Cell-matrix interactions and cell-cell junctions during epithelial histo-morphogenesis in the developing mouse incisor

The continuously growing rodent incisor develops mainly along its antero-po sterior axis. The labio-lingual asymmetry which characterizes this tooth is initiated at the cap stage and increases further during the cap to bell tr ansition (ED14 to ED16) when histogenesis of the enamel organ proceeds. His tology, transmission electron microscopy ITEM), and immunostaining were use d to document the changes in the basement membrane (BM) as well as the modi fications of epithelial cell-matrix and cell-cell interactions during this period. The expression of plakoglobin, desmoglein and E-cadherin at ED14 su ggested that the main cell-cell junctional complexes were adherens junction s. The expression of desmoglein and TEM observations suggested a progressiv e antero-posterior stabilization of the enamel organ by means of desmosomes from ED14 to ED18. alpha6 integrin, BP 230 and laminin gamma2 chain were a ll expressed in the developing incisor but were not always co-distributed. Immunostaining and TEM suggested that only primitive type II hemidesmosomes were present. At ED14, cells of the enamel knot (EK) did not show any spec ific expression for antigens involved in cell-cell interaction. However, st rong staining for the laminin gamma2 chain characterized the BM in contact with EK cells. The BM in the labial part of the cervical loop demonstrated ultrastructural changes: the presence of loops of the lamina densa in this region preceeded the differential expression of the integrin alpha6 subunit and that of the laminin gamma2 chain in the labial/lingual parts of the ce rvical loop. Apoptosis was transiently observed in the contiguous mesenchym e. This affected osteoblasts and also nerve cells close to the labial part of the cervical loop.