Arteries define the position of the thyroid gland during its developmental relocalisation

Abstract

During vertebrate development, the thyroid gland undergoes a unique relocalisation from its site of induction to a distant speciesspecific position in the cervical mesenchyme. We have analysed thyroid morphogenesis in wild-type and mutant zebrafish and mice, and find that localisation of growing thyroid tissue along the anteroposterior axis in zebrafish is linked to the development of the ventral aorta. In grafting experiments, ectopic vascular cells influence the localisation of thyroid tissue cell non-autonomously, showing that vessels provide guidance cues in zebrafish thyroid morphogenesis. In mouse thyroid development, the midline primordium bifurcates and two lobes relocalise cranially along the bilateral pair of carotid arteries. In hedgehog-deficient mice, thyroid tissue always develops along the ectopically and asymmetrically positioned carotid arteries, suggesting that, in mice (as in zebrafish), co-developing major arteries define the position of the thyroid. The similarity between zebrafish and mouse mutant phenotypes further indicates that thyroid relocalisation involves two morphogenetic phases, and that variation in the second phase accounts for species-specific differences in thyroid morphology. Moreover, the involvement of vessels in thyroid relocalisation sheds new light on the interpretation of congenital thyroid defects in humans.KEY WORDS: Thyroid, Zebrafish, Mouse, Arteries, Vegf, Scl, Hedgehog Development 133, 3797-3804 (2006) DEVELOPMENT MATERIALS AND METHODS AnimalsZebrafish work was carried out according to standard procedures and staging in hours post fertilisation (hpf) refers to development at 28.5 to 29°C. The term 'larva' is used in the text for fry that have hatched from the chorion and generally refers to an age older than 72 hpf. The mutant zebrafish line kdr y17 (Covassin et al., 2006) is allelic to the kdr (flk1) lines described previously (Habeck et al., 2002) and develops heart oedema owing to compromised circulation or other specific defects at stages older than 40 hours, so that determination of mutant phenotypes was possible based on morphology. Homozygous cloche mutant embryos (clo s5 ) (Stainier et al., 1995) were also identified before fixation according to their phenotype at 24 hpf. Phenotypic details described in this study were always evident in all homozygous specimens (more than 10 in each experiment).For analysis of short digits (Dsh/Dsh) (Niedermaier et al., 2005) and Xt J /Xt J mutant mouse embryos (Persson et al., 2002) timed matings of mutants were generated. The homozygous phenotypes are distinguishable from wild type owing to severe morphological defects, and in the case of Dsh-Xt J matings we determined the genotype by PCR from extra-embryonic membranes (primer information available upon request). Preparation of specimensIn situ hybridisation on zebrafish was carried out according to standard procedures, using nk2.1a (Rohr and Concha, 2000) as molecular marker for the thyroid primordium. Whole-mount immunohistochemistry with antibodies against thyroid h...

Morphogenesis of the thyroid gland

“…Specifically, in zebrafish Scl or vegf mutants in which the vascular patterning of the pharyngeal region including the thyroid specification zone of the endoderm is severely disturbed thyroid progenitors are readily specified (Alt et al, 2006a). In cloche mutants lacking all vascular progenitors of the anterior trunk the thyroid primordium is present (Alt et al, 2006a). That this also may account for the embryonic mouse thyroid is suggested by observations of a thyroid rudiment present in Tie2 mutants lacking endothelial cells (Postiglione and di Lauro, personal communication).…”

“…human, mouse and chicken) and teleosts (e.g. zebrafish) (Kameda, 1995;Fagman et al, 2006;Alt et al, 2006a). Follicular cells in zebrafish also resemble thyrocytes of higher vertebrates in all other aspects, i.e.…”

“…However, available experimental data clearly indicate that thyroid specification does not require a vascular signal. Specifically, in zebrafish Scl or vegf mutants in which the vascular patterning of the pharyngeal region including the thyroid specification zone of the endoderm is severely disturbed thyroid progenitors are readily specified (Alt et al, 2006a). In cloche mutants lacking all vascular progenitors of the anterior trunk the thyroid primordium is present (Alt et al, 2006a).…”

“…That this also may account for the embryonic mouse thyroid is suggested by observations of a thyroid rudiment present in Tie2 mutants lacking endothelial cells (Postiglione and di Lauro, personal communication). As will be highlighted later on, the nearness to embryonic vessels is instead likely important for later stages of thyroid development (Alt et al, 2006a;Fagman et al, 2007).…”

“…This process is generally described as migration of the thyroid primordium ( Fig 3B). A corresponding translocation takes place in zebrafish embryos although the thyroid tissue does not assemble into an encapsulated organ (Alt et al, 2006a). In the human fetus defective or incomplete caudal movement of the thyroid may be responsible for several clinically important conditions, e.g.…”