Whereas Rara is expressed throughout the developing digestive tract, Rarb expression is spatially restricted: it is found in both the epithelial and mesenchymal layers of the oesophagus and the anterior (cardiac) portion of the stomach, is absent at pyloric and duodenal levels (Figure 6D,E), and reappears more posteriorly at midgut levels [Dolle et al., 1990]. As previously mentioned, Rarg transcripts are specific to the differentiating squamous epithelia in the oesophagus and left wall of the stomach (Figure 3D) [Ruberte et al., 1990]. The Rarb2 isoform is expressed in the developing diaphragm and liver capsule, whereas both Rarb isoforms are expressed in the pancreas primordium (Figure 6D,E). Whether their expression is restricted to a given cell lineage has not been determined. RA produced by the RALDH2 enzyme was recently shown to be required for early development of the dorsal pancreas in the mouse [Martin et al., 2005; Molotkov et al., 2005].
