A Comparison of the Surface Morphology of Milk-Derived and Tissue Culture- Grown Mouse Mammary Tumor Virus

Abstract

When examined in the electron microscope by negative staining and thin sectioning, the surface morphology of the murine mammary tumor virus (MuMTV) obtained from murine mammary tumor cells grown in tissue culture appears to be similar to that of MuMTV obtained from mouse milk. The viral envelope appears to be covered by evenly spaced projections consisting of stalks and knobs. The diameter of the knobs is about 5 nm and the projections are approximately 8 nm apart. However, when milk derived MuMTV is examined using the freeze-drying and freeze-etching techniques, no surface projections can be seen. Instead, the surface appears to be covered with evenly spaced inden tations or pits (1,2). By freeze-drying intact murine mammary tumor cells growing in tissue culture, Demsey et al. (3,4) observed protrusions on the surface of MuMTV particles budding from the cell's surface.