Metastasis and morphology of transplanted BR6 mouse mammary tumours

BR6 mouse mammary tumours were maintained by serial s.c. transplantation into female or castrated male syngeneic hosts. The tumours could be broadly classified according to their structure, and usually remained stable through many passages. Occasionally, sarcomatous changes were seen (64 out of 800 cases). Pulmonary metastases were found in 5 per cent of mice bearing transplants of well-differentiated acinar tumours, in 34 per cent with tumours of the acinar/ductal type, and in 97 per cent with poorly differentiated tumours. There was a similar correlation between lymph node metastases and degree of primary tumour differentiation, the incidences being 0, 0.5 and 22 per cent respectively. Sarcomatous changes were associated with a reduced incidence of metastasis. Local invasion of muscle or peritoneum occurred in 32 per cent of mice with well-differentiated tumours, and in 59 per cent with less differentiated tumours. Selection for increased metastatic potential was not achieved by transplantation of lung nodules to a subcutaneous site.