|MadSci Network: Cell Biology|
In embryos and youngsters, there is a considerable amount of cell movement in the process of tissue formation, but most normal cells in adults do not move around to any great extent. Blood cells are an exception. Movement through the blood stream is passive. However, lymphocytes and other white blood cells leave the circulation and penetrate tissues under some circumstances, such as in response to an infection. In fact, there is constant movement of the white cells as they perform a function called immune surveillance, whereby they seek out invading pathogens and abnormal "non-self" cells such as cancer cells. The process immune infiltration of tissues is called extravasation and is outlined at these sites: http://nic.savba.sk/logos/books/scientific/node38.html http://hsc.virginia.edu/medicine/basic-sci/biomed/ley/main.html Tumor cell metastasis requires similar events to cross the endothelial cell barrier that lines blood vessels.
Try the links in the MadSci Library for more information on Cell Biology.