BONE CELLS AND BONE RESORPTION 489 



is also indicated by the finding that fluctuations in calcium levels 

 in tissue fluids may occur within minutes after changes in para- 

 thyroid secretion in nephrectomized animals ( Talmage, 1956, 1962 ) . 

 This suggests an immediate effect on the rate of resorption already 

 in progress. 



Since it is becoming increasingly apparent that the several types 

 of bone cells represent different functional states of the same cell 

 (Bloom etal, 1958; Young, 1962^, l96Sa, 1963&), and, further, that 

 these cellular specializations are reversible, it is evident that all bone 

 cells contain the genetic potential for bone resorption. In which of 

 these functional specializations is this potential realized? 



There appears no longer to be any doubt that osteoclasts are capa- 

 ble of resorbing bone and calcified cartilage (as well as dentin, 

 cementum, and enamel: Sognnaes, 1959). Some reports claim, how- 

 ever, that osteoclasts may appear after resorption has taken place 

 (Burrows, 1938; Gaillard, 1955<7, 1955t>), or may not appear at all, 

 despite recognizable resorption (Engel, 1952; Storey, 1957; Urist 

 and Deutsch, 1960). Apparently, groups of bone cells in which the 

 resorptive response has been activated tend to coalesce, creating a 

 multinucleated cell, yet need not necessarily do so in order to carry 

 out this function. The ability of osteoclasts to phagocytize whole 

 cells (indicated by the presence of nucleated inclusions within 

 cytoplasmic vacuoles: cf. Arey, 1917; Jaffe, 1933) has been noted. 

 This suggests the presence of a variety of lytic enzymes in these 

 cells. 



Many workers have brought forward evidence which suggests 

 that osteocytes may be capable of limited resorption of their lacunar 

 walls (Jaffe, 1933; Kind, 1951; Heller-Steinberg, 1951; Lipp, 1956; 

 Kroon, 1958; Frost et ah, 1960). In the present material, however, 

 no consistent alteration in osteocytes or their surroundings suggesting 

 a significant activation of the resorptive potential was observed, 

 even following massive doses of parathyroid extract. The loss of 

 glycogen by osteocytes under such conditions (Heller-Steinberg, 

 1951; Laskin and Engel, 1956) probably represents a direct response 

 to the hormone, since no glycogen depletion occurred in chondro- 

 cytes. 



It has occasionally been stated (e.g., Follis, 1952) that osteoblasts 



