THE TWO GREAT GROUPS OF CONNECTIVE-TISSUE CELLS. 37 



Fibroblasts are loaded with dye vacuoles which have only a pale pink color. A few in each cell 

 appear like fat vacuoles, but these are not numerous. The dye vacuoles are round, but more 

 typically triangular, comet-form, rod-shaped, and bizarre shapes, apparently due to elongated 

 vacuoles. They attain considerable size, but are on the average slightly smaller than the macro- 

 phage vacuoles. The linear forms are very common; many of them occur in each cell. They may 

 even be more deeply stained than the vacuoles, though they are not bright red (fig. 82). Neutral 

 red stains the dye vacuoles and emphasizes the linear forms. Janus green stains normal mito- 

 chondria. 



(See also Protocol, Rat 18-2, p. 13.) 



Attention may now be called to a most important physiological difference 

 between fibroblast and macrophage cells. We refer to the reaction which these 

 cells exhibit to supravital applications of weak neutral red (1:10,000). This 

 basic dyestuff would appear to speedily enter both of these cell types with equal 

 avidity, but it is never greatly concentrated in the segregation-apparatus of fibro- 

 blasts. This is true regardless of the fact that a large vacuolar segregation-appara- 

 tus may exist in the fibroblasts (e. g., after vital staining with dye T 148). On the 

 other hand, in a wholly characteristic way, the vacuoles of macrophage cells always 

 concentrate weak yellow solutions of neutral red to a conspicuous color intensity 

 (deep orange or red) . We call this characteristic response of the macrophage the 

 "neutral red reaction. " i It is a fact that such weak solutions of neutral red elec- 

 tively tinge the vacuolar-apparatus of fibroblasts, even though there is a greater 

 concentration of this dyestuff within the vacuoles of macrophages. As we have 

 just remarked, in cases where the type of dye treatment and of dosage establishes a 

 segregation-apparatus strikingly similar or identical in both macrophages and 

 fibroblasts, the "neutral red reaction" applied supravitally to such vitally stained 

 tissues immediately picks out with infallibility the macrophage cells. But if 

 acid dyes may enter the fibroblasts only more slowly than the macrophages, they 

 are also not free to leave these cells as easily as they do the phagocytes. This brings 

 us to a consideration of decolorization. 



Not the least interesting of the phenomena connected with the storage of dye- 

 stuffs is the subsequent liberation of these substances by the cell, so that, save 

 for certain residual effects, a more or less perfect decolorization of the cells 

 and tissues occurs. It is apparent from this fact that the concentration of these 

 materials within the cytoplasmic confines of certain cells is a temporary, not a 

 permanent, effect, and that the factors which operate to establish these deposits 

 must continue in efficacy to insure their permanence. There is reason for believing 

 that this behavior toward dyestuffs is merely a visible instance of the manner in 

 which other materials, not immediately utilizable in the metabolism of the body, 

 are conserved in the storage mechanism constituted by these cells. The dis- 

 covery of what these substances may be is an attractive field for future investiga- 

 tion. Analogous phenomena are found in the discrete miscroscopic deposits of 

 lipoids and glycogen which may occur when the bod) 7 has to contend with an over- 

 abundance of these substances or their alimentary precursors, but the places of 

 storage for these materials, and especially for glycogen, are differently located (e. g., 

 liver, striated muscle-cells, etc.), 2 and we have referred to this merely as an analogy, 



1 We have not been impressed with the advisability, as has Renaut, of separating protein-containing (rhagiocrine) 

 from Bimpler liquid-containing (plasmocriue) vacuoles in the connective-tissue cells. 



'One may also mention the accumulation of hemosiderin in the pancreatio parenchyma cells. 



