16 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



The sections of the long bone show dense collections of macrophages of similar 

 type to those of earlier stages (fig. 9), except that there are not nearly so many 

 young transitional forms, and there are more degenerate cells in places. Wherever 

 there is fragmented tissue to be absorbed, especially shreds of muscle, there are 

 myriads of large macrophages stuffed with blue granules (fig. 10). The crevices 

 between muscle fibers are often crammed with veritable nests of these phagocytes. 

 As formerly, the damaged tissue stains diffusely blue. There is very little exudate. 



It is evident, however, that in some regions tissue destruction is waning. Large 

 patches of young scar tissue appear (fig. 9, s), and in the sections simply cleared 

 without counterstaining one has a striking demonstration that in such areas the 

 phagocytes are much less prominent indeed, they are often relatively incon- 

 spicuous. In figure 9 an area of scar tissue appears contrasted with an area of 

 degenerating muscle fiber, and it is easily seen that in the former the blue staining 

 is very weak, whereas it is exceedingly marked in the latter. The macrophages 

 may be looked upon as assisting to prepare the way for the scar tissue, and they 

 are thus an important factor in repair. 



Upon searching with the high-power lens through the patches of young fibro- 

 blasts (fig. 9, s), one is struck with the great number of phantom-like cells, which 

 appear to be macrophages undergoing degeneration (fig. 11). They resemble 

 similar cells described for the third and fifth days, but arc much more numerous. 

 Their appearance is very characteristic; they often resemble a mass of fishing-net, 

 the knots being represented by the scattered dye-granules. The cytoplasm, at 

 first pale and vacuolated, is in the more extreme types reduced to a mere lacework, 

 and here and there in this are to be found occasional irregular grains of blue, showing 

 that the specific function has not altogether departed. Not infrequently cellular 

 inclusions are found in them, often stained diffusely blue. The cytoplasm may be 

 reduced to tattered remnants. Fragments of cytoplasm, containing a few scattered 

 dye-granules, are sometimes found, marking the remains of a cell which has suffered 

 dissolution. Often the nucleus is stained blue an evidence of cell death. 



The forms of degeneration which have been described are the most extreme. 

 The vast majority of the macrophages in the scarred areas on the sixth day are in 

 this condition. There are, however, cells which are not so advanced in degenera- 

 tion, especially in regions where fibroblasts are less abundant. These contain 

 more dyestuff and usually are less vacuolate. All grades of these are found con- 

 necting the normal macrophage at the one extreme with the disintegrating remnant 

 at the other. 



The reason why these cells drop out of the ranks and break up may be because 

 they have become exhausted in the course of their strenuous activities, for often 

 their cytoplasm is clogged with phagocytized material; they are thus incapacitated 

 for ingestion of the dyestuff. Possibly, too, the cells have become intoxicated by 

 the materials taken up. Again, since these cells are found characteristically in the 

 scarring areas, where waste material has been removed, and consequently where 

 the opportunity for activity even the stimulus thereto has departed, it may be 



