Alkaline phosphatase activity in the periodontal ligament and gingiva of the rat molar: its relation to cementum formation.

Alkaline phosphatase (ALP) is a glycoprotein thought to be involved in processes leading to mineral formation in tissues like bone and cementum. In the rat molar periodontium, several regions are associated with the formation of cementum (periodontal ligament, inner part of the gingiva), whereas other areas are not (e.g., the outer part of the gingiva just beneath the outer oral epithelium). In an attempt to establish how the spatial distribution of ALP activity relates to cementum formation, we assessed the activity of the enzyme quantitatively in the periodontium of the rat maxillary molars, by using the indoxyl-tetrazolium salt method. It appeared that the distribution of enzyme activity in the ligament was heterogeneous, indicating local variations in the phosphate household. Highest activity was found in areas related to mineralization, adjacent to the alveolar bone and cementum. Enzyme activity was higher adjacent to cellular cementum than to acellular cementum. With respect to acellular cementum, a highly significant positive correlation was found between ALP activity and cementum thickness, which indicates a close relationship between local phosphate production and cementum formation rate. An interesting observation in the connective tissue of the gingiva mesial to the first molar was a sharp demarcation between an ALP-positive inner part, adjacent to the tooth, and an ALP-negative outer part, underneath the outer oral epithelium. In the interdental gingiva, the entire connective tissue proved positive for the enzyme, suggesting that this region consists of the combined inner gingival parts of two adjacent teeth.