Effects of radiation on the developing canine thymus

The effects of in vivo and in vitro gamma irradiation of the non-lymphoid components of the thymus were studied in fetal, newborn and juvenile dogs. Thymic fragments were irradiated in vitro (0, 0.5, 1.0, 2.0 and 4.0 Gy) prior to explant culture. Explant outgrowths were classified as epithelial, spindle or mixed cell type and the colony diameters measured. Radiation caused a dose related decrease in both the number an diameter of spindle cell colonies at all ages, and a corresponding increase in the number of epithelial-colonies. Diameter of epithelial colonies showed a dose related decrease in fetuses but an increase in neonates and juveniles. Although the epithlium in the fetus was significantly more radiosensitive than in the neonate, the spindle cell, or mesenchymal, components of the thymus were much more radiosensitive than the epithelium at all ages. Furthermore, in the absence of radiation, non-lymphoid cells of the thymus showed an age related decrease in proliferative capacity in vitro