Recipient thymus was transiently reconstituted by YS/P-Sp–derived progenitor cells. (A) YS and P-Sp cells from Ncx1^−/− and WT embryos were cocultured with OP9-DL1 for 10-12 days and transplanted into the peritoneal cavity of sublethally irradiated NOG neonates. Two weeks after injection, recipient thymuses were analyzed. Although there was no apparent structural thymus in NOG neonates, mice that received a transplant display an identifiable thymus that contains ∼ 1 × 10⁶ cells. When analyzed at 14 weeks after injection, no thymus was detected in any mice that received a transplant (Table 1). The cell numbers of the thymus from C57BL/6 mice that did not receive a transplant were enumerated as a control. (B) FACS analysis of reconstituted thymus from recipient mice. All the thymocytes were donor derived and consisted of CD4⁺CD8⁺ DP cells as well as CD4⁺ SP and CD8⁺ SP cells, with TCRβ expression. The number of mice that received a transplant is shown in Table 1.