Electron Microscopic Study of Spontaneous Mammary Carcinomas in Cats and Dogs : Virus - like Particles in Cat Mammary Carcinomas '

Electron microscopic study of 1 1 spontaneous cat mammary tumors revealed the presence of virus-like particles in 5 of the tumors examined. In three tumors, spherical particles with two concentric shells surrounding an electron-lucent center were found budding from, or free within, the cisternae of the endoplasmic reticulum. In one tumor, spherical particles with four concentric shells surrounding an electron-lucent center were observed budding from the cell membranes or free within the intercellular spaces. In the fifth tumor, both types of particles were present. One or two particles with large, centrally located nucleoids were found within the cisternae of the endoplasmic reticulum in three tumors examined. Whether these particles are etiologically related to the tumors in which they were found or are merely passenger agents cannot be determined at the present time. Eleven spontaneous dog mammary tumors were also studied; no virus-like particles could be found in any of these tumors.