Immunocytochemical and radioautographic evidence for secretion and intracellular degradation of enamel proteins by ameloblasts during the maturation stage of amelogenesis in rat incisors.

In the continuously erupting rat incisor the ameloblasts progress through distinct stages associated with the secretion and maturation of enamel. We have examined the possibility that the so-called "postsecretory" ameloblasts of the maturation stage of amelogenesis remain biosynthetically active and are engaged in the synthesis, secretion, and degradation of enamel gene products. The ultrastructural distribution of antigenic sites for enamel proteins was studied within enamel organ cells during the early maturation stage of amelogenesis in rat incisors by using the protein A-gold immunocytochemical technique and rabbit polyclonal antibodies developed against mouse amelogenins. All regions of amelogenesis from late secretion through the first complete modulation from ruffle-ended to smooth-ended ameloblasts were examined. Specific immunolabelling was found within the rough endoplasmic reticulum, Golgi saccules, secretory granules, and lysosomes of ameloblasts throughout these regions. The heaviest intracellular immunolabelling was found within secretory granules and lysosomes (multivesicular type). Quantitative analyses showed that the Golgi saccules and the multivesicular lysosomes of modulating ameloblasts were generally less immunoreactive compared to similar organelles in ameloblasts secreting the inner enamel layer. Radioautographic studies confirmed that ameloblasts of the maturation stage incorporated 3H-leucine and 3H-methionine and secreted labelled proteins into the enamel layer. Grain counts indicated that ameloblasts from the first ruffle-ended band incorporated about two-fold less 3H-methionine and secreted about tenfold less labelled proteins into the enamel compared to ameloblasts secreting the inner enamel layer. The results of this study confirm that ameloblasts do not terminate biosynthesis and secretion of enamel proteins once the final layer has been deposited on the surface of the developing enamel. They continue to form and release new proteins during the maturation stage which intermix with older proteins laid down initially during the secretory stage of amelogenesis. Secretory activity for enamel proteins has been detected in ameloblasts up to at least the second ruffle-ended phase of maturation, at which point the enamel matrix is partially soluble in EDTA.