THE LIQUID MEDIUM IN TISSUE CULTURE

Summary In the past, largely on an empirical basis, it was found that the most effective liquid media for culturing animal tissue cells consisted of mixtures of physiological saline solutions, tissue extracts (preferably from embryonic tissue) and serum or plasma. The marked influx of investigators with primarily biochemical interests into the tissue culture field in the last few decades has led to determined efforts to learn more of the precise relationships between the composition of these media and the metabolism of the tissues. Since both of these are so complex, and the equilibrium between them is such a delicate one, it is perhaps not surprising that many apparently contradictory results are seen in the current literature. The place of physiological salt solutions in tissue cultures appears to be a relatively simple matter, and their functions are reasonably well understood. The present confusion relates to the role of the tissue extract and serum components of the medium. For certain types of cells there is evidence that a tissue-extract nucleo-protein fraction, supplemented by a surprisingly small group of smaller molecule nutrients, is adequate to support growth, at least for short periods. Various possible mixtures of these smaller components may be effective as a supplement in the same system, although it is probable that optimum combinations exist, again depending on the cell type and the culturing conditions. The tissue nucleoprotein fraction cannot be considered as a ‘growth hormone’, in the conventional sense of that term, since other cell types, or even superficially similar cell types taken from other animal donors, are able to maintain themselves satisfactorily when this fraction is completely absent, at least if serum proteins are present in the medium. It is thus evident that quantitative comparisons of data found by various researchers are of very dubious validity unless it is certain that identical culturing techniques were applied, using identical cell systems. On a more positive note, however, it seems probable that the analytical approach to the liquid medium problem is bringing the long-sought goal of a controlled composition ‘synthetic’ medium for cultured animal cells much nearer to attainment for certain systems sharply defined as to cell type and culturing procedures. There are also indications that ‘growth’, biochemically speaking, as defined by the cell division in a culture, may be a separate process from ‘growth’ as defined by increase in culture mass, protein content or similar criteria. In other words, the chemical and physical conditions required to aid in inducing or in completing the mitotic cycle are not necessarily directly related to the nutritional requirements of growing and maturing individual cells in a healthy culture. There is some evidence that extracts made from tissues of older animals are, in many culture systems, as effective as extracts of embryonic tissue. Since, on the whole, such tissues are more accessible, their wider use by tissue culturists should be investigated more extensively. The role of serum in culture media is still not clear. In some cases it is apparently not required at all, in others it has a beneficial effect. Many of the reported contradictions as to the value or lack of value of serum in culture media are probably related to its variability in composition and state of preservation, and to the age and health of the donor animal. Until more experiments are done with better defined materials, resolving these conflicting results does not appear to be possible. A synthetic medium which would either fully support growth and development of all tissues, or one which would allow growth and development in limited but defined systems, is still the most logical goal as a reference medium and as the starting point for the systematic study of all phases of culture behaviour. The progress toward this goal has been definite but limited. With the tools now available for fractionating biological mixtures, synthesizing compounds of biological interest, and elucidating mechanisms of metabolism, the realization of a completely controlled medium, and possibly a completely synthetic medium, appears now to be merely a matter of time and continued effort. The advances in knowledge of tissue growth and behaviour that have already been achieved in the absence of such a medium are very impressive, and a constantly accelerating rate of significant research results is to be expected as the interest in this field increases.