Bone tissue, with its dynamic microenvironment featuring osteoclastic bone resorption, angiogenesis and matrix degradation, appears to facilitate proliferation of tumor cells after the onset of bone metastasis. In this study, we examined metastatic lesions in the femora of BALB/c nu/nu mice two weeks after intracardiac injection with human breast carcinoma MDA-231 cells. Histopathological observations showed the metastatic lesions close to the chondro-osseous junction, and revealed MDA-231 cells loosely intermingled with different cell types such as osteoblasts, fibroblastic stromal cells, osteoclasts and endothelial cells. In the metastatic nest, many tartrate resistant acid phosphatase (TRAPase)-positive osteoclasts accumulated in direct contact with or were close to alkaline phosphatase (ALPase)-or receptor activator of NF-κB ligand (RANKL)-positive osteoblastic cells. It seems likely that osteoclastogenesis is mediated through cell-to-cell contacts with ALPase-and RANKL-expressing osteoblastic cells. Formation of many capillaries lacking complete basal membranes and pericytes ratified the results of in situ hybridization, which revealed intense expression of VEGF in tumor nests, and therefore, indicated ongoing tumor-induced angiogenesis. The tumor cells possessed matrix metallo-proteinases (MMPs)-1 and -9, and frequently extended their stout cytoplasmic processes into fragmented fibrillar components of the growth plate cartilage, implicating degradation of cartilaginous matrix. Thus, osteolytic bone metastasis has demonstrated pathological features as tumor-induced angiogenesis and degradation of extracellular matrix, in addition to osteoclastogenesis. This complex interplay between tumor cells and host tissues may enable and nourish the establishment of a microenvironment that facilitates tumor progression.Bone metastasis is an important clinical entity in patients suffering from some sorts of malignant tumors, and blood circulation comes out as a vector for tumor cells to spread and establish metastatic lesions. Noteworthy is the venous drainage from the breast directly into the vertebral venous plexus, which may explain the high rates of bone metastasis by breast carcinoma cells. Bone microenvironment per se also seems to play an important role, facilitating colonization and proliferation of tumor cells. Breast carcinomas show a trend toward bone metastasis, especially if compared to fewer bone metasta-