32 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



marrow-cavity and periosteum of bone and in the tissue of the immediate vicinity. 

 When an injury occurs, such as a fracture, it is quite possible to think of these 

 "resting-wandering" cells as undergoing extensive rapid multiplication until the 

 aggregation of phagocytes which is so marked a feature of the early stages of the 

 repair process is produced. But to accomplish this result the proliferation would 

 have to be enormous, and it is noteworthy that but few mitotic figures were to be 

 found among the macrophages. 



Again, it is known that endothelial cells can, under the stress of inflammation, 

 take on phagocytic properties, as shown by their reaction to colloidal dyestuffs 

 (MacCurdy and Evans, Tschaschin, and others), and hence it may be assumed 

 that some of the macrophages are recruited from these elements. The reticular 

 cells of bone-marrow, too, are known to be phagocytic, and these may contribute 

 their quota to the sum total of the "poly blasts." 



It would be difficult, however, to conclude that any or all of these sources could 

 account for the tremendous local increase in trypanophilic cells, as seen, for instance, 

 on the third day, even providing for the immigration of considerable numbers 

 from adjacent tissues. Again, such an hypothesis would have no place for transi- 

 tional cells of the type seen in figure 8, and of these there are all grades, from the 

 finished polyblast large, filled with enormous blue granules, and with a relatively 

 small nucleus all the way down to what appears to be the parent cell. This is a 

 small mononuclear element resembling a lymphocyte. The parent cells contain no 

 dye-granules, but they soon gain phagocytic ability, as shown by their rapid 

 increase in size and the larger and larger amount of dye which they take up (fig. 8). 

 The transitional cells are particularly abundant in the early stages, and most of 

 all on the third day. 



The idea that some at least of the macrophages are really metamorphosed 

 lymphocytes has been steadily gaining in the literature. Even in normal connective 

 tissue, transitional forms may be found linking together the small round amoeboid 

 lymphocytes with the clasmatocytes (Ranvier, Tschaschin) and suggesting the deri- 

 vation of the latter from the former. The origin of the macrophages of the "taches 

 laiteuses" in the rabbit is indicated by finding transitional cells connecting them 

 with a small lymphoid element (Tschaschin). Maximow (1907, 1909 1 ) has estab- 

 lished the relationship of these wandering cells by embryological researches. The 

 same author (1902, 1909'-) has long maintained that the hypertrophied mononuclear 

 phagocytes or polyblasts of areas of inflammation are largely derived from lympho- 

 cytes attracted thither from the tissue-spaces and from the blood-stream, and 

 Tschaschin brings forward evidence to confirm this view the lymphocytes, it is 

 assumed, rapidly undergoing a metamorphosis, their powers of phagocytosis becoming 

 intensified, as shown by their progressive increase in ability to ingest vital dyes. 

 Tschaschin has illustrated a series of transitional forms in his figure 11, Taf. vn, 

 which strikingly resembles that shown in figure 8. Maximow (1916), according to 

 Downey (1917), has even been able to bring about the development of typical vitally 

 staining polyblasts from lymphocytes in tissue-cultures of lymph-nodes of young 



