Identification of p-Ts in the IL-7R⁺subpopulation of FB cells. Single cells from IL-7R^– and IL-7R⁺ populations from FL and FB of 12- or 13-dpc fetuses were cultured under MLP assay conditions. Progenitor types were determined by analyzing the cells generated in each well. The frequency and total number of different types of progenitors in IL-7R^– subpopulation of 12-dpc FL and FB cells (A), IL-7R⁺ subpopulation of 12-dpc FL and FB cells (B), and IL-7R⁺ subpopulation of 13-dpc FL and FB cells (C) are shown. The scales on the top and bottom of figures represent the numbers of progenitors detected among the 100 cells assayed and the estimated total number of progenitors per fetus, respectively. In calculating the number of progenitors per embryo, the cell numbers and proportions of the subpopulations in FL and FB shown in Figure 1 were used. Data are representative of 3 to 5 independent experiments.