24 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



they are relatively few and inconspicuous. The apparent reduction in their num- 

 ber here is due in part at least to the increased proportion of other cells as well as 

 to the fact that the tissue becomes more open in structure. The phagocytes are to 

 be regarded as having a hand in the preparation of the marrow-tissue habitat. 



Farther out in the callus the spaces are large, and many are occupied by enor- 

 mous blood-sinuses with walls composed only of endot helium. Here many of the 

 trabeculse are thin and are evidently being broken down; others are thickened and 

 are being built up. This. is the most active area of bone resorption. In this region 

 hordes of reticulum macrophages, brightly stained and of increased size, form a 

 striking picture and are the most outstanding feature of this stage. Their district 

 of greatest concentration, as compared with the last stage, has shifted peripherally, 

 in company with the district of greatest callus-erosion. The preference of the 

 macrophages for regions of bone resorption is all the more significant because it is 

 consistent with the behavior of these cells in earlier stages. 



The morphology of the macrophages and their relationship to the surrounding 

 structures need no special description here, for they are similar to those of the last 

 two stages described. Careful measurements of the longest diameters of ten of 

 the largest cells gives an average length of 9.95 microns thus showing that the 

 cells have become somewhat hypertrophied. The dye-granule content is similar 

 to that of earlier stages (fig. 15 c). Some associated polymorphonuclear leucocytes 

 are found in the tissue-spaces, but no dye-granules appear in them. Giant-cells 

 are quite frequently found, but they also contain not a trace of dye. Often they 

 give the impression of being a mere scrap-heap of old osteoblasts and bone-cells, 

 the residue of bone-erosion (Arey, 1917). 



In the skull of this stage macrophages of large size, and conspicuous staining 

 inhabit the culliix spaces. Large blood-sinuses in the spaces are also a feature here. 



On the whole it may be concluded that at the fifteenth day little or no tissue 

 is being destroyed outside of the bone, for the number and staining intensity of the 

 macrophages have been reduced almost to normal and there is an absence of demon- 

 strable moribund tissue. In the callus, however, the reticulum macrophages are 

 especially abundant and phagocytic in the regions of bone destruction, and appear 

 to be actively engaged in phagocytizirig the products of this process. 



TWENTIETH-DAY STAGE. (S 12-2), (S 13-2). 



The cleared ribs of the twentieth day (fig. 5) are very different from those of 

 earlier stages, as the third and fifth days, for the blue investment has become very 

 thin and but few granules, representing macrophages, are to be seen in it with the 

 binocular. What blue there is seems to be largely a diffuse staining of the perios- 

 teum and scar-tissue. The size of the fracture-site is much reduced and the callus 

 has undergone a good deal of resorption, the part now being of a slender spindle- 

 like form. The interior of the bone is occupied by a large-meshed callus, containing 

 some blue cells, and the medullary canal is evidently being restored. Other ribs 

 at this stage show less perfect approximation, but essentially the same features. 



