10 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



surround the remnants of this, and even squeeze in between the fibers. Under 

 the high power they swarm among the fibroblasts and polymorphonuclear leuco- 

 cytes, around the broken bone-ends, in the open spaces everywhere. In the more 

 intact part of the muscle, between the fibers, the macrophages are often spindle- 

 shaped or stellate. Many of these are doubtless the so-called resting-wandering 

 cells or clasmatocytes. In all areas, but particularly where the tissue is fragmented, 

 cells of more rounded or oval shape are found. These are similar to those described 

 for the second day, but are much more abundant. 



It is at once seen that there are great variations in the dimensions of the macro- 

 phages. The largest are enormous cells, studded with granules of irregular size; 

 the average long diameter of 10 of these was 23.3 microns. They are distinctly 

 more voluminous than the greatest macrophages of the second day and contain 

 more dyestuff, thus indicating a gain in phagocytic efficiency. Their shape varies; 

 some are rounded or oval, while others are very irregular in outline elongated, 

 with long processes sometimes constricted from the main part of the cell. They 

 resemble the "polyblasts" which Tschaschin found in areas of inflammation in 

 the rat vitally stained with isamin-blue and in the rabbit vitally stained with try- 

 pan-blue. The cytoplasm, aside from the dye-granules, is for the most part clear. 



It is significant that there are all degrees of size in the dye-containing cells, 

 from this extremely large macrophage to quite small cells which harbor very little 

 dye; this type again grades off into a small round cell with a relatively large nucleus 

 and but little cytoplasm, which resembles a lymphocyte. In the carmine-stained 

 preparations they present a large, deeply staining nucleus and little cytoplasm. 

 They contain no dyestuff. We may select transitional cells from almost any part 

 of the field and arrange them in an order of size, as in figure 8. From the appear- 

 ance presented by such a series it will be at once inferred that an hypertrophy of 

 the cytoplasm of this small round cell has taken place and that coincident with 

 this hypertrophy there has been an acquisition of the power of phagocytosis; for 

 dye-granules, at first small in number and size, begin to make their appearance in 

 the enlarged cytoplasm. These granules gradually become larger and more numer- 

 ous as the cytoplasm increases in size, and reach their maximum development in 

 the largest polyblasts. There seems to be a direct correspondence between the 

 size of the macrophage and the amount of dyestuff it contains, on the one hand, 

 and the extent of its phagocytic activities on the other; for it may be assumed that 

 the reaction of the phagocyte toward the dyestuff in degree as well as in kind- 

 is an index of its behavior toward the material which it is specialized to ingest. 



During this metamorphosis the nucleus undergoes little change in size, so that 

 it is extremely small in comparison with the enormous mass of the cytoplasm. 

 In the early changes of the lymphocytoid cell the nucleus often becomes bilobed 

 or even binucleate. Superficially it may resemble a polymorphonuclear leucocyte. 

 In the mature forms no multinucleate or giant-cells were observed. 



The transitional stages which have been described suggest that at least some 

 of the macrophages have been derived from the lymphocyte-like cells and this 



