14 



Internal Remodeling of Compact Bone 



FRANKLIN C. McLEAN. Department of I'hysiology. I'niversity of 

 Chicago, Chicago, Illinois 



ROBERT E. ROWLAND,* Radiological Physics Division, Aigonne Na- 

 tional Laboratory, Argonne, Illinois 



BONES increase in length by the functioning of the growth appara- 

 tus, inchiding the epiphyseal cartilage plate and the metaphysis of 

 the long bones. They increase in diameter bv apposition of new 

 periosteal bone, while the marrow cavity is being enlarged by re- 

 sorption at the endosteal surface. In addition to the changes in size 

 and shape of the bones, as a result of the remodeling incident to 

 growth, there is continuous internal remodeling throughout the life 

 of the individual; this serves an important physiologic function, 

 essential to homeostatic control of the calcium level in the blood 

 plasma, and therefore to life itself. 



In compact bone there is first the formation of absorption cavities, 

 described by Tomes and De Morgan in 1853. The appearance of 

 these cavities is associated with the presence of osteoclasts, and the 

 cavities are extended until they assume the form of tunnels. Because 

 of certain similarities to rock boring by biologic organisms, as de- 

 scribed elsewhere in this volume (chapters 1, 2, and 3), and be- 

 cause of the possible significance of absorption or resorption cavities 

 in the evolution of the skeleton, it seems desirable to treat the sub- 



° Present address: Department of Radiation Biology, Sehool of Medicine and 

 Dentistry, University of Rochester, Rochester, New York. 



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