[3H]Choline Uptake and Turnover into Membrane and Extracellular Matrix Phospholipids, Visualized by Radioautography in Rat Incisor Dentin and Enamel

In order to study the uptake and fate of [3H]choline into cellular and extracellular phospholipids in the forming part of mandibular rat incisors, radioautography was carried out after treatment with the iodoplatinate reaction which retains phospholipids. Thirty minutes and 1 hour after the intravenous injection of the radiolabeled precursor, grain density in secretory odontoblasts and ameloblasts was not significantly above background labeling whereas dentin was actually labeled. Therefore, at this early period, odontoblasts cannot be responsible for the secretion of phospholipids incorporated into dentin, and intercellular diffusion of components originating from blood could explain this early dentin labeling. After 2 hours, odontoblasts and ameloblasts were labeled. In cells, grain density reached a maximum at 4 hours, reduced at 24 hours, and strongly decreased at 4 days. In predentin and enamel, grain density peaked at 24 hours and diminished at 4 days. However, in the forming enamel 4 days after the injection, labeling was twice as high as in any other compartment. Altogether, the results highlighted two distinct pathways for phospholipids in dental mineralized dental tissues: a first one shows evidence of early incorporation of [3H]choline into dentin resulting from intercellular diffusion independently from odontoblasts secretion, whereas inside the forming enamel, higher labeling and longer retention of choline-containing membrane components were detected between 4 hours and 4 days. This suggests an accumulation of membranes that are not subjected to rapid turnover in contrast with other dental compartments.