining appears concentrated in foci or granules (B,C). In panel A, vertical lines indicate a picture element captured at a slightly different focus. (E-M) 48 hpf embryos; detection of endogenous peroxidase activity with FITC-tyramide followed by the detection of granules with SB demonstrates a perfect coincidence of the 2 stainings. (E-H) High magnification of cells stained with both SB (E,G) and FITC-tyramide (F,H) near the ear. (I,J) Lateral right views of the eye and ear regions. Arrowheads point at the 3 cells shown at higher magnification in (E-H). (K-M) ventral tail (rostral to the left): DIC optics (K), fluorescence (M), and overlay (L). ca indicates caudal artery; e, ear; and r, retina. Scale bars, 15 μm in panels B and E, 40 μm in panel K, and 50 μm in panels I and J.