480 R. W. YOUNG 



The Cellular Component in Resorption 



The presence of osteoclasts was a characteristic, but not inevita- 

 ble, accompaniment to demonstrable resorption. In some cases, 

 osteoprogenitor cells (but never osteoblasts) were present in such 

 regions. There was no evidence of decalcification (in agreement 

 with Sognnaes, 1959), changes in organic mass (in contrast to 

 Greulich, 1961; Takuma, 1962), or modifications in staining proper- 

 ties of the organic matrix beneath osteoclasts. Occasionally, bone 

 surfaces with increased affinitv for the PAS reagent were observed 

 in animals treated with parathvroid extract (Heller-Steinberg, 1951; 

 Gaillard, 1955^, 19555; Laskin and Engel, 1956; cf. Kroon, 1958), 

 but this could not be correlated with demonstrable resorption ( Fig. 

 22). Frequentlv, a single osteoclast could be observed apparently 

 engaged in resorbing a segment of matrix containing components of 

 varying age, degree of calcification, histological organization, etc. 

 (Figs. 3, 7, 9, 11, and 21). No cvtoplasmic ingestion of calcium salts 

 by osteoclasts was detected, nor was glvcine-H^ concentrated by 

 these cells, even when thev were present on hea\'ih' labeled bone 

 surfaces (cf. Arnold and Jee, 1957). 



Osteoclasts were generallv characterized by large, pale-staining 

 nuclei, prominent, RNA-containing (pyroninophilic, RNase-digesti- 

 ble) nucleoli of high organic mass (Fig. 7), and variable but 

 appreciable cytoplasmic RNA, In normal animals ( seldom in hvper- 

 parathyroid rats ) some osteoclasts were seen with scanty cytoplasm 

 and small, dark-staining nuclei, with insignificant nucleoli. Meta- 

 chromatic (toluidine blue) granules and numerous small, PAS-posi- 

 tive granules (retained after incubation in saliva) were observed 

 within the cytoplasm of osteoclasts. Nucleated inclusions, contained 

 within cytoplasmic vacuoles, were a prominent feature of osteoclasts 

 in hyperparathyroid animals, but were rare in controls. The included 

 nuclei generally had little associated cytoplasm, and were small, 

 dark-staining or pyknotic, of high organic mass, and characteristically 

 lobulated (Figs. 15 to 17). 



There is considerable variability in the size, shape, and arrange- 

 ment of osteocytes in normal bone (Figs. 1, 2, 11, and 18). In regions 

 of resorption, no unusual modifications in osteocytes or their sur- 



