Neoplasia is the uncontrolled multiplication of abnormal cells which do not respond to normal homeostatic controls and yet they are poorly recogas being abnormal by the body's normal defence mechanisms. This allows them to proliferate, invade locally, undergo metastatic spread and ultimately cause clinical signs of disease or even death.

The key features of a neoplasm which differentiate it from other forms of cell growth are:

(1) excessive tissue growth
(2) unresponsive to normal control mechanisms
(3) not dependent upon initial stimulus to be present for continual growth.

Growth factors are the major regulators of mammalian cell growth in the body and they act via receptors on the cell surface. It is known that some genes which can induce the development of neoplasia (oncogenes), work through their action on growth factors (GF), GF receptors or proteins involved in transferring GF signals across the cell membrane (Slauson & Cooper 1990).

Neoplastic cells (particularly those in malignant tissue) are different from normal cells and in vitro demonstrate changes in biochemistry, antikaryotype and cell surface characteristics.

Neoplasms are classified as benign or malignant based upon their characteristics (see Table 6.1).

Although there are some factors known to be associated with the development of neoplasia there is no single common biological mechanism that explains why cancer occurs and the initiation and progression of any specific neoplastic growth is influenced by many factors - most of which have still to be fully elucidated.