Tumor cells display a characteristic set of features that distinguish them from normal cells. These traits allow the individual cells to form a tumor mass and eventually to metastasize to other parts of the body. We will briefly consider the changes that affect cell functions and then discuss some of the capabilities that must be acquired by the tumors as a whole to enable them to grow and spread.
A wide range of changes occur during the transformation of a normal cell to a cell capable of forming a cancerous growth. All cancer cells acquire the ability to grow and divide in the absence of signals that are normally 'turned on' and/or in the presence of signals that are normally 'turned off'. There are also detectable changes in the physical properties of the cells. These changes include the following:
Cytoskeletal changes: The distribution and activity of components in the cell infrastructure, namely microfilaments and microtubules, may change. These alterations change the ways in which the cell interacts with neighboring cells and alter the appearance of the cells. Changes in the cytoskeleton also affect cell adhesion and movement (motility).
Cell adhesion/motility: The reduction of cell:cell and cell:extracellular matrix adhesion allows large masses of cells to form. As described in the section on cell division, cancer cells are able to continue to grow even when surrounded by other cells. The alterations in cell adhesion also impact on the ability of the cells to move. Cancer cells must be able to move and migrate in order to spread, and cell adhesion plays a major role in regulating cell movement.
Nuclear changes: The shape and organization of the nuclei of cancer cells may be markedly different from that of the nuclei of normal cells of the same origin. This change in appearance may be useful in the diagnosis and staging of tumors.
Enzyme production: Cancer cells often secrete enzymes that enable them to invade neighboring tissues. These enzymes digest away the barriers to migration and spread of the tumor cells.