Fig. 2. T4 immunostaining in the early (4.5-5 dpf) zebrafish larva. (A-I) Selected cross-sections at 4.5 dpf, T4 immunostained (H,I, high magnification). (A) Overview. Cross-section through the head at an anteroposterior level between D and E (compare with schematic drawing in J). Cartilage is highlighted by colours as in J (yellow, first branchial arch; magenta, second branchial arch; green, basibranchial cartilage that is composed of derivatives of all branchial arches). The arrow shows immunostaining in the anterior non-follicular domain, the arrowhead a pigment cell. Insert indicates adjacent control section, processed for immunostaining without first antibody. Note absence of immunostaining (arrow). The arrowhead indicates the same pigment cell as in the adjacent section. (B-G) Selected sections from one embryo at different anteroposterior levels as indicated in J. Arrows indicate immunostaining in the thyroid. (H) Close up of the anterior non-follicular T4 domain. (I) Close up showing a follicle further posterior. (J) Schematic drawing of a zebrafish larval head of 4.5 to 6 dpf, ventral view, showing the skeleton, parts of the circulatory system and the thyroid. The approximate positions of the sections shown in B-G are indicated as grey bars. Blue, thyroid/T4 immunostaining; brown, third to sixth branchial arches; green, basibranchial cartilage; magenta, second branchial arch; orange, heart and ventral aorta; yellow, first branchial arch. bb, basibranchial cartilage; ch, ceratohyale; co, colloid; h, heart; me, Meckel’s cartilage; pq, palatoquadrate; va, ventral aorta.