Fig. 4.

Age dependence of the recombination process. The effective TdT level $0\le \alpha \le 1$ is estimated as an interpolation parameter between the recombination statistics of embryonic and mature animals. (A) Mean effective TdT $\overline{\alpha }$ at various ages (17 d embryo and 4 d, 21 d, and 42 d postbirth), from different tissues: thymus and periphery. Periphery is taken from blood, except for day 4 for which it is taken from spleen. The data points are compared with a minimal model of thymic entry, residence, and output, with a sharply increasing effective TdT level $\alpha (t)$ represented by the dashed line (Materials and Methods). A, Inset shows recombination entropy as a function of age, as predicted by the model. (B) The variance of $\alpha$, which measures the level of mixing in the repertoires, is shown as a function of its mean for both data (symbols) and the prediction of the minimal model (lines). The black line shows the maximal possible variance $\overline{\alpha }(1-\overline{\alpha })$. Numbers represent age from birth in days.