Intrathymically Injected Hemopoietic Stem Cells Can Differentiate into All Lineage Cells in the Thymus: Differences Between c‐kit+Cells and c‐kitCells

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Abstract
To investigate whether hemopoietic stem cells (HSCs) can differentiate into all lineage cells even in the thymus, we injected two types of HSCs (c‐kit+ and c‐kit cells) obtained from C57BL/6 Ly5.1 mice directly into the thymus of 7.5 Gy‐irradiated C57BL/6 Ly5.2 mice. When c‐kit cells (low density/lineage−/CD71−/major histocompatibility complex class Ihigh/Sca‐1+/Thy‐1low/ c‐kit) were injected, donor‐derived (Ly5.1) cells were detected on day 8 after intrathymic (i.t.) injection, and the number reached a maximum on day 24 after injection. Granulocytes and macrophages were also detected on day 8 after injection. However, B220+ B cells were observed on day 13. Eighteen days after i.t. injection, the injected lobes showed red color due to the synchronous development of erythroid cells. Histological studies revealed the development not only of erythroid lineage cells but also of megakaryocytes in the thymus. In contrast, when c‐kit+ cells were injected, a significant number of donor‐derived cells were detected on day 5 after i.t. injection (three days earlier than in the case of c‐kit cell injection). The differentiation into erythroid lineage cells was also observed six days earlier than when c‐kit HSCs were injected. These findings suggest that c‐kit HSCs are more primitive than c‐kit+ HSCs, although both can differentiate into all lineage cells after i.t. injection.

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