Polarity and membrane transport in osteoclasts.

The osteoclast is a highly polarized non-epithelial cell. The apical pole of the cell is determined by the cell's attachment to the extracellular matrix. This attachment forms the sealing zone, delimiting the subosteoclastic bone resorbing compartment. The apical membrane of the cell forms the ruffled-border, which contains some specific membrane proteins and a proton pump ATPase, which acidifies the apical compartment. Newly synthesized lysosomal enzymes are vectorially transported into this apical compartment bound to mannose-6-phosphate receptors. The basolateral membrane is highly enriched in sodium pumps with beta and alpha 1 subunits. Associated with the acidification process is the carbonic anhydrase found in the cytoplasm and membrane-associated and a bicarbonate-chloride exchanger in the membrane.2 These features put the osteoclast in the same functional category as the kidney tubule intercalated cell and the gastric oxyntic cell, both of epithelial origin, which secrete acid in a polarized fashion.