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Fig. 2 han is expressed in tissues surrounding the thyroid primordium. (A-C) Whole-mount embryos, anterior is up; (D-F) sections. Labelling of panels is as in Fig. 1. (A,B) The thyroid primordium (arrow in B) is adjacent to the outflow tract of the heart (arrows in A). Two expression domains between the first and the second branchial arch (black arrowheads) flank the thyroid on the same anteroposterior level. Red arrowheads point to han (hand2) expression in the fin buds. (C,D) Thyroid hormone (TH) immunostaining visualises arch-associated neurons (AANs) in the putative carotid body primordium (arrowheads). Double staining of hand2 and TH (D) shows that both expression domains are adjacent to each other. (E) hand2 is also expressed in the endoderm (red arrowheads). Notice the strong expression in cells next to AANs (black arrowheads), and in the heart (arrows). (F) For comparison, see the thyroid marker in F (combined with the heart marker MF20). h, heart.