These mitogens are released by other cells, usually located nearby in the tissue. This dependence on exogenous, mitogenic signals ensures that the proliferation of individual cells within a tissue is dictated by signals received from other cells nearby. The resulting interdependence of cells makes possible the maintenance of normal tissue architecture. A cell responds to external mitogens via a complex signal-processing pathway. Mutant, or deregulated, forms of these growth-promoting genes are found in a variety of tumors.
Many tumors secrete substantial amounts of mitogenic growth factors.
This secretion is not observed in normal cells and reflects a deregulation of the expression of the growth factor genes. Once released into the extracellular space, these polypeptide mitogens may be recognized and bound by growth factor receptors on the tumor cell surface. The activated receptors proceed to induce proliferation of the tumor cell. This autostimulatory, or autocrine, feedback loop appears to be essential for the continued growth of many types of cancer cells. Among other consequences, the cells no longer depend on mitogens of exogenous origin.