Fig. 6. Thyroid relocalisation in vertebrates involves two morphogenetic phases. Schematic drawing summarising morphogenetic aspects in thyroid development of vertebrates. (A-C) Selected stages of thyroid development. Red, cardiovascular system; light green, pharyngeal epithelium; dark green, thyroid tissue. Cranial is towards the left. A and B are lateral views, schematics in C are dorsal or ventral views. (A) Evagination of the primordium from the pharyngeal epithelium on the level of the outflow tract of the heart in zebrafish or the aortic sac in mice, respectively. (B) First phase of relocalisation, visualised by the black arrow, close to the outflow tract or the aortic arch. (C) Distribution of thyroid tissue in larval stages (zebrafish), late embryonic stages (mouse) or adult (echidna). A second phase of relocalisation as shown for zebrafish and mouse is indicated by arrows. The directionality of growth of thyroid tissue in zebrafish is uncertain (bi-directional arrow), as ventral aorta and thyroid grow concomitantly. In the adult echidna, the thyroid is positioned at the base of the cervical arteries. Despite a lack of embryological data, it is tempting to assume that in this species a second phase of morphogenetic changes is reduced or missing. The midline is indicated by broken lines. ao, aortic arch; as, aortic sac; ot, outflow tract.