568 



G. NICHOLS, JR. 



- E C F p H 7.4 



-CELu LAYER 



INNER FLUID 

 LAYER pH6.8? 



CALCIFIED 

 COLLAGEN 



Fig. 7. A schematic view of the "interface" between the calcified col- 

 lagenous matrix of bone and the circulating extracellular fluids (ECF) (see 

 text). In the in vitro svstem described here, the incubation medium substitutes 

 for the extracellular fluid. As conceived at present, movement of ions between 

 calcified matrix and extracellular fluids is driven by diffusion along concentra- 

 tion gradients only. Such ion movement (and exchange) occurs, not directly 

 between calcified matrix and extracellular fluid, but rather via the "inner fluid 

 layer"; hence, a three-compartment system. The "inner fluid layer" is thought 

 to be of very small relative volume and indeed may be partly or totally filled 

 with an extension of the fibrillar and amorphous matrix in which the cells are 

 embedded. Despite the ability of ions to diffuse rapidly between this inner 

 layer and the extracellular fluids, some considerable diff^erences in its ionic 

 composition could be maintained under steady-state conditions thanks to its 

 very small volume in relation to cellular metabolic activitv and the high change 

 density present in close proximity to the mineral crystals. One possible dif- 

 ference — in pH — is indicated in the diagram with a question mark. Some 

 evidence in favor of this idea is shown in Fig. 8. 



