Liver metastasis is established by metastasis of micro cell aggregates but not single cells

Cancer cell engraftment in the target organ is necessary to establish metastasis. Clinically, lymph node metastasis of single cells has been confirmed using cytokeratin staining. In the current study, a LacZ-labeled cancer cell line was used to visualize intrahepatic metastasis of single cells or liver micrometastasis. KM12SM-lacZ stably expressing LacZ was prepared with a highly metastatic colon cancer cell line, KM12SM. KM12SM-lacZ was injected into the spleen of nude mice and following 1 week the spleen was excised. The liver was then examined for metastasis following 1, 2 or 3 weeks. Confirmation of liver metastasis was completed by observing the grade of metastasis. Grade-1 metastasis (DNA level), human DNA in liver tissue was detected; Grade-2 metastasis (metastasis of single cells), confirmed by X-gal staining; Grade-3 metastasis (histopathological micrometastasis), diagnosed by light microscopy and Grade-4 metastasis (typical metastasis), easily detected macroscopically or by hematoxylin and eosin staining. The Grade-1 metastasis detection rates 1, 2 and 3 weeks following splenectomy were 50, 100 and 100%, respectively. Grade-2 metastasis was not detected by microscopy. The Grade-3 metastasis detection rates for 1, 2 and 3 weeks were 75, 100 and 100%, respectively. Micrometastasis was observed in the portal vein lumen and wall. The Grade-4 metastasis detection rates were 50, 100 and 100% for 1, 2 and 3 weeks respectively. Cancer cells were present in vessels surrounding the main tumor. In conclusion, a specific number of cancer cell aggregates may be necessary to establish hematogenous metastasis.