Cellular activity in the developing palate of the rat assessed by the staining of nucleolar organiser regions

To provide information concerning biosynthetic activities within the mesenchymal cells of rat palatal shelves, the numbers of nucleolar organiser regions (NORs) were detected using an Ag-NOR staining technique within tissues obtained from coronal sections of 30 fetal heads prepared for light microscopy. The study involved comparisons between stages of palatogenesis and between location along the palatal shelves, anterior and posterior regions being analysed to correspond to the presumptive hard and soft palates. For both anterior and posterior regions, the mean Ag-NOR count/cell was initially high (approximately 3.4 anteriorly and approximately 3.0 posteriorly). The count/cell decreased significantly immediately after elevation but increased again after shelf reorientation and complete fusion. When comparisons were made between anterior and posterior regions of the palatal shelves, no differences were discerned at a stage prior to elevation (day 15) and a stage after the shelves had elevated and completely fused (day 17). Immediately post-elevation (day 16.5), however, the mean count/cell was significantly higher anteriorly. Thus the degree of mesenchymal cell activity, as expressed in terms of protein biosynthesis, can change in the developing palate according to both site and stage of development.