DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 15 



The callus of the fifth day shows evidences of development in its greater 

 thickness and in its longer trabeculse of bone. The spaces between the bony plates 

 are larger, and a pronounced feature of this stage is the presence of large blood- 

 sinuses within these spaces, especially in the region next to the original bone. They 

 are distinctly more capacious than those of earlier stages. Many of these sinuses 

 have strands of endothelium projecting into the lumen, or even completely crossing 

 it; again, the wall is often irregular and instances are easily found where a smaller 

 capillary is being incorporated into a larger one. Some of the intertrabecular 

 spaces show several small capillaries, and it is evident that the larger sinuses are 

 formed from the fusion of two or more smaller vessels. 



As in the third-day stage, cells faintly stained with small blue granules are 

 found in the spaces of the callus. They are somewhat more numerous and a little 

 more distinctly stained than in the earlier stages, and are largest and most strongly 

 stained in the intervals between the walls of the blood-sinuses and the bone, and 

 hence the area close to the original bone is most generously supplied with them. 

 Often the endothelial cells contain dye-granuljs. None of these cells are at all to 

 be compared in size and strength of staining with the large phagocytes of the 

 degenerating tissue. 



The same parallel is thus present here as in the last stage, viz, the excavation 

 of the spaces of the callus which goes hand in hand with the number and phago- 

 cytic ability of the trypanophilic cells. 



Quite a lot of cartilage is found in the callus of this stage, but it shows nothing 

 of interest from the standpoint of vital staining. 



Summing up the fifth-day stage, the most striking feature, as at the third day, 

 is the great number of large macrophages vigorously at work in clearing out the 

 waste material at the site of the injury. There is not such an active development 

 of macrophages, for fewer transitional forms are seen. That defunct tissue is being 

 cleared away is evident from the decreased amount of diffusely stained debris. 

 Fibrous tissue is developing. 



Trypanophilic reticulum cells, showing a little increase in numbers and stain- 

 ing powers, are found in the expanding callus spaces and often envelop the large 

 thin-walled blood-sinuses, which are now a prominent feature of the larger spaces. 



SIXTH-DAY STAGE. (S 11-2). 



The sixth-day stage as seen in the cleared ribs is characterized by the same 

 dense cloud of large macrophages enveloping the site of the. fracture. Plugs of 

 macrophagic tissue fill the open ends of the bone. The broken surfaces appear to 

 stain only faintly and also to be becoming round and thin ; in this we have evidence 

 that the dead bone is disappearing. Debris is slight in amount. The callus is more 

 sharply outlined and is optically denser. 



The cleared skull shows the same local increase in the trypanophil cells, the 

 space between the insert and the regions about the edge of the trephine opening 

 being literally crowded with them. Some diffusely staining debris is seen. 



