IT BM transplantation results in increased WT c-Kit^high thymic precursors and a high progenitor niche occupancy. (A) The phenotype of immature donor thymocytes was assessed in ZAP-70^−/− mice 25 weeks after IT administration of WT CD45.1⁺ BM progenitor cells. CD25/CD44 profiles of donor CD45.1⁺ and recipient CD45.1⁻ TN thymocytes were evaluated to distinguish TN1, TN2, TN3, and TN4 populations, and representative dot plots are shown. Control dot plots of TN thymocytes from CD45/1⁺ WT and ZAP-70^−/− mice not receiving a transplant are shown. (B) The presence of early progenitors within gated CD44⁺/CD25⁻ TN1 thymocytes was assessed by c-Kit staining. (C) Graph shows the absolute numbers of c-Kit^high thymocytes within the TN1 subset of WT, ZAP-70^−/−, and intrathymically reconstituted ZAP-70^−/− mice. The graph in the inset shows the relative percentages of c-Kit^high thymocytes within the donor and recipient populations from ZAP-70^−/− mice that received an IT transplant (n = 5). **P = .008. (D) Hematoxylin and eosin staining of transverse thymus sections from WT, ZAP-70^−/−, and intrathymically reconstituted ZAP-70^−/− mice. Thymi from WT, but not ZAP-70^−/−, mice are normally structured with densely packed cortical regions and less dense medullary regions, whereas thymi from intrathymically reconstituted mice show the formation of an extensive medulla.