STRUCTURE-FUNCTION RELATIONSHIPS IN OSTEOCLAST 501 



branching, which can sometimes be seen to terminate in sacUke 

 vesicles bounded by a single membrane (Figs. 7 and 9). Generally 

 there are multitudes of other vesicles, structurally identical, in the 

 near-by cytoplasm. At the other end of the scale ( Figs. 2, 5, 7, 8, and 

 10) are broader channels leading into large vacuoles, probably pino- 

 somes ( Hancox and Boothroyd, 1961 ) . 



Bone salt crystals can be recognized in both the finer and the 

 broader channels and also in the more distal pinosomes. Their ap- 

 pearance in such places has been described in most of the published 

 work on the electron microscopy of osteoclasts (Scott and Pease, 

 1956; Gonzales and Kamovsky, 1961; Hancox and Boothroyd, 1961; 

 Dudley and Spiro, 1961; Cameron and Robinson, 1958). 



The occurrence of base collagen fibrils between folds of the ruffled 

 border does not seem to have been encountered by other workers. 

 Collagen occurs constantly, however, in our material (Figs. 5, 7, 

 10, 11, and 12). The fibrils seem to protrude from the surface of the 

 calcified matrix and are closely invested by ruffled border folds. The 

 interfold spaces continue outward into the cytoplasm in fine chan- 

 nels which may branch before ending in vesicular sacs. More rarely, 

 collagen cross-banding can be identified in the larger and more distal 

 pinosomes ( Fig. 10 ) . Partly demineralized fibrils are also seen. 



Discussion 



Turning next to the possible functional implications of these struc- 

 tural findings, two points particularly seem worth discussing, namely, 

 the presence of free or detached bone salt crystals and the occur- 

 rence of collagen fibrils apparently denuded of their mineral. 



Figures 2 to 1.2 are electron micrographs made from unstained sections of 

 osmium-fixed bone embedded in Araldite. 



Fig. 2. Low-power electron micrograph of part of an osteoclast, at right, 

 in contact with a trabeculum of bone labeled BM. The position of zone C is 

 indicated by that letter, and the arrow points to the amorphous cytoplasm typi- 

 cal of this zone, which is seen at higher magnification in Figs. 3 and 6. The 

 position of the ruffled border (RB) of zone A is arrowed. (X 8000.) 



Fig. 3. Bone matrix (BM) at left; apposed to it is the amorphous, inert- 

 looking cytoplasm typical of zone C. It contains a few ribosomes and vesicles. 

 A short, blunt cytoplasmic channel is arrowed. (X 45,000.) 



