Figure 8

Pharyngeal organs fail to migrate in Alk5/Wnt1-Cre mutants. At E14.0, the thymus was not detectable in the superior mediastinum (asterisks) in Alk5 mutants (D), when compared to controls (A). Serial sectioning revealed that the thymic primordia had failed to descend and were seen bilaterally in the upper pharyngeal region (E, F) surrounded by neural crest derived mesenchyme (blue staining cells in E). In controls, the parathyroid glands were properly associated with the thyroid glands (arrows in C), while in Alk5 mutants the parathyroid glands were associated with the thymic primordia (arrows in F). However, both controls and mutants expressed parathyroid hormone (PTH) at comparable levels (blue staining in C and F). A and D, hematoxyllin and eosin staining; B and E, R26 R lineage tracing assay – counterstaining with eosin; C and F, section in situ hybridization for PTH – counterstaining with eosin. T, thymus; Th, thyroid; asterisks in D depict the absence of the thymic primordia; asterisks in E and F depict the tongue.