Tissue Growth and Tumor Growth

Abstract

Tumors in general and cancers in particular are tissues growing under special conditions. A comparison between the laws determining tumor and tissue growth will deepen our knowledge of the physiology of tumors as well as of tissues. There exist similarities and analogies in the behavior of both, and there are differences either real or due to a gap in our knowledge. The differences will suggest new problems. In this brief resume no attempt at completeness will be made, and we will have to be content with mentioning some of the salient points. We shall first compare the causes of tumor and tissue growth, then the reactions of the host organism against tumor and tissue growth, and lastly, certain phenomena observed primarily in the tumor cells. We will have to inquire how far the last mentioned phenomena are duplicated in tissue growth. 1. Tissue growth is initiated by external factors affecting a more or less complicated system and leading to chains of reaction which may be identical, or at least similar, even in cases in which the primary causes differ. These primary causes are called “formative stimuli” in order to distinguish them from functional stimuli. It is probable that essentially both kinds of stimuli are identical and act in a similar manner. The difference in the result of both kinds of stimulation depends probably on the quantitative differences in strength and time, and on the differences in the systems on which they act. 2. The primary causes initiating tissue growth may be either physical or chemical. The physical factors are either mechanical or rays which are held back by the tissues. The chemical ones are essentially substances produced somewhere in the organism. Certain parasitic organisms which cause tissue and tumor growth act partly through chemical agencies which they produce; they may in addition exert a mechanical effect. In regard to substances produced outside of the organism and causing local tissue growth after application to certain tissues, it is doubtful how much their action is a specific chemical one and how much it is due to secondary physical factors released through injury of the tissues. Some pathologists assumed mechanical factors to be the only ones which can elicit tissue growth under experimental or pathological conditions. This view is evidently erroneous. Pathological or experimental phenomena are merely a modified interaction of phenomena occurring in the normal life of the organism. Chemical stimuli undoubtedly play a considerable part in the normal growth phenomena of the organism. This alone is presumptive evidence that they have a similar function under pathological conditions. 3. It can be shown that the stimulus brings about a series of changes in the affected tissue; these changes are partly chemical, as evidenced by increased oxidation, or physical (increased water content of the cells). They lead to a greater sensitiveness of the cells. Corresponding to their chemical and physical changes, morphological changes take place. These changes follow a definite curve. A new stimulus reaching the cell after a certain stage of this curve has been reached acts therefore on a different system and must produce different results. It has been shown that small quantities of various rays acting on various tissues may produce stimulation, while large quantities cause destruction. In conjunction with Doctor Spain we have shown that larger defects may produce a more energetic response of tissues than smaller defects; they represent a stronger formative stimulus. Successive formative stimuli lead only to a limited extent to increased reaction. Adaptive tissue changes implying an increased resistance to the stimulus are called forth. Thus a limitation in the proliferative effect is produced.