Much of what has been learned concerning the properties of cancer cells has been developed from studies of a variety of cell lines that have been adapted for growth in tissue culture or in appropriate experimental animals. A variety of mammalian (chiefly rodent) and human cancer cell lines have been developed for this purpose. It has to be kept in mind that the properties of these highly selected experimental systems may differ from those that one might expect to find occurring in primary tumours in patients. Despite this caveat, it is apparent that many principles that have been learned from the experimental systems have been valuable in the understanding of human malignancy.
Any line of tumour cells that has adapted to growing in tissue culture or through serial transplantation in animals will have been subject to an extremely rigorous selection process. Normal (i.e. nonmalignant) fibroblasts in tissue culture will die out after they have undergone a number of sequential cell divisions (approximately 50). This appears to be the case no matter how carefully the culture conditions are established. This also appears to be true for many cancer cells in that there appears to be an upper limit to the number of sequential divisions they will undergo before becoming senescent and dying. Some tumour cells, however, become ‘immortalized’ and they will replicate indefinitely in the right type of environment. Recently, it has been suggested that an important step in the process whereby cells become immortal is related to the expression of the enzyme telomerase.