Frequency Analysis of the T Cell Precursors in the Thymus

A frequency determination of the T cell precursors in murine adult and fetal thymuses as well as in the bone marrow and fetal liver was made. Cells were serially diluted and injected into deoxyguanosine-treated fetal thymus lobes with a microinjector, and the lobes were cultured for 12 to 21 days. The lobes in which T cell development did not occur were discriminated from those in which T cells developed, and the precursor frequency was determined by Poisson probability distribution analysis. The precursor cell frequencies in adult bone marrow cells (2.4 × 10-5) and fetal liver cells ( 1.7 × 10-4) were comparable to those determined previously in in vivo intrathymus transfer experiments. The present study further shows that only a small fraction of fetal thymus cells (0.9-5.0 × 10-2), CD4-8- adult thymocytes (1.6 × 10-2), and Thy-1 low positive adult thymocytes (3.3 × 10-4) retain the precursor activity.