18 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



mandizing properties. Although reticulum cells containing a few small blue gran- 

 ules are seen in the younger spaces, the staining is stronger and the cells more evi- 

 dent in the older spaces. Here the dye is fairly well marked. There has been no 

 increase in size, and but little in staining intensity as compared with the third and 

 fifth day stages. The blue-stained cells frequently lie between the endothelium 

 of the blood-sinuses (fig. 12 BS) and the trabecuta of young bone (fig. 12 c). The 

 endothelium itself often contains granules of dyestuff. The vital-staining is 

 nowhere at all comparable with the brilliant coloration of the extraosseous macro- 

 phages. The reticulum cells are very small as compared with the larger macro- 

 phages, and the dye-granules are also comparatively small. 



A feature of this stage, as shown in the carmine-stained preparations under the 

 oil-immersion lens, is the large number of mitotic figures which occur in the try- 

 panophilic cells (fig. 13). These are especially numerous in the outer regions of the 

 callus, where the cells are multiplying rapidly. They are reticulum cells, and it is 

 evident that their manner of multiplication is by karyokinesis. The presence of 

 dye-granules within the cytoplasm is not incompatible with mitosis, as was shown 

 for the Kupffer cells by Evans, Bowman, and Winternitz (1914). 



Giant-cells are extremely rare. Only one small giant-cell was found after an 

 extended search. No dye-granules were to be seen in it. 



In the skull sections of S 11-2 callus is slight in amount. Reticular phagocytes 

 are beginning to appear in it. 



The other member of the six-day stage, S13-1, shows essentially the same 

 features. The callus occupying the marrow cavity is particularly well developed 

 and innumerable trypanophilic reticulum cells are found in the intertrabecular 

 spaces. The original bone in the vicinity of the callus has a worm-eaten appear- 

 ance; in these spaces the vitally stained reticulum cells are quite conspicuous and 

 numerous. In addition there are here a fair number of giant-cells, some of large 

 size. None contain dye-granules. The blood-sinuses of this region are very large. 



In brief, then, it is noted that at the six-day stage there is the same relation- 

 ship apparent between resorption of the callus and the presence of trypanophil 

 cells. Of special interest is the presence of well-stained cells in the spaces which 

 have been eroded in the old bone. The obvious relationsliip between the large 

 blood-sinuses and bone-resorption is also noteworthy. 



In reviewing the sixth-day stage we have to note the persistence (in areas 

 where tissue destruction is evidently proceeding rapidly, especially in moribund 

 muscle) of enormous numbers of large and very phagocytic macrophages. They 

 are of the same type as in preceding stages, except that transitional forms are much 

 less frequently seen than on the third day, so that fewer young cells are being 

 called out. In areas where destruction of tissue has ceased scar-formation is 

 well under way, and here the loose fibrous tissue contains immense numbers of 

 relatively weak-staining but often voluminous macrophages, which appear to be 

 degenerate or involution forms. A few of them are found among the active phago- 

 cytes. Thus, in this stage, tissue resorption is gradually ceasing, as shown by 



