The Two Faces of Resorption 



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amount of calcium (about 15 gm/day/horse) and an excessive amount of phosphorus 

 (about, 3V2 times as much). 



The animals were killed after periods of 4, 7, 10, 12 and 30 weeks of abnormal 

 feeding. The clinical observations have been similar to those already published by 

 Krook and Lowe (1964). After 4 weeks, very minor signs of osteitis fibrosa were 

 present but the parathyroids were enlarged about 3 times. After 8 weeks, extremely 

 severe osteitis fibrosa was recognized and in the colourful language of the senior 

 veterinary pathologist (K), "the parathyroids were almost as big as the testicles of a 

 dachshund". 



The bone response was uneven throughout the body. On the other hand, the 

 horses which were the youngest at the onset of the experiment were the most severely 

 affected. Changes in the mandibles and maxillae were observed sooner and progressed 

 at a more rapid rate than those in the metacarpi (Krook and Lowe, 1964) and long 

 bones. Increased radiolucency was particularly evident in the bones of the skull and 

 face. Complete resorption of the lamina dura dentis was evident after 30 weeks. 



Comparative alpharadiographs of the periodontal portion of the mandible and 

 maxilla examined in the light of histochemical staining and with the knowledge of 

 the mechanism of bone growth pro- 

 vided by radioautography (Belan- 

 GER and MiGicovsKY, 1963 b) have 

 been very instructive. 



Short term studies have demon- 

 strated that only mesenchymal 

 cells, preosteoblasts or "osteopro- 

 genitors" (Tonna, 1965; Belanger 

 and MiGicovsKY, 1963 b; Young 

 1963) are originally labeled. The 

 presence of radioactive osteoblasts 

 and osteocytes occurring later is an 

 indication of growth movements 

 and also of tissue replacement in 

 areas where the adult condition has 

 already been achieved. 



In 3 weeks-old chicks, the re- 

 placement rate of metaphyseal 

 trabeculae of the tibia has been 

 established at 4 days (Belanger 

 and MiGicovsKY, 1963 b). In the 

 lamina dura of the horse jaw, this 

 time is unknown. However, a con- 

 stant cell movement must be simi- 

 larly taking plase from the border 

 of the osteonic canal, towards the 



areas of lower tissue density where osteolysis occurs and where the cells die. If osteo- 

 lysis and peripheral accretion are balanced, a static histological, histochemical and 

 radiological picture is maintained. On the other hand, if osteolytic stimuli such as 

 hyperparathyroidism are present, resorption Is presumably greater than accretion. 



