VI - PREFACE 



have the biochemical capacity to digest the organic conchioHn 

 shell matrix. 



Vertebrate hard tissues prone to biological destruction are not 

 entirely uniform with regard to the nature of their organic scaf- 

 folding and inorganic building blocks. Presumably there may also 

 be different molecular bonds which bridge the two, i.e., the organic- 

 inorganic linkage which renders the biological whole — be it shell, 

 pearl, ivory, or bone — something far more complex ( as well as more 

 beautiful) than a simple summation of the chemical parts. The com- 

 plexity of analyzing this problem has been illustrated h\ a number 

 of biological systems described in this volume. 



All together at least half a dozen destructive influences appear 

 to be at work: acid demineralization, chelation, enzymatic diges- 

 tion, proteolysis, molecular bond disruption of the organic-inorganic 

 linkage, cellular ingestion, possibly phagocytosis, physical motion, 

 and mechanical friction. A combination of two or more if not all 

 of these mechanisms may well exert their influence at some stage of 

 destruction within one and the same biological system. The more 

 readily understood physical forces are at work in the case of large 

 multicellular organisms, e.g., the twisting motion of rock-boring 

 bivalves and the drilling action of the snail rasping holes in a shell 

 region partially softened by decalcification. Yet anyone who has 

 observed the cinematographic recordings of the lively process of 

 experimental bone resorption in tissue culture will have a vivid 

 impression that the osteoclast — aside from its complex biochemical 

 apparatus — does in fact move around in a slow-motion "twist," rub- 

 bing its pseudopodia along the presumably softened walls of an 

 eroding Howship's lacuna. 



Though the discussion is primarily focused on the destructive as- 

 pects of hard tissue biology, it is noteworthy that a variety of sys- 

 tems in fact exhibit closely related constructive (biopositive) and 

 reparative phenomena; in other words, that we are dealing with 

 interrelated three-way processes: formation, destruction, and re- 

 formation at the cytological level; mineralization, demineralization, 

 and remineralization at the molecular level; in short, cellular and 

 chemical remodeling. 



In coral reef remodeling the extensive "erosion" of the dying 



