Primary cells and transformation

Cells taken from an animal and placed in culture are termed primary cells until they are subcultured (Chapter 4). Primary cells, if successfully established in culture, will multiply and will require regular subculturing. However, specialised or terminally differenti- 

On reaching confluence a primary culture may be subcultured into two or four new bottles and this subculturing may be repeated at about weekly intervals for several months. In such a culture the cells may remain diploid and retain many characteristics of the initial explant. This is a cell line, and several different kinds of cells may be present and some of the characteristics may prove unstable. The cells may be cloned (Chapter 7) and some clones may exhibit a stable phenotype. Such is the WI 38 cell strain, a commercially available strain of human embryonic lung cells, many identical cultures of which were frozen after only a few passages. 

There are several stages to the transformation process. Some cell strains (e.g. mouse 3T3 cells) have gained only the property of immortality, whereas in others this is accompanied by an increased rate of cell division and a decrease in contact inhibition reflected in the ability to grow to high cell densities. NIH 3T3 cells undergo further spontaneous transformation at a rate which depends on their recent history (Rubin and Xu, 1989). Thus late passage cells subcultured into a nutritionally poor medium show very numerous foci representing clones of cells with reduced contact inhibition. Such cells often show reduced nutritional requirements relative to primary cells. 

This is the situation with BHK 21 hamster fibroblasts (Macpher-son and Stoker, 1962 Stoker and Macpherson, 1964) which still have the correct diploid chromosome number but which are believed to have the incorrect chromosome complement. These cells do, however, exhibit a certain amount of contact inhibition of movement ( 2.4.2) and may be further transformed by treatment with polyoma virus (e.g. to form Py Y cells Fig. 2.1) or SV40 (to form SV28 cells). 

Highly transformed cells no longer require to grow attached to a substratum ( 2.4) and can be cultivated in soft agar. Such cells will often form tumours when injected into suitable animal hosts. 

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