Figure 1.

A highly schematic model of normal development to explain how the observed transdetermination might occur in the lungs of transgenic mice expressing a constitutive activator of Wnt signaling. (a, b) The mesoderm associated with the endodermal epithelium normally emits Wnt signals in two regions, at two different times. (c) Because the competence of the endoderm - its potential to form different tissues - changes during this period, this results in the activation of different homeotic genes, and the formation of different tissue types. But artificially prolonged signaling would activate both homeotic genes, with unpredictable consequences that might include the formation of cell types belonging to both tissues.