Fig. 1. Reconstruction of the thyroid gland in adult zebrafish. (A) Cross-section of an adult zebrafish head in the ventral gill region at the level of the second branchial arch (Haematoxylin and Eosin staining). The arrows indicate the thyroid follicles. (B-F) High magnification views of thyroid follicles of adult zebrafish. (B) Haematoxylin and Eosin staining. The arrow shows the follicular epithelium. (C,D) Giemsa staining. The arrows show the follicular epithelium in C and the nuclei of the epithelium in D, the arrowhead indicates a vesicle at the apical surface of the follicular cells. (E) PAS staining visualising the follicular epithelium (arrow) in medium pink and the colloid in light pink; the arrowheads indicate vesicles at the apical surface of the follicular cells. Cartilage is stained strongly. (F) Immunostaining with an antibody against T4 reveals strong reactivity of the follicular cells (arrow). (G) Reconstruction of the distribution of thyroid follicles in the adult zebrafish head. Thyroid follicles in blue, heart and ventral aorta in red, gill arches in yellow. ca, cartilage; co, colloid; g, gills; mo, mouth/pharynx; va, ventral aorta.

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