An Electron Microscope Study of Spontaneous Mammary Carcinomas in a Subline of Strain DBA Mice2

Abstract

Spontaneous mammary carcinomas of DBA/212 mice proliferated milk-agent particles more abundantly than any tumors studied so far. Acinar spaces were usually filled with the characteristic 100 mµ particles containing nucleoids, and the cell membranes which border the spaces were lined with the forming particles in all stages of development. Particle formation appeared to be at the cell membrane which thickens on the inside and protrudes to form a hemisphere, then further thickens and forms a horseshoe-shaped spheroid, and finally a very thick-shelled sphere which pinches off from the cell. The dense material of the sphere then rearranged itself to form a nucleoid, 20 to 40 mµ in diameter, surrounded by a membranous sac which appeared to be the same morphologically as the cell membrane. While the particles were still attached to the cell they were remarkably constant in diameter, but in the acinar spaces many of them appeared swollen and some contained two nucleoids. Within the cytoplasm of these tumor cells there was an abundance of dense, thin-shelled 60 to 70 mµ spheres of type A, as described by Bernhard. The spheres were usually associated with cytoplasmic vacuoles, but were rarely found near the cell border. Except for these particles and vacuoles, the cells which were so actively exfoliating the milk agent appeared normal and healthy.