The crown odontoblasts of rat molars from primary dentinogenesis to complete eruption.

The involution of crown odontoblasts after primary dentinogenesis in teeth of limited eruption is discussed. The odontoblasts of rat first lower molars were analyzed morphometrically from the tenth day to the 40th day of age, i.e., from the late phase of primary dentinogenesis to complete eruption. All the organelles underwent atrophy, but at different rates. In particular, the membranes of the endoplasmic reticulum decreased progressively in surface area from day 10 to day 40, whereas those of the Golgi apparatus decreased significantly between day 10 and day 14, and then remained practically unchanged in size. The volume of the lysosome compartment never increased beyond that during primary dentinogenesis. The profile length of the endoplasmic reticulum in each observed cell section was taken as an estimate of secretory activity. At day 40, this organelle was smaller in approximately 95% of the cells than it had been in any cell at day 10. These results suggest that cell atrophy may occur without any increase in the degradation processes of the cytoplasmic components and that the organelles along the secretory pathway may have independent regulatory systems. In the odontoblasts, as in several types of secretory epithelial cells, only a small fraction of the cells is engaged in appreciable secretory activity. This occurs, however, when the overall activity of the same cell population is relatively low.