DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 37 



the tissue of the Haversian canals of the old bone) early make their appearance. 

 First seen on the third day, they soon expand, the largest spaces naturally occupying 

 the oldest part of the callus, situated, of course, in the vicinity of the original bone. 



With the hollowing-out of these spaces numbers of the reticulum cells con- 

 tained in them gradually acquire the power to ingest and store colloidal dyestuffs. 

 Faintly stained cells were found in the spaces in the third-day stage. This mani- 

 festation of phagocytic activity on the part of the cells of the reticulum is intimately 

 associated with the breaking down of tissue consequent upon the excavation of the 

 callus spaces. It is noteworthy that the brightest staining occurs in the cells of the 

 largest spaces, and especially in those at the edge of the old bone an indication that 

 the greatest phagocytic activity is resident in areas of greatest tissue destruction. 



As new spaces open out, trypanophilic cells, at first very weakly stained, 

 appear in them, so that the number of these cells gradually increases, keeping pace 

 with the growing volume of the callus recesses. Cell multiplication is by mitosis, 

 as is proved by finding karyokinetic figures among them, even in dye-containing 

 cells. Also, with advance in age of the reticulum cells, there is usually a slight 

 concomitant progressive increase in their phagocytic potentiality which goes hand 

 in hand with the gradually accelerated callus erosion; for, in the fifth and sixth- 

 day stages, the cells in the older and larger spaces are somewhat more brightly 

 stained than corresponding cells of earlier periods. No noteworthy increase in size 

 of these phagocytes is so far apparent. Thin- walled blood-sinuses of large size appear 

 in the callus-spaces of this period, and are a marked feature. A favorite position 

 of the macrophages is between the walls of these vessels and the osseous plates. 



In no area of the callus during this period is the staining phenomenon of the 

 reticulum cells at all comparable with that of the extraosseous macrophages; 

 indeed, the reticulum cells are quite small and inconspicuously stained as compared 

 with the deeply dyed "polyblasts" of degenerating soft tissue, so that it seems 

 more rational to look upon them as cells whose powers are as yet potential rather 

 than actively functioning as elements, indeed, capable of developing very efficient 

 phagocytic ability on short notice. Though their service in phagocytizing the 

 products of tissue breakdown during this early stage is recognized, it is felt that 

 the amount of colloidal waste resulting from callus erosion can not be very great, 

 since the total callus destruction is as yet small. The main feature of the callus 

 during this phase is construction (rather than destruction) of osseous tissue. 



ACTIVITY. 



The history of the callus during the period from the tenth to the twentieth 

 day inclusive is of the greatest interest. The first part of this interval is charac- 

 terized by both development and destruction of bone, for as the callus expands in its 

 more outlying regions the older parts are worn away, as shown by the progressively 

 widening spaces. Gradually peripheral growth declines, it being practically non- 

 existent by the twentieth day, but very active destruction of the trabeculae is 

 maintained throughout. During the first few days the area of most active bone 

 destruction is in the older portions of the callus around the original bone, but as 



