The thymus and parathyroids develop from third pharyngeal pouch (3rd pp) endoderm. Our previous studies show that Shh null mice have smaller, aparathyroid primordia in which thymus fate specification extends into the pharynx. SHH signaling is active in both dorsal pouch endoderm and neighboring neural crest (NC) mesenchyme. It is unclear which target tissue of SHH signaling is required for the patterning defects in Shh mutants. Here, we used a genetic approach to ectopically activate or delete the SHH signal transducer Smo in either pp endoderm or NC mesenchyme. Although no manipulation recapitulated the Shh null phenotype, manipulation of SHH signaling in either the endoderm or NC mesenchyme had direct and indirect effects on both cell types during fate specification and organogenesis. SHH pathway activation throughout pouch endoderm activated ectopic Tbx1 expression and partially suppressed the thymus-specific transcription factor Foxn1, identifying Tbx1 as a key target of SHH signaling in the 3rd pp. However, ectopic SHH signaling was insufficient to expand the GCM2-positive parathyroid domain, indicating that multiple inputs, some of which might be independent of SHH signaling, are required for parathyroid fate specification. These data support a model in which SHH signaling plays both positive and negative roles in patterning and organogenesis of the thymus and parathyroids.
Author contributions
V.E.B. designed the experiments and performed the analysis under the supervision of N.R.M.; J.D.O. confirmed results and contributed images to Figs 2, 8 and 9; J.G. contributed images to Fig. 3; J.G. and I.R. contributed Fig. S3; E.R.R. and N.R.M. conceived the project; V.E.B., J.G., E.R.R. and N.R.M. wrote and edited the manuscript.
Funding
This work was supported by National Institutes of Health grants [R01HD056315 and R01AI107096] to E.R.R. and N.R.M. Deposited in PMC for release after 12 months.
