DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 9 



these into the precallus. This material is directly continuous with the old bone. 

 Ossification is so slight that it is difficult to find manifestations of it. Occasionally 

 spaces, containing one or two osteoblasts, occur between the spicules of bone. 

 Vessels have hardly begun to invade this precallus, a small capillary being found 

 but rarely in its peripheral layer. 



The principal features of the second-day stage, then, may be summed up as 

 follows: (1) the presence of increased numbers of vitally stained macrophages in 

 the tissues surrounding the fracture which have been damaged by the trauma, 

 and in the exudate associated with this; (2) the diffuse blue staining of dead and 

 damaged tissue; (3) the first indication of the callus. 



THIRD-DAY STAGE. (S 11-1). 



The next stage studied was that at the end of 3 days' repair. In the cleared 

 ribs (fig. 3) it is at once seen that there has been a great change at the site of the 

 fracture, which appears very much more intensely stained than at the earlier stage. 

 As at the second day, the position of the approximated ends of the bone is marked 

 by an opaque blue irregular mass, representing the diffusely stained areas of injured 

 bone and soft parts; beyond this the outline of the bone-shaft may be easily followed. 



But the most characteristic thing is the dense, cloud-like, blue sheath which 

 surrounds the fracture-area and gives to it a swollen appearance. Upon examining 

 the specimen critically with the binocular, the blue staining may largely be resolved 

 into granules, each of which represents a macrophage. The appearance is as though 

 the normal resident macrophages had become immensely multiplied in the pre- 

 ceding 24 hours, or as though great numbers of macrophages had invaded the 

 area from other parts. Many of the cells occur in rows between the muscle strands, 

 giving rise to the appearance of blue granular streaks. 



Between this investment of macrophagic tissue and the bone there is a clearer 

 area, and as this is followed in either direction along the bone it is seen to underlie 

 a membrane which is continuous with the periosteum. No definite periosteum is 

 found on the bone which it covers. It represents the young callus. Its vigorous 

 proliferation around the ends of the bone has caused the swelling which has resulted 

 in an outpushing of the macrophagic zone. 



In the long bones an intense blue staining at the site of the bone-wound is 

 extremely conspicuous in the fresh condition. As in the ribs and skull, it is due to 

 the diffuse staining of injured tissue as well as to the macrophagic invasion. The 

 sections of the long bones show a diffuse blue-staining of the broken ends (shown 

 in black in fig. 7, B), and also a dense blue-staining of the exudate and loose debris, 

 of which there is abundance. This dense staining is even more marked than on 

 the second day. 



These sections are very striking. A good example of a specimen field is seen 

 in figure 7. This is a low-power photomicrograph from a tangential section through 

 the fracture-area; in it myriads of blue cells (shown as black dots M) crowd about 

 the broken bone and through the surrounding injured tissues and extravasated 

 blood. There is a great deal of fragmented muscle and the blue macrophagic cells 



