Radioautographic studies on amelogenesis.

Radioautography has been used to visualize various aspects of morphogenesis and differentiation in the continuously erupting rat incisor. Formation of the entire incisor involves continuous production of "tooth segments" at the growing end of the tooth, each of which undergoes a similar history of development as it is carried by eruption towards the oral cavity. The sequence of differentiation which characterizes the life cycle of the ameloblasts was timed using 3H-thymidine, a precursor of DNA. The cells pass through presecretory, secretory and maturative stages whose collective activity results in the layer of mature enamel. 3H-Amino acids, as precursors of proteins were used to evaluate the protein synthetic activity of ameloblasts, before, during and after they produced the layer of enamel. Quantitative analysis (grain counts) of the differential utilization of 3H-proline and 3H-tyrosine by the various types of ameloblasts suggests that the cells produce structural proteins throughout their life cycle, but they produce enamel proteins only in the zone of secretion. The data further suggest that near the end of the presecretory zone structural proteins are used in the formation of Tomes' processes and that during secretion structural proteins contribute to the persistent growth of those processes as the rods are lengthening. Sugars such as 3H-N-acetylmannosamine and 3H-fucose were used to examine glycoprotein formation by ameloblasts. In the secretion zone labeled glycoproteins were not present in the enamel layer, but were confined to the cell bodies and Tomes' processes of ameloblasts at time intervals up to 4 hours after injection. This was contrary to the behaviour of extracellular proteins labeled with 3H-amino acids which left the cell and were present in the enamel at similar time intervals. The distribution of labeled sugars was indicative of turnover of membrane-associated glycoprotein possibly related to growth. This was interpreted as further support for the concept of a lengthening Tomes' process which remains embedded in the enamel until it is obliterated by the forming rod. Preliminary attempts were made to define the role of hormones in tooth development. Specific receptor sites for 125I-insulin were localized to the endothelial lining of capillaries in the papillary layer during maturation. Although the significance is unclear, the potential of this tool in studying dental morphogenesis is considerable.