STRUCTURE-FUNCTION RELATIONSHIPS IN OSTEOCLAST 499 



on the basis of striictiiral differences. Three of these will be discussed 

 below; their main characteristics are as follows. 



Xone C 



The cytoplasm overlying the matrix is usually poor in organelles 

 such as ribosomes, rough or smooth-walled sacs and vesicles, mito- 

 chondria, etc.; it has a finely granular amorphous appearance (Figs. 

 2, 3, 4, and 6). The cell membrane follows the contour of the bone 

 edge closely except that here and there it seems to form short, blunt 

 channels leading inward ( Fig. 3 ) . Loose or detached bone salt crys- 

 tals can sometimes be distinguished within these (Fig. 6). 



Appearances suggest that cvtoplasm in this area was neither 

 elaborating any secretion nor pouring it forth onto the bone surface 

 at the moment the cell was fixed. It is interesting that Dudley and 

 Spiro ( 1961, Fig. 13 ) obtained a similar cytoplasmic picture in what 

 they describe as "endosteal lining cells," which they regard as form- 

 ing a more or less inert covering membrane. 



Zone B 



This constitutes a transition zone whose length is very variable; 

 it may be practically absent. The chief characteristics are the pres- 

 ence, first, of cytoplasmic vesicles gathered near the cell membrane, 

 and, second, of more numerous and longer channels leading in from 

 the surface. Bone salt crystals can be recognized in these, and they 

 have obviously traveled some distance from the matrix ( Fig. 4 ) . 



Zone A 



This, of course, is the "brush" or "striated" border area of light 

 microscopy (Kolliker, 1873; Kroon, 1954). Scott and Pease (1956) 

 seem to have been the first to describe its ultrastructure. They 

 thought it similar to the folded membrane of macrophages described 

 by Palade ( 1955 ) and called it the "ruffled" border; this name is 

 generallv employed now in electron microscopy. 



In our material the border consists of a complex system of cyto- 

 plasmic folds and projections separated b\ cleftlike spaces of varia- 

 ble appearance, and an associated system of cvtoplasmic vacuoles 

 and vesicles. At one end of the scale are relativelv fine canals, often 



