Position- and time-restricted amelogenin gene transcription was analysed in developing tooth organs using in situ hybridization with asymmetric complementary RNA probes produced from a cDNA specific to the mouse 26 × 10(3) Mr amelogenin. In situ analysis was performed on developmentally staged fetal and neonatal mouse mandibular first (M1) and maxillary first (M1) molar tooth organs using serial sections and three-dimensional reconstruction. Amelogenin mRNA was first detected in a cluster of ameloblasts along one cusp of the M1 molar at the newborn stage of development. In subsequent developmental stages, amelogenin transcripts were detected within foci of ameloblasts lining each of the five cusps comprising the molar crown form. The number of amelogenin transcripts appeared to be position-dependent, being more abundant on one cusp surface while reduced along the opposite surface. Amelogenin gene transcription was found to be bilaterally symmetric between the developing right and left M1 molars, and complementary between the M1 and M1 developing molars; indicating position-restricted gene expression resulting in organ stereoisomerism. The application of in situ hybridization to forming tooth organ geometry provides a novel strategy to define epithelial-mesenchymal signal(s) which are believed to be responsible for organ morphogenesis, as well as for temporal- and spatial-restricted tissue-specific expression of enamel extracellular matrix.

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