THE RECOVERY KINETICS OF LYMPHOHEMATOPOIETIC TISSUES IN NFS MICE AFTER SPLIT-DOSE X-IRRADIATION

The recovery kinetics of lymphohematopoietic tissues was studied after split doses (4 X 1.7 Gy) of X-irradiation in NFS mice, which are high ly susceptible to radiation-induced lymphoma. Thymic and splenic weigh ts had decreased to a similar extent (42% and 43% of normal) 24 hours after irradiation. Thymic weight began to recover after a continuous d ecrease for 3 weeks and had attained an almost normal level by 8 weeks after irradiation. In contrast, splenic weight had recovered to more than 70% of the control 1 week after irradiation but recovery was dela yed thereafter. The number of thymocytes was remarkably reduced (26% o f the normal) 24 hours after the last irradiation and was still signif icantly lower (60% and 69%) than normal 4 and 8 weeks after irradiatio n. The nucleated marrow cell number was not as significantly reduced ( 56% of normal) as thymocyte number 24 hours after exposure and had rec overed to an almost normal level by 4-6 weeks after irradiation. The n umber of femoral CFU-S (spleen colony-forming units) had decreased to 1.7% of the normal level by 24 hours after the last exposure, then sho wed logarithmic growth. However, recovery ceased 6 weeks after irradia tion, at which point the CFU-S number was about 60% of the normal leve l. These results suggest that disordered lymphohematopoiesis in the th ymus, spleen and bone marrow persists for an extended period of time a nd that this disorder occurrs in connection with a reduction in the fe moral CFU-S number.