Abscopal and Direct Effects on Calcium Mobilization, Alkaline Phosphatase Levels, and Dentin Formation following X-Irradiation of either the Rat Incisor or the Thyroid-Parathyroid Region

¹ Department of Radiation Biology, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, Pennsylvania

Two hundred and sixteen female rats were placed in three equal groups using a random group design. One group of 72 rats was sham-irradiated and maintained as a control. Rats in the two experimental groups received 900 r of X-radiation to either the area of the developing maxillary incisors (head) or to the thyroid-parathyroid region. Each animal was given an intraperitoneal injection of radiocalcium (⁴⁵calcium) immediately following irradiation. Eight animals from each group were sacrificed at 1, 4, 8, 12, 17, 20, 24, 28, and 32 days post-irradiation.

At sacrifice, the rats were weighed, examined for evidence of gross pathology, and the two maxillary incisors, right femur, and other requisite tissues were taken for determination of their radiocalcium, total calcium, and/or alkaline phosphatase content.

An additional 43 animals were used for the correlation of histological changes with the above-measured parameters and to obtain the excretion patterns for the injected radiocalcium in each group. Sixteen of these animals were observed clinically until 96 days post-irradiation.

Alterations in the odontogenic response were evaluated on the basis of calcium distribution in the animals and radiographic, histologic, and alkaline-phosphatase-level changes in the maxillary incisor teeth.

Both experimental groups, the head-irradiated and the thyroid-parathyroid-irradiated animals, showed similar patterns in mean radiocalcium uptake by their maxillary incisors. A significant decrease in radiocalcium uptake at 4 days post-irradiation was followed by a generalized increase in ⁴⁵calcium uptake over control values after this period. The head-irradiated animals, however, showed distinct radiographic and histologic evidence of odontogenic disturbance in the developing maxillary incisors after 8 days post-irradiation. This disturbance was manifested as an area of reduced radiographic film density and histologically as an area of disruption of the odontoblastic cell layer, osteodentin formation, and/or the presence of a dentinal niche. No such evidence of odontogenic disturbance was noted in either the control or thyroid-parathyroid-irradiated animals.

The extractable alkaline phosphatase levels in the maxillary incisors of the head-irradiated animals were reduced significantly from control incisor values over most of the experimental period. Little difference from control values was noted in enzyme level of the incisors from the thyroid-parathyroid-irradiated animals. It was also noted that no relationship seems to exist between the level of this enzyme in the maxillary incisor and the degree of tooth calcification.

From the results of this investigation, it was concluded that a definite difference in odontogenic response of the maxillary incisors occurred following direct (head) irradiation of this structure as compared to the abscopal or indirect effects on tooth formation from irradiation of the thyroid-parathyroid area. However, it was further concluded that both experimental groups showed altered calcium mobilization patterns in the maxillary incisors as compared to the control animals. Furthermore, few differences in these patterns were observed between the two experimental groups.