EMPERIPOLESIS OF MARROW-CELLS WITHIN MEGAKARYOCYTES IN THE BONE-MARROW OF SUBLETHALLY IRRADIATED MICE

The incidence of megakaryocytic emperipolesis was studied in the bone marrow of normal and X-irradiated mice. Two groups of mice received to tal body irradiation with a single dose of 5 Gy and one of the two gro ups had been treated with a radioprotective drug, ethiofos (WR-2721), before irradiation. Mice from a third group remained unexposed to irra diation and served as controls. The Wright-Giemsa stained bone marrow smears were analyzed every 5 days during a 30-day period, starting 1 d ay after irradiation. The number of megakaryocytes exhibiting the phen omenon was determined and expressed as an average value for every expe rimental group. The frequency of megakaryocytic emperipolesis was less than 15% of megakaryocytes from control smears but increased to 34% i n mice that had only been irradiated and to 43% when mice were treated with WR-2721 before irradiation. In the last case, i.e., irradiation and treatment with a radioprotective drug, a positive correlation betw een the macrocytic megakaryocytes and elevated emperipolesis was noted . Under light microscopy, there were no signs of phagocytosis; engulfe d cells remained unaltered with their normal structure intact. Granulo cytic, erythroid, and lymphoid cells appeared to be the most frequent marrow cells engulfed by mature megakaryocytes. The number of incorpor ated cells in one megakaryocyte ranged from 1 to 3, though occasionall y more than 6 were seen in macrocytic megakaryocytes. Based on our fin dings and on a review of the associated literature, we believe emperip olesis is an interesting cellular phenomenon related to the fast passa ge of marrow cells across the marrow-blood barrier, especially through the cytoplasm of megakaryocytes in response to an increased demand fo r cell delivery. The high demand for cell delivery which occurs after irradiation may cause certain mature bone marrow cells to take a trans megakaryocyte path to enter the circulation of the blood. Irradiation seems to have an immediate effect (observed after 24 h) on emperipoles is, suggesting that a humoral factor is involved in the pathogenesis.