484 R. W. YOUNG 



State of Matrix in Sites of Preferential Resorption 

 in Normal Rats 



No relationship was apparent between sites of preferential resorp- 

 tion and the age, degree of calcification, amount of organic mass, 

 histological organization, or chemical variations (differential stain- 

 ing) of the calcified matrices of bone and cartilage. For example, 

 although no difterences could be detected among the calcified car- 

 tilage intercellular partitions in regard to these variables, some of 

 these partitions are resorbed in the zone of cartilage invasion, while 

 others are spared. At the distal extremities of the metaphvseal tra- 

 becule, resorption simultaneously removes the calcified cartilage 

 cores and the thin osseous coating, despite difterences in the age 

 ( the cartilage is slightly older than the bone ) , histological organiza- 

 tion, organic mass content, and relative amounts of matrical con- 

 stituents (Figs. 3, 8, and 9).* Similarly, in the skull, resorption of 

 the walls of the diploic spaces involves removal of both woven and 

 lamellar bone, without regard to their age ( the woven bone usually 

 is older: Young, 1962i>) or any of the other variables studied, in- 

 cluding degree of calcification (Figs. 4, 5, 11, and 13). 



Sites of Preferential Resorption in Rats Treated 

 with Parathyroid Extract 



Bone resorption was markedly accelerated in all animals which 

 received parathyroid extract, as indicated by the regional disappear- 

 ance of calcified matrices in PTE-injected animals, compared with 

 untreated controls. The greatest resorption occurred in regions which 

 in uninjected animals could be identified as sites of normal resorp- 

 tion. In addition, resorption tended to spread from normally resorp- 

 tive zones into adjacent areas. 



For example, an increase of resorption in the zone of cartilage 

 invasion resulted in the occasional partial or apparently complete 

 detachment of metaphyseal trabeculae from the calcified intercel- 

 lular partitions of the growth cartilage. At the distal end of the 

 metaphyseal spongiosa, resorption was also accelerated, noticeably 



* In some species significant differences in calcification occur as well ( Owen et ah, 

 1955). 



