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 x 10-5) and fetal liver cells (1.7 x 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 x 10-2), CD4-8- adult thymocytes (1.6 x 10-2), and Thy-1 low positive adult thymocytes (3.3 x 10-4) retain the precursor activity. © 1993 Academic Press, Inc.