Normal human cells, such as skin cells called fibroblasts, can be grown in the laboratory provided the growth medium is optimised. Fibroblasts are attached to a glass or plastic surface, but when the surface is completely covered, they stop growing - a phenomenon known as contact inhibition. The cells can be sub-cultured by removing them from the surface with with the enzyme trypsin, diluting them in fresh medium and transfering them to fresh containers. Initially it was thought that these cells could grow indefinitely, but this turned out not to be true. Extensive experiments by Leonard Hayflick in the 1960s showed beyond doubt that human fibroblasts have a finite lifespan in culture. After 50-60 population doublings - each broadly equivalent to one cell division - the rate of division is reduced, the cells become senescent and then cease growth altogether. This is commonly referred to as the "Hayflick limit" to growth. Hayflick realised that this provides an experimental system to study ageing at the cellular level, in many ways that cannot be achieved by the study of cells in the body. It was known that many cancer cells did not have the same limit to growth. Cultures of these cells could be grown indefinitely; they had become immortalised. Thus the use of cells in culture also provides a powerful method to study the transformation of normal cells into cancer cells.
When Robin Holliday was appointed Head of the Genetics Division at the National Institute for Medical Research at Mill Hill, London, in 1970, he decided to establish a laboratory to study the ageing of cultured human cells, including their transformation to immortal cell lines. The main thrust of the research was to determine whether molecular defects occur during ageing, particularly in proteins and DNA. This work provided support for the view that cellular ageing may be due to a breakdown of information transfer between macromolecules. It was also shown that the lifespan of human fibroblasts is not strictly programmed, as separate cultures vary in their lifespan. The commitment theory of cellular ageing proposed that cells that are intitially immortal (such as stem cells) give rise with a given probablity to cells commited to senescence. Experimental evidence for this was obtained. Human cells do not become immortalised spontaneously, but they do with low frequency after infection with the virus SV40. Transformed lines of normal cells and cells with particular genetic defects were obtained, and these have been used in laboratories world-wide.
In his Sydney laboratory it was discovered that the naturally occurring small dipeptide carnosine has remarkable effects on human cells. The normal features of senescence are suppressed, and their lifespan is increased. Also, under appropriate conditions, carnosine is toxic to cancer cells. In mixed cultures of normal and cancer cells, only the normal cells survive.
Historical review:
Holliday, R. Twenty years of ageing research at the Mill Hill laboratories. Experimental Gerontology 37, 851-857 (2002)