Development, morphology, and progression of mammary tumors during and after fertile life in BALB/cfRIII mice.

A study was done on 368 BALB/cfRIII mammary tumor-bearing breeding female mice that are of the BALB/c genotype and carry milk-transmitted RIII murine mammary tumor virus (MuMTV) infection initiated by foster-nursing. All tumors were characterized by chronologic, biologic, and morphologic analyses. Mammary tumors occurring during pregnancy-lactation periods were mostly pregnancy-dependent plaques showing organoid tubular structure, sudden growth during late pregnancy, regression after delivery, and slow progression. The frequency of pregnancy-dependent tumors per 100 mice at risk increased steadily from first (1 tumor) to sixth (38 tumors) pregnancy-lactation period. Mammary tumors occurring after fertile life were mostly characterized by an irregular, slow growth and a type B morphology. The frequency of these tumors per 100 mice at risk per 100 days of survival also increased regularly in females undergoing from one (73 tumors) to six (117 tumors) pregnancy-lactation periods. Data were discussed in the light of the pattern of MuMTV release through BALB/cfRIII milk.