No abstract

The existence of a possible local control of CFU turnover was studied in mice in which one tibia only was irradiated (LR mice) and in mice in which one tibia was shielded during whole‐body irradiation (‘TBR’mice). In both the LR and ‘TBR’mice, the increased CFU turnover continued in the irradiated tibiae even after the time when in the unirradiated (shielded) tibiae it returned to normal levels. The early temporary decrease in the CFU numbers in the shielded tibiae of ‘TBR’mice is attributed to a temporary demand for increased differentiation rather than to migration of CFU. The direct control of CFU turnover appears to be local in nature, in contrast to the stimulus for CFU differentiation.

Measurements to determine the kinetic status of the morphologically unrecognizable haemopoietic precursor cells in the bone marrow are frequently carried out using techniques which inhibit or destroy cells in the DNA‐synthetic (S) phase of the cell cycle. For example, tritiated thymidine (3H‐TdR) has for many years been recognized as a highly specific label for DNA synthesis and, as such, administration of large doses of 3H‐TdR has often been used, both in vitro and in vivo, to kill cells in S. Assay of the surviving cells has then given a measure of the proportion of the total cells which are in the S‐phase of the generation cycle. Other compounds which have been used for the same purpose are: 125Iodo‐deoxyuridine (125I‐UdR), another S‐phase specific label, or hydroxyurea (HU) which prevents entry of cells into S and inhibits or kills cells already in S (Sinclair, 1965). For a variety of reasons, different laboratories tend to make different choices of the agent to be used for this purpose. As a result, it has sometimes proved difficult to marry data obtained from different sources. In the course of using 3H‐TdR, tritiated uridine (3H‐Ur), 125I‐UdR and HU in attempts toevaluate the kinetic status of bone marrow stem cells, it has become clear that their use is not straightforward and this paper presents data which illustrate some of the pitfalls associated with their use.