The Two Faces of Resorption 7 



Osteoclasts which had been sparsely observed at 4 and 7 weeks, were more 



numerous at 12 weeks. They attained almost tumoral frequency at 30 weeks and 

 appeared most active along the abnormal new trabeculae. 





5. The role of osteoclasts 



These observations bring us to consider the role of osteoclasts and the factors 

 which are responsible for the occurrence of these cells. 



In an excellent review, Hancox (1956) lists a series of generalized and localized 

 factors as responsible for the appearance of osteoclasts. In the first group, hyperpara- 

 thyroidism, hypervitaminosis A, the administration of glucose or lead and a diet low 

 in calcium have been reported. "Remodeling", bone fractures, ectopic bones or teeth, 

 degenerated deciduous roots, circulatory disorders, infections, introduction of inert 

 material in the root canals of teeth, have been recognized as so many local factors. 



The introduction of Plutonium (Arnold and Jee, 1957) or Yttrium (Neuman 

 et ai, 1960; Jowsey, 1963) into the organism, has produced beautiful demonstrations 

 of surface resorption in the presence of osteoclasts. 



Grafts such as performed by Barnicot (1948) and also in vitro transplantation 

 and culture of bone specimens (Gaillard, 1955 a, b; Goldhaber, 1962) have been 

 followed by stimulation of osteoclastic activity. 



Irving and Handelman (1963) have recently published interesting results with 

 devitalized autologous bone grafts. A multinucleated giant cell reaction was initiated 

 from mesenchymal cells whether the grafted bones were mineralized or demineralized. 

 The host response did not seem to be affected by parathormone. 



All these events and also the results of our current observations on horses seem to 

 have something in common: The presence of abnormal skeletal tissues either bone, 

 cartilage or the components of the teeth. 



