Intravital microscopy through an abdominal imaging window reveals a pre-micrometastasis stage during liver metastasis.

Cell dynamics in subcutaneous and breast tumors can be studied through conventional imaging windows with intravital microscopy. By contrast, visualization of the formation of metastasis has been hampered by the lack of long-term imaging windows for metastasis-prone organs, such as the liver. We developed an abdominal imaging window (AIW) to visualize distinct biological processes in the spleen, kidney, small intestine, pancreas, and liver. The AIW can be used to visualize processes for up to 1 month, as we demonstrate with islet cell transplantation. Furthermore, we have used the AIW to image the single steps of metastasis formation in the liver over the course of 14 days. We observed that single extravasated tumor cells proliferated to form "pre-micrometastases," in which cells lacked contact with neighboring tumor cells and were active and motile within the confined region of the growing clone. The clones then condensed into micrometastases where cell migration was strongly diminished but proliferation continued. Moreover, the metastatic load was reduced by suppressing tumor cell migration in the pre-micrometastases. We suggest that tumor cell migration within pre-micrometastases is a contributing step that can be targeted therapeutically during liver metastasis formation.