Apoptosis and necrosis occurring during different stages of primary and metastatic tumor growth of a rat mammary adenocarcinoma.

The pattern of spontaneous apoptosis and necrosis was investigated in an untreated, transplantable rat mammary adenocarcinoma (MTLn3) throughout the natural course of primary and metastatic tumor growth. The occurrence of spontaneous apoptosis was different when comparing primary to metastatic tumor growth. In the primary MTLn3 tumor growing at the mammary fat pad inoculation site we observed an inverse association between tumor growth and apoptosis. As the primary tumor increased in size, the extent of spontaneous apoptosis decreased. In contrast, an increase in apoptosis was associated with tumor growth of MTLn3 metastases in the axillary lymph node and the lung. In regard to necrosis, a similar pattern of increased necrosis was associated with tumor progression in both primary and metastatic tumors. Differences between primary and metastatic tumors in their pattern of spontaneous apoptosis may have important implications for the design of clinical treatment strategies.