Ultrastructural features of the intestinal absorption of mouse mammary tumor virus in newborn BALB/cfRIII mice.

The retrovirus mouse mammary tumor virus is present in mouse strains with a high incidence of mammary tumors as a causative agent. It is produced mainly in the mammary glands of sexually mature females and is milk-transmitted to newborns. The fate of the mouse mammary tumor virus is almost unknown. Where it enters, how it is distributed, and where it remains latent, remain unresolved problems. This study tries to answer the first of these questions. Viruses are for the most part digested in the stomach. Very few well-preserved B particles, i.e., the infective particles, are allowed to enter through a process of endocytosis, mainly in the newborn-type epithelial cells. These are epithelial cells with a very rich absorptive apparatus, characteristic of newborn rodents. The adult-type absorptive cells and the M cells of the Peyer's patches might be partly involved.