They are regulated by parathyroid hormone, which activates them, and calcitonin and OPG, which repress them.
FLG 29.1 is an immortalized line of precursors of this type of cell.  These cells lie in a hollow cavity called the Howship's lacuna and have a characteristic homogenous cytoplasm with a foamy appearance. They express the cysteine protease cathepsin K, which catabolizes elastin, collagen, and gelatin. They are regulated by parathyroid hormone, which activates them, and calcitonin and OPG, which repress them. They are also stimulated by interleukin-6, a protein that is overexpressed in osteoporosis. Their ruffled border contains a proton pump that plays a key role in these cells' dissolution of hydroxyapatite. For 10 points, what is this type of cell that is responsible for releasing calcium from bones into the blood, a process known as bone resorption?
FLG 29.1 is an immortalized line of precursors of this type of cell.  
osteoclasts