How Do Bone Cells Secrete Proteins

Osteoclasts (OCs), which are the exclusive bone resorbing cells, degrade skeletal matrix by forming an intimate relationship with the bone surface. Thus, when OCs attach to bone, they produce an actin-rich sealing zone representing a gasket-like structure, which isolates the resorptive milieu from the general extracellular space. This “resorptive microenvironment” contains a ruffled border, the unique bone-degrading organelle of the OC, which consists of a complex, villous-like organization of the plasma membrane. This structure appears only in resorbing cells and is the product of signals derived from the bone matrix. These signals polarize as yet undefined acidified vesicles containing the OC vacuolar H+ATPase towards the bone-apposed plasma membrane, into which they insert, thereby increasing its complexity. The ruffled border is thus the most definitive marker of the resorbing osteoclast.