Cytotoxic immigration of granulocytes into megakaryocytes as a late consequence of irradiation

The immigration of neutrophilic granulocytes into megakaryocytes was studied in the bone marrow of normal and X-irradiated beagles under various exposure conditions. Two groups of dogs received homogeneous total-body irradiation. One group received a dose of 1.6 Gy and the other received a dose of 2.4 Gy (midline tissue). A third group was irradiated from the left side of the body only. This exposure resulted in an inhomogeneous total-body irradiation (entrance dose 3.8 Gy, exit dose 0.9 Gy). A fourth group of animals received partial-body irradiation with a dose of 11.7 Gy delivered to the anterior two-thirds of the body, thereby subjecting about 70% of the hemopoietic marrow to irradiation. Dogs of a fifth group remained unexposed to irradiation and served as controls. The marrow was analyzed in sections of the ribs approximately 1 year after irradiation. The total number of megakaryocytes in one section was evaluated. The number of megakaryocytes showing granulocytes in their cytoplasm was determined and expressed as a percentage. This phenomenon can be explained as cytotoxic immigration of granulocytes into megakaryocytes. It was observed in approximately 1-2% of the megakaryocytes in the marrow of normal dogs. One year after irradiation the value increased to 10-26%. It was observed that neutrophilic granulocytes penetrated only into the large mature megakaryocytes in which the nuclei were mostly pyknotic. This phenomenon may be considered as a late effect of irradiation.