Effect of radiation upon the embryonic thymus

The effect of a MLD of total body irradiation on the embryonic thymus of the opossum was analyzed. The destructive effect depended on cell types in the thymus when irradiated. Small lymphocytes immediately after formation were more sensitive to irradiation than older small lymphocytes. Hassall's corpuscles and epithelial cells were most resistant to the destructive effect of irradiation. Mitoses were most suppressed at 16 hours after irradiation. The suppressive effect was greatest when the thymus was filled with small lymphocytes at the time of irradiation. Embryos irradiated when the mitotic index was highest had an elevated mitotic index. Irradiation of epithelial thymus delayed first appearance of lymphocytes about 2 days. The sequence of appearance of lymphocytes in the embryonic thymus after irradiation recapitulated normal embryogenesis. Regeneration of lymphocytes in older embryos was modified because of the persistence of small lymphocytes after irradiation. These cells eventually disappe...