Mouse incisor tooth organs express the genes responsible for enamel extracellular matrix formation exclusively on the labial surface of the organ. A previous investigation has suggested that lingual inner dental epithelium of mouse incisor did not contain potential ameloblasts. The present work extends our histological observations, by analyzing the presence of mouse amelogenin mRNA in heterotopic mouse incisor tissue recombinations using in situ hybridization to 35S-labelled asymmetric complementary RNA probes from a cDNA specific to the mouse Mr 26 x 10(3) amelogenin. Labial polarized ameloblasts located in front of the lingual predentin layer were associated with numberous hybridization signals indicating an increased density of amelogenin mRNA. In contrast, the lingual inner dental epithelium in contact with labial predentin never showed amelogenin hybridization signals, indicating the absence of amelogenin transcripts. These results confirm our observation that the lingual inner dental epithelium does not contain potential ameloblasts, since these cells do not transcribe nor accumulate mRNA amelogenin even when in contact with a favorable microenvironment provided by the labial predentin.