Studies of how cancer cells move have shown that there are many chemicals dissolved in the tissues that can attract cancer cells. Some are produced because of interactions between the host tissue and the cancer cell. These factors would explain how cancer cells know in which direction to move. However, unless cancer cells could move only in short bursts that depended on the local concentrations of these chemicals, other mechanisms for the movement must exist. The answer to this problem is that malignant cells have the ability to produce their own chemicals that stimulate their own motility, the so-called autocrine motility factors. These allow the cancer cell to move independently of the host tissues.
Observations of cancer cells show that, to move, they send out leg-like protrusions, or pseudopodia, into the surrounding tissues. These pseudopodia are responsible for finding the attracting chemicals in the surrounding tissue. They contain high concentrations of receptors for the matrix proteins so that they can attach to those proteins, just as the cancer cell initially attaches to the laminin and fibronectin of the basement membrane. Part of the movement of the cell, then, could be by a series of attaching and letting go of the surrounding tissues, propelling it along. It would be useful if the pseudopodia also contained some of the enzymes to dissolve the matrix to allow the cell to move through it.
Entering the secondary site
When circulating tumour cells reach their target organ they either stick to the fining cells of a small blood vessel in that organ or a clump of cells becomes wedged in the vessel. The cancer cells then attach to the basement membrane beneath the fining cells and the cells seal them off from the bloodstream. What follows is a process similar to that at the beginning, where the cancer must dissolve the basement membrane and move into its new home.