All primitive and larval granulocytes express PU.1. (A) Whole-mount immunodetection of PU.1 with a polyclonal antibody directed against zebrafish PU.1. Left lateral view of a 14-somite embryo counterstained with DAPI, showing the immunostained cells in the rostral-most, “myelopoietic” lateral mesoderm (). At this early stage, the staining is only nuclear. (B) Coimmunodetection of PU.1 (green) and L-plastin (red) in the yolk sac of a 24 hpf embryo; nuclei stained with DAPI. (C-F) Immunostaining for PU.1 at 5 dpf; (C,D) Ramified nongranulocytic myeloid leukocytes in the trunk epidermis; an asterisk points at the nucleus, positioned at one end of the cell; (E) caudal fin; (F) CHT (, rostral end). (G-L) Immunostaining for PU.1 after Sudan Black staining at 48 hpf (G-I, L) and 72 hpf (J, K). All SB⁺ cells also express PU.1. For example, in the pericardial region (G), posterior (H) and ventral to the ear (J), and in the CHT (K); in panel K, a vertical line separates 2 parts captured at a slightly different focus. The PU.1⁺, SB cells are probably macrophages, notably in the retina and brain (I). y indicates yolk sac; p, pericardial area; e, inner ear; n, notochord; and ca, caudal artery. Scale bars, 200 μm in panel A, 15 μm in panel B, 20 μm in panels C,G,L, 10 μm in panel D, 5 μm in panel E, and 50 μm in panel F.