 distribut transfus tritiat cytidin-label leukocyt red cell bone marrow normal irradi rat normal rat total bodi irradi 550 x-rai fate bone marrow label nucleat red cell transfus peripher blood observ autoradiograph label nucleat cell lymphocyt-like cell migrat marrow parenchyma normal anim 2 cell 1 000 parenchym cell hr irradi 27-51 hr migrat appear increas time parenchym area difficult defin label red cell rare observ enter parenchyma despit presenc nonlabel erythrocyt parenchym structur irradi vascular bed irradi increas greatli parenchyma diminish minimum 10 percent control 75 hr despit chang architectur marrow averag densiti label cell area marrow correspond valu expect assumpt free-flow circul techniqu allow distinguish time free-flow circul intact sinusoid area sinusoid wall destroi 