As has been indicated above, it is not always possible to determine whether bone formation, following transplantation of a tissue to a favorable environment, is the result of proliferation and osteogenetic activity of transplanted cells, or whether the cells of the host, under the influence of induction as a result of the transplant, are responsible for the appearance of new bone. In an attempt to resolve this difficulty and for the purposes of exposition, there is presented a diagram (Fig. 12) which illustrates the probability that one or both of the mechanisms are operating in any given instance.
In the upper portion of the diagram are shown those tissues in which new-bone formation is almost certainly the result of the osteogenetic activity of the transplanted cells (tissue cultures, periosteum transplanted to the eye); in the lower portion are devitalized tissues which cannot conceivably give rise to new bone by proliferation, and which form one of the strongest arguments for the phenomenon of induction. Between these extremes are various tissues which may act by one or both mechanisms, and an attempt has been made to arrange these in some semblance of order to illustrate the probabilities in the case of each tissue. At the middle of the scale it appears highly probable that cancellous bone and fibrocartilaginous callus may proliferate with formation of new bone and also induce bone formation by their effects upon the cells of the host.
There is thus proposed a resolution of a basic problem of osteogenesis by acceptance of the postulate that post-foetal new-bone formation, instead of depending solely upon one of the two mechanisms which have been proposed, may occur through the effect of either of these mechanisms, or by a combination of the two.
Wadsworth General Medical and Surgical Hospital, Veterans Administration Medical Center, Sawtelle, Los Angeles; Department of Physiology, University of Chicago, Chicago, Illinois