20 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



The sections of the long bones of the tenth day (S6-1, S6-2) present very little 

 remaining debris. Occasionally a few shreds of muscle are found, showing evidences 

 of degeneration. Here macrophages are fairly numerous, though much reduced 

 in number as compared with earlier stages. These phagocytes are of the mature 

 type, with very few transitional stages. Comparatively few involution forms are 

 seen. Scar-formation is advanced. 



The callus of the long bones (S6-1, S6-2) at this stage shows little increase in 

 volume. The most interesting feature is seen in the hollowed-out spaces near the 

 original bone; for here, especially in the cleared and uncounterstained sections, 

 one is struck with the conspicuous appearance of the many vitally stained cells. 

 Even under the low-power they are very evident, as shown by figure 14, drawn 

 from a section from SG-1. and are much more brightly stained than in the earlier 

 stages. As before, they are often distributed around the thin-walled blood-sinuses. 

 They are not in actual contact with the callus. They are found in the callus of the 

 marrow cavity as well as in that of the external surface of the bone. The degree 

 of staining intensity gradually diminishes as the outer district of the callus is 

 approached, and in the more compact areas of callus, where apparently no 

 resorption is occurring, the blue cells are very few and insignificant in staining. 



A description of the general morphology of the typical reticulum macrophages, 

 as seen with the high-power lens, may here be given. It applies, with slight varia- 

 tions, to the cells of the succeeding stages up to and including the twentieth day. 



Like all reticulum cells they are characterized by a number of processes flat 

 and wide, or extended and threadlike through which they are directly continuous 

 with their neighbors. The cell-body is oval or stellate, and often elongated and 

 flattened. The nucleus is fairly large and usually rounded or of oval outline, though 

 it may be notched. 



The trypanophilic reticulum cells at the tenth-day stage are the same as at 

 the earlier stages, except that they are often much more brilliantly stained and 

 hence may be regarded as much more actively phagocytic. They are quite evi- 

 dently true reticulum cells, and not invading elements from other regions of the 

 body, for their protoplasmic connections with other reticulum cells, which may or 

 may not contain dye-granules, can easily be made out. The size of the well-stained 

 cells varies; the tendency seems to be to develop a cell of fairly uniform dimensions 

 The average diameter of ten of the largest cells at the tenth-day stage, as taken 

 between the widest extremities of the dye-granule content, was found to be 7.6 

 microns. They are thus considerably smaller than the largest extraosseous macro- 

 phages, though larger than the macrophages of ordinary bone marrow. The 

 arrangement of the cells in the spaces is the same as that in stages already described, 

 or as in the later stages. 



The degree of staining varies considerably; some cells have a mere sprinkling of 

 small dye-granules, while others are stuffed full of granules of larger size (fig. 15, a), 

 and there are all degrees of variation between these extremes. Usually the nucleus 

 is completely inclosed in a zone of blue granules, but not infrequently cells are 



