Bone resorption by osteoclasts
Parathyroid hormone (PTH) binds to receptors on osteoblasts, causing them to form receptor activator for nuclear factor κ-B ligand (RANKL) and to release macrophage-colony stimulating factor (M-CSF). RANKL binds to RANK and M-CSF binds to its receptors on preosteoclast cells, causing them to differentiate into mature osteoclasts. PTH also decreases production of osteoprotegerin (OPG), which inhibits differentiation of preosteoclasts into mature osteoclasts by binding to RANKL and preventing it from interacting with its receptor on preosteoclasts. The mature osteoclasts develop a ruffled border and release enzymes from lysosomes, as well as acids that promote bone resorption. Osteocytes are osteoblasts that have become encased in bone matrix during bone tissue production; the osteocytes form a system of interconnected cells that spreads all th