Catecholamine cell groups in the peripheral nervous system of zebrafish embryos. Whole-mount embryos or transverse sections labeled with anti-TH antibody or antisense TH RNA probe. (A) TH-positive cells (indicated with black arrowheads) adjacent to the rostral hindbrain of 22S embryo—dorsal view. Rhombomeres r3 and r5 are labeled by Krox-20. (B–D) Ventral (B and D) and lateral (C) views of THir cells in 50-h (B and C) and 72-h (D) embryos. Arrowheads indicate THir cells posterior to the eyes. Rostral is up in D. (E) Transverse section through the pharyngeal arches of a 96-h fry showing THir cells located between the sternohyoid muscles of arch 3. (F) Prim-25 (36 h) embryo showing THir cells located at the interface between primordial arches 2 and 3, rostral to the gill arches (labeled in black with zn-8 antibody). (G) Putative sympathetic THir cells ventral and lateral to the rostral notochord of a 96-h fry. (H) Sympathetic (sym) THir neurons in a 96-h fry. Abbreviations: a, aorta; aac, arch-associated CA cells; cb, ceratobranchial; fp, floor plate; g1, 2, 3, and 4, gills 1, 2, 3, and 4; H, hyoid; M, mandibular; m, muscle; n, notochord; P2 and P3, pharyngeal arches 2 and 3; sh, sternohyoid; sym, sympathetic neurons; tr, trigeminal ganglia; tv, transverse ventral muscle; v, blood vessel. Scale bar: 80 μm (A), 100 μm (B, C, F), 50 μm (D, H), and 25 μm (E, G).