Final answer:
Bone resorption requires the activity of osteoclasts, which break down old bone while osteoblasts form new bone.
Step-by-step explanation:
The cell responsible for bone resorption, or breakdown, is the osteoclast. Osteoclasts are multinucleated cells that break down old bone while osteoblasts continually form new bone. This ongoing balance between osteoclasts and osteoblasts is responsible for the constant reshaping of bone.