DETERMINATION OF THE THRESHOLD VALUE OF RESIDUAL HEMOPOIETIC STEM CELLS IN MICE AFTER LARGE DOSE ~(60)Co IRRADIATION
Shen Ming-yan and Chu Jen-pao (Institute of Radiation Medicine,Academy of Military Medical Sciences,Beijing)
Hemopoietic stem cells (HSC) are the precursors of blood cells.They have both extensive capacity for self-renewal and the potential for differentiation to all cell lineages including myeloid and lymphoid series. In the bone marrow syndrome of radiation injury,the survival or death of the individual exposed to irradiation in the LD_(1-99)/30 days range is determined by the number of residuas HSC.Therefore,HSC have become an impor-tant part of study in radiobiology and radia-tion hematology. Our data indicate that spleen colony-form-ing units (CFUs) have a much high radiosen-sitivity to γ-rays.Their dose-response curve is an exponential straight line,with a D_0 value of 0.94 Gray after total body irradia-tion (TBI).LD_(50)/30 days of the mice is 8.31 Gray.We have used the dose-survival curve of CFUs and the survival rate of reci-pients which received HSC by transplantation to determine the threshold value of residual stem cells in 8.5 Gray γ-ray irradiated mice.Such dose is considered as critical absolute lethal dose.At this dose level,the number of residual stem cells is estimated about 20 CFUs or 200 HSC,equivalent to 1×10~(-4) of total hemopoietic stem cells.This number of HSC is an effective dose in rescuing the lethally irradiated syngenetic mice. To our knowledge,the number of CFUs involved in the response of ionizing radiation can be used as a quantitative index for the estimation of irradiation dose and early diag-nosis for radiation disease.Furthermore,the threshold value of stem cells can be used as a basis for bone marrow transfusion and trans-plantation.