An unholy alliance—Colon cancer cells in situ co-opt fibroblasts in surrounding tissue to break out

It means cancer "in place" but a carcinoma "in situ" often does not want to keep its place. Standing between a cancer cell in situ and the surrounding tissue of fibroblasts and extracellular matrix is the basement membrane, a thin sheet of fibers that normally cradles the cells above it. The basement membrane is also the frontline physical barrier that keeps primary tumors from spreading into the matrix below. Perforating the basement membrane is a cancer cell's first move toward invasion, but how?