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Inflammatory exudate (SE) cells were collected from subcutaneous coverslips in mice and transferred into lethally irradiated (1,000 r) recipients. Eight days after transplantation 59Fe incorporation in the spleen and bone marrow was significantly greater than in controls treated with the suspending medium only. One hundred percent of mitoses were of the T6T6 karyotype in the marrow and spleen when SE cells were obtained from CBA/T6T6 donors. The repopulating potential of SE cells, however, lagged significantly behind that of bone marrow cells and the failure to observe consistently macroscopic spleen colonies calls into question whether the observed regeneration was due to pluripotent stem cells. Radioautographic studies with 3H-TdR showed that the majority of SE cells had recently been generated, but long-lived, noncycling cells of lymphoid and monocytoid morphology were also present in the exudate.

Lymphocytes in thymic cortex and germinal centers of lymphoid tissues are labeled intensely with generally labeled tritiated deoxycytidine [G-3H]dCyd whereas they are weakly labeled with methyl tritiated deosythymidine [methyl3H]dThd of the same specific activity, not only by single injection but also by an intensive injection schedule. [G-3H]dCyd can be used to label short-lived lymphocytes strongly, although not specifically. The distribution patterns of labeled lymphocytes were different depending on the injection schedules of [G-3H]dCyd. [G-3H]dCyd can be used as a precursor molecule for cytosine and also thymine found in DNA. The ratios of radioactive thymine to crytosine measured biochemically on DNA extracted from radioactive lymphocytes labeled by the various schedules indicate strongly that short- and long-lived lymphocyte populations have different abilities to utilize pyrimidine nucleosides for DNA synthesis.

Healing skin wounds were studied in a series of parabiotic rats. The femurs of one parabiont of each pair were shielded whilst both animals were given 800 r from a Co(60) source. The animals were wounded 3 days after irradiation. Each animal with partially shielded marrow was then given tritiated thymidine intraperitoneally daily while the cross-circulation was arrested by clamping. After the thymidine-(3)H had cleared the blood, the clamp was released. Animals were sacrificed, and wounds were prepared for radioautography 1, 2, and 6 days after wounding. In the wounds of the shielded animals thymidine-(3)H was observed in epidermis, endothelium, leukocytes, fibroblasts, and mast cells. Only neutrophilic leukocytes, monocytes, and lymphocytes were labeled, as determined by light and electron microscope radioautography, in the wounds of each nonshielded parabiont. None of the many fibroblasts present were found to contain label in the wounds of the nonshielded parabionts through the 6 day period. These observations provide further evidence that wound fibroblasts do not arise from hematogenous precursors and, therefore, must arise from adjacent connective tissue cells.