Differential expression of Sonic hedgehog along the anterior–posterior axis regulates patterning of pharyngeal pouch endoderm and pharyngeal endoderm-derived organs

Abstract

Previous studies have implicated Sonic hedgehog (Shh) as an important regulator of pharyngeal region development. Here we show that Shh is differentially expressed within the pharyngeal endoderm along the anterior-posterior axis. In Shh-/- mutants, the pharyngeal pouches and arches formed by E9.5 and marker expression showed that initial patterning was normal. However, by E10.5-E11.0, the first arch had atrophied and the first pouch was missing. Although small, the second, third, and fourth arches and pouches were present. The expression patterns of Fgf8, Pax1, and Bmp4 suggested that pouch identity was abnormal at E10.5 and that Shh is a negative regulator of these genes in the pouches. Despite the loss of pouch identity and an increase in mesenchymal cell death, arch identity markers were expressed normally. Our data show that a Shh-dependent patterning mechanism is required to maintain pouch patterning, independent or downstream of arch identity. Changes in the distribution of Bmp4 and Gcm2 in the third pouch endoderm and subsequent organ phenotypes in Shh-/- mutants suggested that exclusion of Shh from the third pouch is required for dorsal-ventral patterning and for parathyroid specification and organogenesis. Furthermore, this function for Shh may be opposed by Bmp4. Our data suggest that, as in the posterior gut endoderm, exclusion of Shh expression from developing primordia is required for the proper development of pharyngeal-derived organs.

Retinoic acid is required for endodermal pouch morphogenesis and not for pharyngeal endoderm specification

“…Other genes, such as pax9, shh, and foxi1 appear to be involved at later stages for endodermal pouch maintenance and growth. Disruption of these genes results in gene expression changes in the endodermal pouches (Peters et al, 1998;Nissen et al, 2003;Moore-Scott and Manley, 2005).…”

Morphogenesis of the thyroid gland

“…The significance is illustrated by the fact that more widespread Shh repression leads to ectopic pancreatic differentiation and that targeted over-expression of Shh in the prospective pancreas primordium partly diverts the lineage program of progenitors towards an intestinal fate (Apelqvist et al, 1997). That Shh might play a similar role in thyroid development by restricting the endoderm adopting a thyroid fate is suggested by the finding that Shh is strongly expressed in the adjacent foregut epithelium but not at all in the Nkx2-1-expressing progenitor cells forming the thyroid placode (Fagman et al, 2004;Parlato et al, 2004;Moore-Scott and Manley, 2005).…”

“…In Shh deficient embryos, which have asymmetric pharyngeal and carotid vessels, bilobation fails and the thyroid persists as an undivided mass on one side of the neck (Fagman et al, 2004;Alt et al, 2006a). This is of fundamental interest because Shh is not expressed in the median thyroid primordium neither at the placode stage (Fagman et al, 2004;Parlato et al, 2004;Moore-Scott and Manley, 2005) nor in the bud before it fuses event with the UB (HF and MN, unpublished results of genetic fate mapping using Shh-Cre/ROSA26R recombination). The effect is therefore most likely non-cell autonomous, i.e.…”

Maintenance of Thymic Epithelial Phenotype Requires Extrinsic Signals in Mouse and Zebrafish

“…Earlier studies have indicated how the Foxn1 expression domain might be established in the epithelium of the third pharyngeal pouch. In mice lacking Shh, the expression domain of Foxn1 is broadened at the expense of that marked by Gcm2 expression (the future parathyroid) (4). It is therefore possible that Shh expression initiates specification of the parathyroid that requires Gcm2 for further development.…”

“…Based on in vitro studies, it has also been suggested that Wnt signaling plays a role in activating Foxn1 in the prospective thymic rudiment (5). Interestingly, the future thymic anlage is marked by bone morphogenetic protein (BMP) 4 expression, while that of the future parathyroid is marked by Noggin expression (6), suggesting that BMP signaling might be involved at least in maintaining the reciprocal specification of parathyroid and thymus anlagen in the pharyngeal pouch. This hypothesis predicts that BMP signaling is required for the maintenance of Foxn1 expression and that loss of BMP signaling in the thymic anlage might convert it into parathyroid tissue.…”