Interplay of thymus and bone marrow regeneration in x-irradiated mice

aim of the prepresent investigation was to study the modifying effects of bone marrow cells on regeneration, after X-irradiation, of thymus and bone marrow cell populations. Data are presented which indicate that the cellular composition of the thymus and, in particular, the frequency of the stem cells in the organ at the time of radiation exposure determines thymic regeneration for about two weeks after irradiation. After this period, regeneration depends on new precursors from the bone marrow which have previously seeded the thymus. In contrast to the thymus, cellular restoration of the bone marrow is already initially dependent on the number of protected or transplanted marrow cells. Two phases in the recovery of thymic PHA-reactivity after irradiation were observed: one initial phase which is independent on the number of the available bone marrow cells, and a subsequent phase during which PHA-reactivity is slightly increased in mice irradiated with partly protected bone marrow in comparison to in total body irradiated animals. During the entire observation period, PHA-reactivity remains at a low level not exeeding 50 % of that in untreated mice. In contrast the thymus is fully repopulated with regard to the number of nonreactive cells. Alternative pathways of thymocyte development within the thymus are discussed. Bone marrow X cells were shown to be as sensitive to in vitro treatment with a specific H-2 antiserum as were lymphocytes from normal bone marrow. This finding was teken to indicate that the X cells represent a particular lymphoid cell type. A xenogeneic rabbit-anti-mouse embryo antiserum was more toxic to pre-irradiated bone marrow, with high proportion of X cells, than to bone marrow from untreated mice, using in vitro cytotoxicity test. A possible embryonic character of the X cells is discussed. (author)