Beyond Neutrality—Ecology Finds Its Niche

THE complexity of the process of carcinogenesis has led to the necessity of reducing as far as possible the factors involved in experimental carcinogenesis if there is to be any chance of elucidating the mechanisms at work. One obvious way of cutting down influences from outside the tissue undergoing carcinogenesis is to maintain it in tissue or organ culture for the appropriate period. Unfortunately up to the present the maximum period for which organs have been kept alive in vitro is well below that necessary for most forms of carcinogenesis, including chemical carcinogenesis. The establishment of " cell lines " in vitro has enabled the period of in vitro culture to be suitably extended and malignant change in such lines is now well established. TUnfortunately the very process of establishment of such cell lines involves the production of changes in the reaction of the cells to the environment which predispose to malignant change or even bring it about (Earle, 1943). Such cell lines are therefore more closely related to malignant than to normal tissue and their behaviour throws little light on the processes which occur in vivo. More recently malignant change has been induced (Berwald and Sachs, 1965) in mixed embryo tissues maintained for much shorter periods in vitro. Such circumstances more closely resemble those found in vivo, but tissue relationships can scarcely be said to be normal and it is not certain that early embryonic tissues behave in a comparable way to adult tissues in respect to carcinogenic stimuli. It is generally felt that the long period needed for chemical carcinogenesis indicates that this is a serial process, whether a biphasic one as suggested by Berenblum and Shubik (1947) or more complex. There is certainly much evidence that one dose of a hydrocarbon carcinogen leads to immediate tissue changes which may be expressed as tumour formation at a much later date with or without further treatment of some sort. In the present work it was decided to study this initial stage in vitro, but to sidestep the difficulties of long term organ culture by implanting the tissues subsequently into other animals in which they could survive long enough to express any changes brought about by the carcinogen during the in vitro period. It could further be determined whether the initial changes were such that they could be detected in vitro, e.g. as alterations in histological appearances, in correlation with subsequent tumour production.