The Recovery Kinetics of Lymphohematopoietic Tissues in NFS Mice after Split-dose X-irradiation

Abstract

The recovery kinetics of lymphohematopoietic tissues was studied after split doses (4 X 1.7 Gy) of X-irradiation in NFS mice, which are highly susceptible to radiation-induced lymphoma. Thymic and splenic weights had decreased to a similar extent (42% and 43% of normal) 24 hours after irradiation. Thymic weight began to recover after a continuous decrease 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 delayed thereafter. The number of thymocytes was remarkably reduced (26% of the normal) 24 hours after the last irradiation and was still significantly lower (60% and 69%) than normal 4 and 8 weeks after irradiation. The nucleated marrow cell number was not as significantly reduced (56% of normal) as thymocyte number 24 hours after exposure and had recovered to an almost normal level by 4-6 weeks after irradiation. The number 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 showed logarithmic growth. However, recovery ceased 6 weeks after irradiation, at which point the CFU-S number was about 60% of the normal level. These results suggest that disordered lymphohematopoiesis in the thymus, spleen and bone marrow persists for an extended period of time and that this disorder occurrs in connection with a reduction in the femoral CFU-S number.