填空题

Scientists have learned more about the way cancer spreads from one place in the body to another. Once it spreads, it gets more difficult to (36) . Cancer is a disease of uncontrolled .cell growth. (37) _ thinking is that cells simply break off the main cancer, travel through the blood and grow in other organs. But the new (38) suggest there is more to this process. Researchers found that cells from the main tumor send out messengers. These prepare the new organ for a (39) tumor. It works this way. The primary tumor releases proteins called growth factors into the blood. They (40) cells at the target organ to produce a sticky protein, called fibronectin. Fibronectin attaches to the surface of bone marrow cells. The result is a kind of (41) area for cancer to arrive and grow into a secondary tumor. The bone marrow cells help make vessels for blood to pass through and (42) the cancer cells. The researchers believe that without the bone marrow cells, the tumors could not land on the new (43) and grow. Scientists from Cornell University in Ithaca, New York, led the research. It involved laboratory mice. (44) . The mice were then injected with lung cancer cells. The researchers were surprised when the bone marrow cells reached the lungs days before any cancer cells. They found that the bone marrow cells arrived to prepare the lungs for the cancer to spread. Other tests led to similar findings. (45) . They say knowledge of the process could lead to new ways to fight cancer. Cancers can be caused by genetic or environmental conditions, or a combination. (46) . That includes better diet, exercise and no smoking.