New bone cells are mainly replaced by specialized bone cells called osteoclasts and osteoblasts , with help from osteocytes that coordinate the whole process. Osteoclasts remove old or damaged bone, and osteoblasts fill in that space by building fresh bone tissue.

Key bone cells

  • Osteoclasts
    • Large cells that break down and “eat” old or damaged bone (bone resorption).
    • Their work clears space so new bone can be laid down.
  • Osteoblasts
    • Builder cells that produce new bone matrix and minerals, creating new bone tissue.
    • They are directly responsible for forming the new bone that replaces what osteoclasts removed.
  • Osteocytes
    • Former osteoblasts that became embedded in the bone they built.
    • They act like sensors, detecting stress or damage and signaling osteoclasts and osteoblasts to remodel bone.

How replacement actually happens

  • Bone is constantly remodeled in small units where osteoclasts first remove old bone, and osteoblasts then refill that area with new bone; this cycle runs throughout life.
  • Hormones (like estrogen and parathyroid hormone) and mechanical forces (exercise, weight-bearing) strongly influence how active these cells are, affecting how fast bone is broken down and rebuilt.

TL;DR:
The body replaces old bone cells through a coordinated team: osteoclasts remove old bone, osteoblasts build new bone, and osteocytes manage and signal when and where this should happen.

Information gathered from public forums or data available on the internet and portrayed here.