The morphology of murine oncornaviruses following different methods of preparation for electron microscopy.

The effect of different preparative procedures for electron microscopy on the size and shape of murine oncornaviruses has been studied. With conventional negative staining procedures using neutral sodium phosphotungstate, both murine mammary tumor virus and murine leukemia virus appeared in head-and-tail forms, with a peak head diameter of 122 and 130 nm, respectively. Negative staining with uranyl accetate gave round virions with peak diameters of 148 and 130 nm. Prefixed virus was round with peak diameters of 141 and 130 nm, respectively, in phosphotungstate, and 148 and 117 nm, respectively, in uranyl acetate. With thin sections, the peak diameters were 143 and 123 nm. The preservation of the spherical shape of the virus was obtained by glutaraldehyde fixation dehydration in alcholic solutions of uranyl acetate, and critical point drying. Under these conditions the viruses had peak diameters of 99 and 82 nm, respectively. The size of murine mammary tumor virus has always been found to be larger than murine leukemia virus in all preparations except for negative staining with neutral sodium phosphotungstate. Shadowing of the virion preparations revealed considerable flattening of the particles in all cases except for critical point drying. Negatively stained preparations did not cast any shadow, and thus thethickness of the particles could not be evaluated. Virus can be reversibly converted from spherical to head-and-tail forms by altering osmotic strength. Under most of the conditions used, murine mammary tumor virus gave a bimodal size distribution with significant numbers of particles that were smaller than the major virus size.