CELLULAR-ACTIVITY IN THE DEVELOPING PALATE OF THE RAT ASSESSED BY THESTAINING OF NUCLEOLAR ORGANIZER REGIONS

To provide information concerning biosynthetic activities within the m esenchymal cells of rat palatal shelves, the numbers of nucleolar orga niser regions (NORs) were detected using an Ag-NOR staining technique within tissues obtained from coronal sections of 30 fetal heads prepar ed for light microscopy. The study involved comparisons between stages of palatogenesis and between location along the palatal shelves, ante rior and posterior regions being analysed to correspond to the presump tive hard and soft palates. For both anterior and posterior regions, t he mean Ag-NOR count/cell was initially high (approximately 3.4 anteri orly and approximately 3.0 posteriorly). The count/cell decreased sign ificantly 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 afte r the shelves had elevated and completely fused (day 17). Immediately post-elevation (day 16.5), however, the mean count/cell was significan tly higher anteriorly. Thus the degree of mesenchymal cell activity, a s expressed in terms of protein biosynthesis, can change in the develo ping palate according to both site and stage of development.