Secretory activity as a function of the development and maturation of ameloblasts

The biosynthetic and secretory activity of rat incisor ameloblasts was studied by grain count analysis of radioautographs at various times following a single injection of either 3H-methionine, 3H-leucine, or 3H-glycine. Experiments were also carried out with leupeptin, a thiol and serine proteinase inhibitor which blocks degradation of proteins within lysosomes. The results from this study indicate that the biosynthetic and secretory activities of ameloblasts increase steadily as the cells differentiate (presecretory stage) and start to form the enamel layer (secretory stage). Secretory activity reaches a peak when the ameloblasts form about one-third of the eventual thickness of the enamel, and remains at this high level until shortly before they start to form the outer and final layers of enamel. Secretory activity then drops rapidly as the cells undergo postsecretory transition, and declines slowly thereafter as the shortened ameloblasts modulate continuously along the surface of the maturing enamel. Ameloblasts appear to biosynthesize more proteins than are secreted. The excess proteins are degraded rapidly in lysosomes and the amino acids reutilized for production of new exportable and/or structural proteins.