Patterns of haemopoietic recovery after stress--II. Treatment with fluorouracil.

The mouse haemopoietic system is not permanently damaged by repeated injections of cytotoxic fluorouracil. It contains approximately normal numbers of nucleated femoral and spleen colony-forming cells (CFUs) after seven monthly injections of the drug and normal numbers of high proliferation potential colony-forming cells (HPP-CFC) after five serial injections. Furthermore, the mouse is fully fertile after seven injections of fluorouracil. The mouse recovers quickly after treatment because it regenerates from cells which were not killed by the drug. Within 14 days of treatment with fluorouracil there are almost twice the normal number of femoral macrophage and high proliferation potential colony-forming cells (M-CFC and HPP-CFC). These numbers then fall but are returning to normal 6 weeks after the drug was administered. In this quick recovery the response of the haemopoietic system differs from its response to the loss of blood cells caused by sub-lethal irradiation, or lethal irradiation, or treatment with busulphan. When mice are treated twice with fluorouracil, the second injection 14 days after the first, the number of femoral M-CFC two days after the second injection, is 16-fold the number in controls, but the number of femoral HPP-CFC is only twice the number in controls. When the interval between the two injections is 21 days, the number of femoral M-CFC is almost 8% of that of mice treated once, but the number of HPP-CFC is 67%. The characteristic response of each type of cell to repeated treatment with fluorouracil is probably due to the number of its precursors which are killed by the drug and to the interval between successive injections. A second injection of fluorouracil, 28 days after the first, speeds the growth rate of HPP-CFC. Their doubling time is 6 h shorter than that of mice treated once. Haemopoietic tissue from mice treated repeatedly with fluorouracil can only outgrow normal marrow under certain conditions. The nature of these conditions and the mechanisms involved are discussed in the light of contradictory findings.