Thus perivascular colony formation was the predominant pattern of Homatropine Bromide growth by carcinoma cells in experimental in vivo metastasis assays. We verified that the colonies resulted from proliferation of vascular-associated tumor cells between 3 and 7 d by measuring tumor area and with BrdU immunohistochemistry. Similar to prior studies, these microcolonies appeared to rely on pre-existing vessels for growth. First, proliferation of tumor cells was observed within 1 week of injection and prior to any evidence of neoangiogenesis. Second, vessel morphology appeared largely normal, however, vessel density was significantly lower in tumor-involved areas of the brain. Finally, to verify the perivascular preference of brain micrometastases was not biased by intravascular delivery of tumor cells, we characterized a syngeneic model of spontaneous brain metastasis. Brain sections were examined for spontaneous micrometastases 5�C7 weeks after orthotopic injection of 4T1-GFP cells into the mammary fat pad. The growth and morphological characteristics of these colonies were indistinguishable from those derived from intracardiac delivery of cells demonstrating both intact GLUT-1positive vasculature and angiotropic spread upon adjacent capillaries. To assess the clinical relevance of frequent vascular cooption in experimental metastasis, we asked whether tumor cells within human brain metastasis specimens displayed a similar vascular association. In brain micrometastases and tumors with carcinomatous CNS spread that could be considered at the early stages of parenchymal colonization, the patterns were similar to those in the experimental models above, with the tumor cells in these brain metastases appearing to track along the blood vessels. Quantitation of vascular cooption in these cases from primary tumors of Benzoylaconine varied origin revealed that 98.2% of metastatic brain colonies were vascular-associated. Based on clinical pathologic indices, there was little clear morphological evidence characteristic of tumor angiogenesis in these cases. Therefore, early growth of brain metastasis microcolonies in experimental models and human clinical specimens occurs via intimate interactions with the existing neurovasculature. This growth can occur immediately after extravasation and does not require neovascularization. We sought to generate experimental situations in tissue culture analogous to the intraparenchymal.