DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 33 



and adult rabbits by the addition of tissue-extracts. Similar views as to the lym- 

 phogenic origin of the macrophages are expressed by other authors. In addition 

 to the lymphocytoid cells, most workers recognize the participation, in the forma- 

 tion of the wandering mononuclear phagocytes of inflammation, of the resting- 

 wandering cells of the tissue-spaces, the endothelial cells, and, in the blood-forming 

 organs, the cells of the reticulum, etc. 



It is probable, therefore, that while some of the large macrophages of the 

 inflammatory region at the fracture-site come from the reticulum of the marrow 

 and from endothelial cells, and that even more are mustered from the ranks of the 

 resting-wandering cells of the surrounding tissues, yet the vast proportion develop 

 from the small lymphocyte-like cells. Though the lymphocytes of the tissues 

 (histiogenous lymphocytes) doubtless supply some of these, by far the greater 

 proportion probably arises from the lymphocytes of the blood-stream, which have 

 wandered from the vessels into the inflammatory zone (hematogenous lymphocytes) . 



We may summarize the discussion of the source of the cells by saying that at 

 the beginning of the inflammatory process cells of the types above mentioned are 

 already present in the wound-area, and although these function actively, and even 

 increase in effectiveness, they soon become inadequate to the demands made upon 

 them. Cells from the surrounding tissues, too, may be presumed to wander to the 

 inflammatory region; in these we may recognize representatives of the resting- 

 wandering-cell type and also the closely related "histiogenous" lymphocyte. But 

 even these reinforcements are insufficient for the performance of the work, and the 

 vast bulk of the phagocytes, as we have seen, come by way of the blood-stream. 



Thus the macrophages of the inflamed tissue in the vicinity of wounded bone, 

 though derived from cells of diverse morphological type, are yet united by the 

 possession of a common physiological potentiality which manifests itself in a uni- 

 form response to a common call to arms. This response consists in the metamor- 

 phosis of these cells into enormous and rapacious phagocytes and in the assump- 

 tion by the latter of an important service in the treatment of the waste products 

 occasioned by tissue-injury. This phagocytic response is graphically demonstrated 

 by the trypanophil reaction. We must postulate a progressive adaptation on the 

 part of the cytoplasm of the mobilized cells until there is produced a mechanism 

 of the highest efficiency in the function of phagocytosis. 



It is a striking fact that the working units of this phagocytic tissue, although 

 derived from different sources, resemble one another so closely that the riper forms 

 are indistinguishable. Indeed, this fact has been commented upon by Tschas- 

 chin (1913), who, speaking of the resemblance of the "polyblasts" derived from 

 resting-wandering cells to those from the lymphocytes, states (p. 388) : "gegen Ende 

 des zweiten Tages der Entzlindung die Polyblasten nach der Quelle ihrer Entstehung 

 nicht mehr unterschieden werden konnen." 



MOBILIZATION. 



Why, it may be asked, do the macrophages congregate at the fracture-site? 

 What is the influence which causes a cell situated near the area of inflammation to 



