26 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



Again, the presence of numerous hypertrophied and highly phagocytic macro- 

 phages in regions where active bone resorption is proceeding is significant. The 

 relationship is similar to that in growing bone. 



The same general morphology as heretofore is noted. Even here there are 

 many non-trypanophilic reticulum cells. As before, too, the voluminous thin- 

 walled blood-sinuses are a feature of the areas of disintegrating callus. Bone 

 growth at this stage is limited to the reinforcement of the permanent osseous 

 trabeculse. But few osteoclasts were found. No dye-granules were discovered in 

 them. 



Specimen S 13-2 presents essentially the same features in the callus. Here, 

 too, the macrophagic picture is very striking. This specimen shows an interesting 

 condition in the cartilage, which sometimes appears in the callus, as noted at earlier 

 stages, for it is now becoming ossified. In this process the cartilage is modified and 

 hollowed into spaces in which appear blood-sinuses whose environment is charac- 

 terized by reticulum macrophages, similar to those found in osseous spaces. Osseous 

 tissue is built around the remnants of modified cartilage as in developing cartil- 

 age bone. Trabeculae so formed are resorbed like the usual trabeculse of the callus, 

 and the macrophagic tissue is concentrated similarly here. A formation essentially 

 the same as the familiar epiphysial plate, with typical rows of enlarged and modi- 

 fied cells, was observed. The reticular macrophages play the same part here as in 

 callus or growing cartilage bone, for they congregate in regions where tissue is being 

 broken down, as noted by Shipley and Macklin (1916 2 ). 



This period represents the high-water mark of the macrophagic activity in 

 the callus, and from this time forward there is a gradual diminution in numbers 

 of the cells and in their phagocytic power, as shown by the brightness of the stain- 

 ing. The decline of the macrophagic tissue is coincident with the gradual fall in 

 the rate of bone-erosion, and with the cessation of this process the macrophages 

 of the reticulum revert to the state of the macrophages of ordinary bone marrow. 

 Though retaining in some measure the power of ingesting and storing colloidal 

 dyestuffs, they are then nevertheless relatively small, few, and weakly staining. 



Summarizing as to the twentieth-day stage, it may be stated that there is no 

 destruction of tissue in any of the specimens outside of the callus. Extraosseous 

 tissue destruction has ceased coincidently with the falling away of the macrophage 

 concentration. In the callus, however, the same interesting participation of the 

 reticulum macrophages in dealing with the waste products of callus destruction is 

 emphasized. Even in the cleared gross ribs the cells may be described in the callus 

 spaces. It is especially evident from the sections that the greatest concentration 

 of macrophagic tissue is always found in areas of greatest callus destruction and 

 that these cells change their location to accompany the erosive mechanism, thus 

 gaining the position of greatest efficiency for the performance of their duties. 



A brief description will suffice for the remaining stages, since the more active 

 processes, outside the bone, have terminated by the twentieth day, and in the callus 

 the stained cells gradually become less and less conspicuous. 



