H3T-labelled blood cells in the CNS response to axotomies at various times after isotope injection.

Numerous investigators, using tritiated thymidine to label mononuclear blood cells, have proposed a hematogenous origin for some of the reactive cells responding to injury in the CNS. However, two questions have remained unanswered: recirculation of H3T at the time of injury with labelling of proliferating local cells; and local reutilization of label released from degenerating cells. In a recently reported experiment using bilateral injuries made at different times (24), a large number os labelled cells responding to the second injury resulted from mitosis of cells already reacting to injury on the contralateral side. Additionally, the possibility that some of the labelled cells could have been newly drived from blood was not ruled out. The present experiment was designed to further explore this possibility. To minimize the problems of H3T recirculation and reutilization, the isotope was injected 3 to 17 days before nerve injury in some of the animals. At sacrifice, labelled cells were found in the nucleus of the injured nerve up to 36 days after isotope injection, with the peak of the reaction occurring at 9 days. Since the isotope was injected 3 to 17 days before injury and because there was no source of reactive cells on the contralateral side, it was felt that these labelled cells were newly derived from blood.