An Electron Microscope Study of Rat Leukemia Induced with Mouse Leukemia Virus (gross).

A comparative electron microscope study was carried out on ultrathin sections of bone marrow, spleen, lymph nodes, and mediastinal tumors from eighteen rats with leukemia induced by mouse leukemia virus (Gross). In organs of all eighteen rats with leukemia, two types of particles were observed: one type, described as virus particles of 990 A average diameter, with an electron-dense center of 630 A average diameter, centrally or excentrically located and surrounded by one or two membranes; the other type, described as vesicular or doughnut-type particles, with an electron-lucent center with an average diameter of 490 A, surrounded by an inner membrane and an outer membrane with an average diameter of 840 A. Occasionally, another membrane (580 A) could be seen between the inner and outer membranes of these particles. Both types of particles were found in the megakaryocytes of bone marrow and spleen, and the virus particles in the intercellular spaces in the lymph nodes, spleen, and in the mediastinal tumors. Occasionally, the particles were also found in cytoplasmic inclusions of cells present in the mediastinal tumors and in the lymph nodes. These particles were observed with equal frequency in these tis sues, but apparently more frequently than in the spleen and bone marrow. Budding of plasma membrane of blast cells leading to particle formation was oc casionally observed in the cells present in all organs examined. Similar budding of the membranes of cytoplasmic channels and of specific granules in the megakaryocytes of bone marrow and of spleen was also observed. Characteristic cylindrical or tubular structures were observed in the megakaryocytes of the bone marrow and spleen of leukemic rats. Segmentation of these structures into doughnut-type particles and budding into similar particles was also frequently seen. No particles of any type or cylindrical structures could be found in either bone mar row, spleen, thymus, or lymph nodes of fifteen normal rats.