28 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



The callus of the fifty-ninth day is very dense. The description of it and of the 

 vitally stained cells it contains is similar to that of the fifty-first day. 



SIXTIETH (S 11-5) AND SEVENTY-FIRST DAY (S5-5) STAGES. 



The cleared rib at 60 days shows little change as compared with that of the 

 fifty-first day. The site of the fracture can hardly be distinguished, so perfect is 

 the repair. In the cleared skull of this stage, and that of the seventy-first day, there 

 is the same diffuse staining of the connective-tissue membrane joining the bony 

 fragments and the same absence of macrophages. The sections of the sixtieth day 

 show nothing of interest in the soft parts. The callus is greater in amount than in 

 the last two specimens described, and hence more time is required for the resorp- 

 tion of the redundant bone. The osseous structure is somewhat less compact and 

 the trabeculse are thinner. There is evidently some osseous resorption still going 

 on. In keeping with this the macrophages are somewhat more numerous and con- 

 spicuous than in the fifty-first and fifty-ninth day specimens. These cells, are, 

 however, comparatively small and weakly stained. Bone-building is going on here 

 and there, as the rows of osteoblasts attest. 



The skull sections show nothing of interest. 



Summarizing the stages from the thirtieth to the seventy-first day, it is to be 

 noted that there is no excess of macrophages outside of the bone. No tissue destruc- 

 tion has taken place during this period except in the callus. At the thirtieth day 

 there are some macrophages to be found in the callus-spaces, but sections of the 

 fifty-first and fifty-ninth days show very few of them, so that bone destruction 

 here may be regarded as almost at a standstill. The spaces are large and the 

 remaining trabeculse are much stouter, so that the principal effort has been directed 

 toward reinforcement of the bony trabeculse rather than destruction of them. 

 The occurrence of a few persisting macrophages in the 60-day stage is to be looked 

 upon as a special case where, on account of the excessive amount of callus, the work 

 of destruction was of longer duration. 



DISCUSSION. 



From the foregoing account of the appearance of the tissues at the site of 

 fractures and trephine wounds in the vitally-stained rat, during successive stages 

 of healing from the second to the seventy-first day, it is plain that there are two 

 distinct phenomena to be considered. In the earlier stages the most notable 

 feature is the tremendous increase in the number of trypanophil cells which occurs 

 in the injured tissue and exudate of the region and about the damaged surfaces of 

 the bone; at a somewhat later period an equally remarkable development of phago- 

 cytic tissue within the spaces of the callus is seen. The discussion falls naturally, 

 then, under two main headings concerned with (1) the phagocytes of the soft parts 

 surrounding the injured bone, or extraosseous macrophages, and (2) the phagocytes 

 of the callus spaces, or intraosseous macrophages. 



