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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.
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