BONE CELLS AND BONE RESORPTION 481 



young rats, there is continued removal of some ( but not all ) of the 

 calcified cartilaginous intercellular partitions in the zone of cartilage 

 invasion at the distal edge of the growth cartilage. (In the central 

 regions of the metaphyseal trabeculae, in the zone of bone apposi- 

 tion, a thin coating of bone is laid down on those cartilage partitions 

 which escape resorption. ) Similarly, in both bones resorption occurs 

 at the distal extremities of the mixed ( bone and calcified cartilage ) 

 metaphyseal trabeculae in the zone of trabecular resorption, keeping 

 pace with lengthening of the trabeculae, which occurs through vas- 

 cular invasion of the growth cartilage (Figs. 8 and 9). On the 

 periosteal surface of the metaphyseal regions of these bones, there 

 is a characteristic region of resorption associated with maintaining 

 the "funnel" shape of the tibia, and the cuwature of the ribs (Fig. 

 3). 



In the crania of these rats (e.g., the parietal bones) resorption is 

 observed endocranially, just within the peripheral bone margin, and 

 to a limited extent ectocranially, in the central region of the in- 

 dividual bones. These findings, largely derived from glycine-H^- 

 injected rats ( cf . Young, 1962Z? ) , are consistent with the distribution 



•Fig. 11. Parietal bone of a 20-day-olcl rat, illustrating formation of diploic 

 spaces by resorption in a region where a segment of woven bone lies midway 

 between the inner and outer tables. The osteoclast on the right is engaged in 

 simultaneous resorption of woven and lamellar bone. Hematoxylin and eosin. 

 (X 320.) 



Fig. 12. The same region shown in Fig. 11, photographed in polarized 

 light. The more irregular arrangement of collagen fibers in woven bone, as 

 compared with lamellar bone, is illustrated by the reduced refractility of the 

 former. (X 320.) 



Fig. 13. Soft x-ray microradiogram of a segment of parietal bone from a 

 23- day-old rat. A thin remnant of woven bone (arrow), characterized by a 

 more irregular disposition of osteocyte lacunae, is slightly more heavily miner- 

 alized than the surrounding lamellar bone. Boundaries of the diploic space 

 are unrelated to these minor diflFerences in the degree of calcification. ( X 200. ) 



Fig. 14. Phase contrast photomicrograph of the central portion of the 

 section from which the preceding microradiogram was made. A layer of osteoid 

 is present on the upper surface of the bone. The osteoid layer is coated by 

 osteoblasts, and is thick enough to contain an osteocyte (arrow). Note that 

 osteocyte canaliculi are visible within the bone, but not in the osteoid. ( X 320. ) 



Figs. 15 to 17. Four cells, apparently phagocytized, are shown within 

 cytoplasmic vacuoles (arrows) in osteoclasts. Chrome hematoxylin-phloxine. 

 (X800.) 



