An osteoclast (from the Greek wors for "bone" and "broken") is a type of bone cell that removes bone tissue by removing its mineralized matrix. This process is known as bone resorption. Osteoclasts and osteoblasts are instrumental in controlling the amount of bone tissue: osteoblasts form bone, osteoclasts resorb bone. Osteoclasts are formed by the fusion of cells of the monocyte-macrophage cell line. Osteoclasts are characterized by high expression of tartrate resistant acid phosphatase (TRAP) and cathepsin K.