Differentiation of mouse T-lymphocytes. 19 Characterization of thymocyte precursors in bone marrow by one G velocity sedimentation.

Bone marrow cells were transferred into 700 R X-irradiated syngeneic mice, and regeneration of the thymus was demonstrated by determining the total amount of DNA in the thymus. The amount of DNA in the regenerated thymus 9 days after X-irradiation paralleled the number of cells injected, the range being 0 to 10(6) cells. These results indicate that regeneration during this period reflects the proliferation of thymocyte precursors existing in bone marrow cells, therefore such can be used as an indicator to determine the number of thymocyte precursors. Thymocyte precursors were separated from spleen colony forming cells and soft agar colony forming cells by one G velocity sedimentation. Thymocyte precursors (more than 4.3 mm/hour sedimentation rate) sedimented faster than did spleen colony forming cells (peak at about 3.0 mm/hour) and Thy-1 antigen was already evidenced on the cell surface as demonstrated by complement dependent cytotoxicity. Soft agar colony forming cells also sedimented faster, suggesting that cells destined to become one type of cell are larger than multi-potential stem cells.