The process of cancer metastasis is dynamic and consists of sequential, interrelated steps. Malignant cells that produce metastases have survived a series of potentially lethal interactions that are regulated by both the intrinsic properties of the tumor cells and host factors. Although some of the steps in this process contain stochastic elements, metastases develop from the nonrandom survival of a few subpopulations of cells that preexist within the parent neoplasm. Metastases can have a clonal origin, and different metastases can originate from the proliferation of different cells. The orthotopic implantation of human cancer cells derived from surgical specimens into nude mice provides a biological model of metastasis. Using this model, clonal analysis of a human renal carcinoma, colon carcinoma, and melanoma has revealed that these tumors are heterogeneous for metastatic properties. Damage to an organ's environment is followed by inflammation and repair, and these homeostatic processes facilitate the proliferation of normal (physiology), and iii some cases, tumor cells (pathology). Accelerated growth of human colon cancer cells was found in hepatectomized nude mice, whereas accelerated growth of human renal cancer cells was found in nephrectomized nude mice. These data suggest that systemic physiological signals can be usurped by neoplastic cells. Collectively, the factors that regulate metastasis are the intrinsic properties of metastatic cells and host factors involved in homeostasis,
