Periodontal Ligament of the Rat Incisor A Quantitative Enzyme Histochemical Analysis of the Distribution of Alkaline Phosphatase Activity

The spatial distribution of alkaline phosphatase (ALP) activity was examined in the periodontal ligament of the continuously growing rat incisor. With the indoxyl-tet-razolium salt method, enzyme activity was demonstrated in undecalcified cryosections, and the amount of reaction product was quantified. ALP activity appeared to be distributed heterogeneously. Its highest activity was found in the bone-related compartment of the ligament. In the tooth-related compartment and the supracrestal extension of the ligament, enzyme activity was significantly lower, but still higher than in the lamina propria ofthe gingiva. In the part of the ligament bordering the cementum, highest activity was found in the apical region just occlusal to Hertwigs epithelial root sheath, where formation ofacellu-lar cementum begins. From there toward the incisal edge, the activity of the enzyme gradually decreased. It is suggested that differences among the various parts of the periodontal ligament are related to local variations in phosphate metabolism and cementum deposition.