BONE CELLS AND BONE RESORPTION 487 



roundings were observed. Occasionally, small groups of apparently 

 enlarged osteocytes, well removed from bone sm-faces, were seen 

 in animals treated with massive doses of PTE. This was an infrequent 

 rather than a common finding. In these hvperparathxroid rats, how- 

 ever, osteocytes were nniformlv depleted of their normal stores of 

 cytoplasmic glycogen (saliva-digestible, PAS-positive material). 

 Chondrocytic glycogen was not noticeably decreased in these ani- 

 mals. 



In regions of osteogenesis in normal animals, newly formed osteo- 

 cytes near the bone surface were characterized by large size, abun- 

 dant cytoplasmic and nucleolar RNA, large, pale-staining nuclei, 

 absence of glycogen, and active synthesis of bone matrix (cf. Young, 

 1962/;, 1963a). A surface layer of osteoid was generally discernible 

 in such regions (Fig. 14). Osteogenic surfaces were inevitably coated 

 with osteoblasts, and were characterized by a granular appearance 

 of the matrix in preparations stained bv Bodian's silver technique 

 (Fig. 18). Within this new matrix, the walls of osteocyte lacunae 

 and canaliculi were not stained. Slightlv farther from the surface, 

 and within all other regions of the bone, they were clearly delineated 

 (Figs, land 18). 



In animals given large doses of parathyroid extract beginning 24 

 hours before sacrifice, no osteogenic surfaces, osteoid, or newly 

 formed osteocytes were detected, owing to the rapid cessation of 

 bone formation. During this process, osteoblasts became depleted of 

 RNA, lost their ability to synthesize bone matrix (cf. Vacs and 

 Nichols, 1962), and reverted to the osteoprogenitor state (Young, 

 1963fl). Osteoprogenitor cells were characterized by a propensity 

 to fuse in resorptive areas, forming increased numbers of osteoclasts, 

 and by regionally accentuated cell proliferation, which led to a sig- 

 nificant thickening of the endosteum and of the cellular lining of 

 the diploic spaces. This proliferative response was associated with 

 the production of a network of argyrophilic fibers (cf. Kroon, 1958). 



Discussion 



The techniques used in this study have made it possible to dis- 

 tinguish a large number of variable features within the mineralized 



