BONE CELLS AND BONE RESORPTION 479 



acterized by a slightly reduced organic mass content (Fig. 5), and 

 by a slightly increased degree of calcification (Figs. 13 and 20). 



Sites of Preferential Resorption in Normal Rats 



Sites of resorption associated with growth remodeling vary among 

 different bones, and in the same bone at different stages during 

 maturation. However, at the costo-chondral junction of ribs, and in 

 the metaphyseal region of tibiae, resorption not only is particularly 

 rapid, but occurs in a characteristic and similar manner, as judged 

 bv the presence of osteoclasts, by the uptake of glycine-H'' (which 

 is minimal in resorptive areas: Young, 1962Z?, 196-3fl), and by the 

 progressive removal of labeled matrix (formed elsewhere) in these 

 resorptive zones (Figs. 8 and 9). In both ribs and tibiae of these 



Fig. 6. Tangential section through the shaft of a rib from an 11-day-old 

 rat injected with 100 units of parathyroid extract 1 day prior to sacrifice. 

 Scattered segments of calcified cartilage matrix are present. Hematoxylin and 

 eosin. (X 320.) 



Fig. 7. Ultrasoft x-rav microradiogram of the region shown in Fig. 6. The 

 cartilaginous remnants are readily apparent, owing to their lower organic mass 

 content as compared with that of the surrounding bone matrix. The prominent 

 nucleoli within the osteoclast nuclei (top right) are characterized by high 

 organic mass content. Arrows indicate two osteoclasts ( or perhaps two segments 

 of a single osteoclast) engaged in the simultaneous resorption of matrices of 

 differing dry mass. ( X 320. ) 



Fig. 8. Tibial metaphysis of a 6-day-old rat sacrificed 2 hours after in- 

 jection of glycine-H'^. The radioactive material, incorporated in large amounts 

 l3y osteoblasts engaged in matrix formation, is concentrated in the zone of 

 bone apposition (a), and is sparse in the zones of cartilage invasion (i) and 

 trabecular resorption (r). Calcified cartilage matrix (arrow) remains un- 

 labeled. Autoradiogram, PAS-hematoxylin. (X 200.) 



Fig. 9. Zone of trabecular resorption in the tibial metaphysis of a 7-day- 

 old rat sacrificed 32 hours after injection of glycine-H''. Heavily reactive bone 

 matrix, formed in the zone of bone apposition in the first 4 hours after injection, 

 is now restricted to the distal extremities of the longest trabeculae (lower 

 left). Resorption has removed the major part of the metaphyseal spongiosa 

 which was present 32 hours previously. Autoradiogram, PAS-hematoxylin. ( X 

 320.) 



Fig. 10. Soft x-ray microradiogram of a segment of parietal bone from a 

 23-day-old rat injected with 150 units of PTE 1 day prior to sacrifice. The 

 diploic spaces have been markedly enlarged by resorption, which has pro- 

 ceeded without regard to regional variations in the degree of calcification of 

 the matrix. (X 80.) 



