Time-related changes in the distribution of 45Ca in the developing enamel of rat incisors as revealed by radioautography.

The distribution and movement of calcium through the enamel organ and into the forming enamel was examined by means of quantitative 45Ca radioautography in rat incisors. Dislocation of radiocalcium in the specimen was minimized during histologic and radioautographic processing by using rapid freeze/freeze-substitution and dry emulsion coating methods. At 30 sec. after the 45Ca injection, distinct peaks of radioactivity occurred in the connective tissue immediately adjacent to the enamel organ and the infranuclear compartment of secretory ameloblasts. An intense labeling also occurred in the superficial layers of the forming enamel extending 15 microns below the surface. The grain density in the distal cytoplasm of secretory ameloblasts increased at later time periods, whereas all other regions of the enamel organ showed a considerable decrease in radioactivity. The radioactivity in the infranuclear compartment of ameloblasts with numerous mitochondria remained relatively high at 2 min. but was abolished by 10 min. after the injection. The grain density in the enamel matrix became much stronger but the labeled regions only extended to 20, 30, and 40 microns below the surface at 2, 10, and 60 min. after the injection, respectively. The application of wet emulsion over similarly prepared sections caused a severe dislocation of radiocalcium in the specimens. These data confirmed the rapid penetration of systemically administered radiocalcium into the surface layers of forming enamel and its slow diffusion to the deeper layers. The time-related changes in relative grain densities at various regions of the ameloblasts support the coexistence of a relatively slow transcellular pathway for calcium through the secretory ameloblast layer.