Preliminary electron microscope study on virus-like particles in a spontaneous mammary tumor of collared lemming.

The present electron microscopic study of a spontaneous mammary tumor (diagnosed histologically as adenocarcinoma) of collared lemming was undertaken in search for the possible presence of particles resembling oncogenic RNA virus particles. The study has demonstrated the presence of a large number of characteristic virus-like particles. The virus-like particles were similar to the ham ster type H virus particles in morphology and intracisternal cytoplasmic location, previously observed in some mouse mammary tumors and human breast cancer. These particles were found budding from and free within the cisternae of the rough endoplasmic reticulum. They measure 1450 A aver age outer diameter with small nucleoids of 700 A diameter. Normal mammary gland tissues of collared lemmings were also examined in the electron microscope. However, no virus-like particles could be found in any of these tissues. Whether these virus-like particles are etiologically related to the mammary tumors of collared lemmings or are merely passenger agents remains to be determined.