DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 11 



hypothetical origin will be discussed later. Mitotic figures were extremely rare 

 among these cells; thus it appears that the cells do not multiply in the fracture- 

 area, but increase by immigration. It is of interest to observe in this connection 

 that in the capillaries of the region of macrophage invasion there are large numbers 

 of lymphocytes; these seem to be present mainly in the large sinusoids. Bay-like 

 diverticula from the capillaries, representing the young vascular sprouts, were 

 observed crowded with lymphocytes, and it may be that these represent important 

 points for the exit of the embryo macrophages from the blood-stream, and the 

 circulation stasis here would be favorable to such a proceeding. The finding of a 

 small clump of lymphocytes in the tissues at the apex of one of these vascular 

 pouches lends support to this supposition. 



That there is an actual destruction of effete tissue is shown by the progressive 

 disappearance of this in the consecutive stages; and this tissue-erosion is coincident 

 with the presence of increased numbers of very large macrophages. Indeed, as 

 will be seen, the numbers and size of the macrophages in a given area are a good 

 index of the degree of protoplasm demolition occurring therein; in other words, 

 there is a direct parallel between the exaltation of the potentialities of the macro- 

 phagic tissue and the absorption of the products of proteolysis. 



Though the macrophages are found at times lying quite close to even in 

 contact with the tissue undergoing lysis, their characteristic position is rather 

 one of complete separation from such tissue, for most of the cells are to be seen lying- 

 free in the colloidal fluid which bathes them. This fluid, which stains faintly pink 

 with eosin, ia necessarily heavily charged with the products of proteolysis. The 

 macrophages are thus favorably situated for functioning in the absorption of this 

 waste material. 



In the third-day stage a macrophage is occasionally found in which, though 

 the cytoplasm is voluminous, the dye-granules are relatively few and scattered. 

 The cytoplasm presents a vacuolate appearance, and in some cases one gains the 

 impression that the cell is filled with phagocytized tissue-waste. It is not unlikely 

 that these cells have been active for a longer period than the well-stained macro- 

 phages. They are most numerous in the areas of young scar tissue where tissue 

 destruction is largely over. Small and intermediate macrophages, as well as the 

 larger sizes, show this meager type of staining. A few seem to be falling to pieces. 



Compared with the typical macrophages these exhausted or involution forms 

 are as yet insignificant in number. They are more abundant in the later stages, 

 as will be seen. 



Polymorphonuclear leucocytes were present in appreciable numbers in the 

 tissues of the fracture-region. They were somewhat irregularly distributed and 

 were often quite closely associated with heavily stained macrophages. Though 

 favorably situated for imbibition of the dyestuff, being at rest or wandering in 

 tissue-spaces whose fluids were thoroughly impregnated with it (Downey, 1917), yet 

 no dye-granules were found in them. Thus, if this cell functions in the absorption 

 of colloids it must do so very sparingly. It also differs from the macrophages in 



