37G F. C. MCLEAN AND R. E. ROWLAND 



mechanism, except that it appears to be identical with osteoclastic 

 resorption in the removal of bone throughout the skeleton. Here 

 the situation seems to be analogous to that in rock boring; in both 

 instances there is speculation concerning the chemical factors in 

 dissolution of calcified tissues; in both cases acid formation and 

 chelation are given consideration as possible mechanisms; in neither 

 have the factors participating in destruction of the hard tissue been 

 elucidated. Rock boring by biologic organisms and tunnel formation 

 bv osteoclastic resorption are sufficiently similar in their nature and 

 their manifestations to suggest a common pathway in the evolution 

 of hard tissues. 



In connection with the comparative biology of calcified tissues, 

 Amprino and Godina ( 1953 ) reported that remodeling of compact 

 bone in the teleost Thijnniis thynnus L. is initiated by the formation 

 of tunnels by osteoclastic resorption. On the other hand, Norris 

 et al. (1963), studying the sea bass Epinephelus striatiis (Bloch), 

 observed resorption apparently in the absence of osteoclasts; primary 

 bone was penetrated by blood vessels, forming tunnels. Acellular 

 bone, arranged circumferentially around thin-walled blood vessels, 

 was then laid down by osteoblastic activity, a thin sheet of osteoid 

 appearing beneath a single layer of osteoblasts. The diameter of 

 the lumen of the acellular osteons varied from 28 to 41 microns. 



There remains the important problem of physiologic regulation. 

 At least two regulatory processes are involved: (1) regulation of 

 initiation and formation of resorption cavities or tunnels; and (2) 

 regulation of the transfer of calcium in such a manner as to maintain 

 the constancy of the Ca++ concentration in the blood, in spite of 

 the rapid turnover of the plasma calcium. 



Little is known concerning the regulatory control of the formation 

 of resorption cavities. Jowsey et al. (1958) have studied the re- 

 modeling of compact bone in parathyroidectomized dogs, and have 

 found that tunnel formation continues after removal of the para- 

 thyroid glands, although at a somewhat reduced rate; in any event 

 a continuing supply of reactive bone is maintained, adequate to 

 support life, and osteoclastic resorption continues. Since osteoclastic 

 resorption, incident to the growth of bone, also continues in the al^- 

 sence of the parathyroids, it appears that osteoclastic activity is not 



