The development of radiation late effects to the bone marrow after single and chronic exposure.

The marrow is a tissue distributed in numerous skeletal parts and works as an organ which is composed of a haemopoietic cell parenchyma and a supporting stroma. The pathophysiological mechanisms involved in the radiation-induced late effects depend mainly on the damage produced to each of these elements. Parenchymal cell damage ends with a failure of the stem cell pool to supply an adequate number of highly differentiated functional blood cells and is clinically manifested as aplastic anaemia or leukaemia. The effects of radiation on the haemopoietic stem cell can be measured by means of spleen colony forming units (CFU-S) in rodents. The self-maintaining capacity of the CFU-S was found to be lower than normal 16 weeks after a dose of 0.64 Gy. In larger animals it is only possible to measure the activity of some of the progenitor cells, estimating the number of granulocyte-macrophage colonies in culture (CFU-GM) as an indicator of stem cell changes. Their number in the blood is about 50 per cent of normal even 160 days after about 0.78 Gy. The stromal cells are also radiosensitive if measured with respect to their capacity to support long-term cell replication in vitro. Marrow fibrosis develops after single, repeated and chronic radiation exposure, and a dose of 40 Gy impairs the capacity of the marrow to support haemopoiesis.