Differences in organization of metastatic and nonmetastatic tumors initiated by the same B16 melanoma clone in mature and young mice.

Subcutaneous transplants of mouse B16 melanoma clone G3.26 grow more slowly, and are markedly more metastatic to the lungs, in mature (greater than 12-month-old) mice than in young (2-month-old) mice. Previous studies suggested that tumors in young mice fail to disseminate viable tumor cells into the hematogenous circulation. To determine if changes in intratumor organization might accompany this altered tumor behavior, G3.26 tumors growing in young and mature mice were examined comparatively at progressive sizes relative to the onset of metastatic dissemination in the older mice. Although the degree of necrosis was comparable in both groups of tumors, vascular density, measured morphometrically in histological sections, was significantly lower in tumors from mature mice at a size when dissemination would be occurring. With the onset of reduced vascular density in tumors in mature mice, there was a substantial increase in the proportion of viable tumor cells that was hypoxic, based on radioresistance and incorporation of the hypoxic cell sensitizer, misonidazole. Quiescent tumor cells, identified by flow cytometry, were also more numerous in tumors from mature mice than in tumors from young mice. Although the importance of these differences in tumor organization to enhanced metastatic behavior is unclear, increased intratumor hypoxia might promote generation of metastatic variants. Alternately, dissemination of tumor cells might be facilitated through a reduced and possibly defective vasculature.