Site of action of drugs in the treatment of osteoporosis. View of bone trabeculae of spongy bone (blue). Osteoclasts (yellow) destroy the bone, while osteoblasts (roses) make bone. On the left is the osteoclast cycle with, from top to bottom, an osteoclast precursor with RANK receptors, a mature osteoclast destroying the bone, an osteoblast in apoptosis. On the right a pre-osteoblast. A mature osteoblast makes RANK-L that will bind to the pre-osteoclast RANK receptor. Estrogen binds to the estrogen receptor of osteoblasts to inhibit osteoclast activity. Top right, a blood vessel and a bowel in sections.