Introduction

This chapter focuses on the oncogenes that have been discovered in avian species and what is known about their mode of action. Since normal cells contain homologous genes known as proto-oncogenes, these are also discussed. Normal somatic cells grow and divide in a controlled manner, such that in an adult animal, the rate of cell division in a given tissue is controlled to enable cell replacement to occur without any large increase in the total number of cells. A number of features distinguish normal cells from transformed or cancerous cells; these are most easily seen in cells growing in culture, but they are also manifest in tumours present in whole tissues. The main differences evident in transformed cells grown in culture are (i) cells are more rounded and often appear in disorganised clusters, (ii) they proliferate in the absence of exogenous mitogenic stimuli such as growth factors, (iii) proliferation is not inhibited by contact inhibition, which occurs with normal cells, (iv) they will grow in liquid medium without adhering to a solid surface, and (v) when injected into animal hosts they give rise to tumours. To understand the basis of oncogenesis, it is first necessary to understand the process of cell division and the normal controls which regulate the cell cycle.

The first evidence for a tumour-inducing virus was from sarcomas in domestic fowl.