Acid Hydrolases, Lysosomes and Bone Resorption Induced 59 



digestive action of the lysosomal hydrolases, simultaneously creating a suitable 

 environment for the action of these enzymes. This environment could be limited to 

 the small area underlying the excretory pole of the cells active in resorption, since 

 the excreted acid will be rapidly buffered by the bone mineral itself when it is 

 diffusing away from the resorption lacunae. Such a buffering would limit consider- 

 ably the extension of the resorption process and confine it to minute areas under 

 direct control of the cells. 



This extracellular function of the lysosomal hydrolases in the digestion of bone 

 matrix could possibly be completed by an intracellular function in the digestion of 

 fragments of matrix taken up in the cells by pinocytosis. Lysosomes are indeed 

 known to be involved in the intracellular digestion of exogenous material engulfed 

 by phagocytosis or pinocytosis in other cell types. Evidence of active pinocytosis in 

 osteoclasts has been shown repeatedly by others. 



Acknowledgements 



Supported by the Fonds de la Recherche Scientifique Fondamentale Collective and 

 the U.S. Public Health Service Research Grant n" MG-08705. The author is Cher- 

 cheur Qualifie du F.N.R.S. He is indebted to Professor C. de Duve for his critical 

 suggestions and for his help in the preparation of the manuscript. 



The tissue culture work was done at the laboratory of Cell Biology and Histology 

 of the University of Leiden thanks to the courtesy of Professor P. J. Gaillard and to 

 a travelling fellowship of the Fondatlon Unlversitalre. Useful advices for the tissue 

 culture technique from Professor P. J. Gaillard and Dr. "W. A. de Voogd van der 

 Straaten are gratefully acknowledged. 



References 



More details and complete bibliography may be found in the following papers: 



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 Vaes, G.: Hydrolytic enzymes and lysosomes in bone cells. In Calcified Tissues. Richelle, 



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