Further Electron Microscopic Studies on the Morphology of the Moloney Agent

By a method combining cryostat sectioning and negative staining, introduced by Almeida and Howatson, cell fragments from the bone marrow of rats with leukemia induced with the Moloney agent were studied with the electron microscope. Virus particles in various stages of maturation and some possessing tails were observed in association with thinly spread fragments of cytoplasm, presumably from megakaryocytes. Phosphotungstate was found to have penetrated into certain particles with resultant visualization of the intermediate membrane and nucleoid. These particles did not possess tails. Some particles that evidenced no penetration of phosphotungstate were also without tails, but the majority possessed the stout, straight tails considered characteristic of the mature form of this virus. These stages in maturation were related to comparable particles in thin sections of megakaryocytes and plasma pellets from leukemic mice and rats. In megakaryocytes all stages in the maturation of particles from early buds with incomplete nucleoids to particles with electron-dense nucleoids and long, straight tails were identified. The evidence to date would appear to support the view that the tails are not artifact but are a real and possibly distinctive feature of this group of viruses. It is suggested that there is a close morphologic similarity between the members of this family of viruses.