Differential effect of host microenvironment and systemic humoral factors on the implantation and the growth rate of metastatic tumor in parabiotic mice constructed between young and old mice.

B16 melanoma cells were injected into the tail vein of young mice, old mice and parabiotic mice constructed between young and old mice, and the number and shape of pulmonary metastases were compared among three experimental groups. In unpaired mice, the number of metastatic colonies in the lungs was 10-fold larger in young than in old mice. In parabiotic mice, the number in young mice was almost comparable with that of unpaired young mice, but the number in old mice approached the level of young mice. Metastatic colonies on the pulmonary surface of young mice were mostly nodular in shape, while those of old mice were flat in shape. The shape of colonies reflecting the tumor growth rate did not change in parabiotic old mice in spite of an increase in number. In young parabiotic mice, the large and intermediate colonies decreased with a concomitant increase of small ones as compared with unpaired young mice. These results suggest that the implantation of metastatic colonies in the lung is mainly dependent on systemic humoral factors and their growth is mainly dependent on the host local factors in the microenvironment, and distinct age changes of both factors greatly influence the metastatic mode of tumors, respectively.