Histochemical characterization of alkaline phosphatase in developing rat teeth and bone

Alkaline phosphatase (EC 3.1.3.1.) in developing teeth and in bone has been studied. Prior to hard tissue function a rather high enzyme activity was noted in differentiating odontoblasts, stratum intermedium, and outer enamel epithelium. A lower activity was observed in the cells of the dental papilla and stellate reticulum. After the onset of hard tissue formation the alkaline phosphatase activity was generally increased. Enzyme activity was also found in the proximal part of tall, secretory ameloblasts. In the short postsecretory ameloblasts a high enzyme activity was noted. At the onset of dentin mineralization there was an increase in enzyme activity in the cells of the subodontoblastic layer. In bone the highest alkaline phosphatase activity was found in osteoblasts. A difference was noted between the alkaline phosphatase of hard and soft tissues by means of the addition of inhibitors to the incubation media. Within the hard tissues it was possible to distinguish between two alkaline phosphatases after pretreatment with heat (56 degrees C) or the addition of specific inhibitors (sodium metavandate, ortho-and pyrophosphate). An isoenzyme which was sensitive to these procedures was demonstrated in odontoblasts and in the pulpal connective tissue. Another alkaline phosphatase isoenzyme, which was resistant to pretreatment with heat or the addition of vanadate or phosphate, was demonstrated in the subodontoblastic cell layer, stratum intermedium and the outer cells of the reduced enamel epithelium.