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The mouse hematopoietic system was subjected to repeated depletion and regeneration either by serial transfer of bone marrow cells through lethally irradiated recipients or by repeated treatment with the cycle-active drug hydroxyurea (HU). The capacity of surviving stem cells to proliferate and self-renew was assayed at intervals by two methods: (a) the spleen colony method; and (b) competitive repopulation of irradiated recipients using chromosome markers, with normal bone marrow cells as an internal control. The progressive decline in stem cell function that occurred during serial transfer of bone marrow and that had already begun after a single transfer was not seen during HU treatment; up to 25 pairs of HU injections given over more than 1 yr had no discernible effect on the number of stem cells present 3 wk after the final injection or on their capacity to self-renew. Within 2 d after exposure to HU, the average self-renewal capacity of surviving stem cells was enhanced. This implies that the drug selectively eliminates poorly self-renewing stem cells and hence that these enter cycle more readily than stem cells with a high self-replicative potential. However, the fact of being in cycle at the time of injection did not of itself affect self-renewal. The results show that serial transfer of bone marrow is not a valid method for studying clonal aging phenomena because it does not fulfill the assumptions on which such studies are based. No evidence was obtained for any intrinsic limitation in the capacity of bone marrow populations for repeated regeneration after HU-induced depletion. However, this does not necessarily imply that individual hematopoietic clones are capable of indefinite expansion because hematopoiesis may (as suggested by the relative resistance of highly self-replicative stem cells to mitogenic signals) proceed on the basis of clonal succession.
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