Effects of irradiation on the nonlymphoid thymus in vitro

Thymic explant cultures were used to study the radiosensitivity of nonlymphoid thymic components in dogs. Thymic fragments from fetal (50 days gestation), newborn, and juvenile (70 days old) dogs were irradiated in vitro at 0, 0.5, 1, 2, or 4 Gy prior to culture. Colonies were classified as epithelial, spindle, or mixed cell type, and colony numbers were counted and diameters measured. Radiation caused a significant dose-related decrease in the number of spindle cell colonies from all ages. There was a corresponding, but smaller, dose-related increase in the number of epithelial colonies. The diameter of spindle cell colonies also decreased with dose, and this was accompanied by a reduction in cell density. While epithelial colony diameters did not change consistently with dose, there was an overall reduction in cell density in these colonies. This was more severe in the fetal than in the juvenile cultures. These results indicate that the putative mesenchymal (spindle cell) components of the thymus are significantly more radiosensitive than the epithelium at all ages and that fetal epithelium is more sensitive than epithelium from postnatal animals. This suggests that radiation-induced thymic epithelial defects reported after prenatal irradiation could be due to a combination of direct epithelial injury and defective inductive interaction between epithelium and the more radiosensitive mesenchyme.