Invasiveness - Malignant Cells Divide at an Increased Rate
Malignant cells lack certain growth-controlling enzymes and this allows them to divide and grow rapidly, outside the control of the body’s normal growth regulation systems. Malignant cells are resistant to apoptosis, which is the normal cell’s mechanism of programmed cell death triggered by DNA damage. They are also capable of dividing without the intervention of external growth factors, and they do not respond to the body's normal signals to inhibit growth.
The abnormal growth of malignant cells is due to mutations in key genes: an example is p53, which is a tumor suppressor gene involved in many cell processes including programmed cell death, cell cycle control and DNA repair. P53 tumor suppressor gene mutations are found in more than half of all cancer cases.
Under the microscope, the nuclei of malignant cells often stain darker than normal nuclei, and a large proportion of cells may be observed undergoing mitosis (cell division), some abnormally.
Not only can malignant cells divide and grow faster than normal cells; they also have the ability to secrete enzymes capable of digesting extracellular matrix material (connective tissue). An important enzyme secreted by malignant cells is major excreted protein (MEP) which has been shown to digest fibrinonectin, collagen and laminin. The destruction of connective tissue in this way by malignant cells allows them to invade healthy tissue more easily.
A rapidly growing collection of malignant cells needs constant nourishment, and neoplastic cells are often capable of angiogenesis (the stimulation of blood vessel growth). Malignant cells are known to express a unique protein known as tumor angiogenesis factor (TAF) which can diffuse at least 5mm through tissue and stimulates the growth of new capillaries to supply the developing tumor with nutrients.