BONE CELLS AND BONE RESORPTION 485 



shortening the trabeculae (Fig. 19). Furthermore, resorption ex- 

 tended into the mid-trabecular region, normally characterized by 

 osseous apposition. In extreme cases, the metaphyseal trabeculae 

 were entirely resorbed. Similarly, an acceleration of subperiosteal 

 resorption in the metaphyseal region of both ribs and tibiae resulted 

 in the thinning of bone at this site. Not infrequently, the shaft in 

 this region was perforated or fractured (Fig. 19). 



In the skull, resorptive sites were related to the age of the animal, 

 as in controls. In young animals (less than 10 days old), an increase 

 in resorption (and in osteoclasts, stained supravitally with neutral 

 red) was most marked endocranially, particularly near the bone 

 margins. Ectocranial resorption near the bone centers also appeared 

 to be increased. In older rats, in which development of the diploe 

 had been initiated, the diploic spaces were noticeably widened in 

 hyperparathyroid animals (Figs. 10 and 22). Surface resorption con- 

 tinued to predominate endocranially (Figs. 20 and 21), but was less 

 prominent than in younger animals. 



State of Matrix in Sites of Preferential Resorption 

 in Rats Treated with Parathyroid Extract 



In hyperparathyroid rats, as in normal animals, there was no re- 

 lation between sites of selective resorption and anv of the several 

 variable features of the calcified matrices which were studied. This 

 finding was typified by the prominent resorption within the diploe 

 of hyperparathyroid rats more than 10 days old. The accelerated 

 removal of bone was characterized by a progressive enlargement of 

 the diploic spaces, without regard to whether the involved matrix 

 was a "younger" region of lamellar bone, or an older region of 

 woven bone with slightly less organic mass, slightly more baso- 

 philic material, appreciably more irregularly arranged collagen, 

 slightly greater impregnation with mineral salts, and larger, more 

 closely packed and irregularly arranged osteocytes (Figs. 10 and 

 22). In some of the older rats, woven bone which was resorbed in 

 this process had been formed several weeks earlier (Fig. 22), 

 whereas that simultaneously removed from the metaphyseal spon- 

 giosa was only a few hours old. 



