Bone Growth, Remodeling, Repair, and Homeostasis

Bone Growth in Width

Appositional growth increases the width or diameter of a bone through the deposition of new bone lamellae onto existing bone or connective tissue. Osteoblasts deposit new bone matrix on the surface of bones between the periosteum and the existing bone matrix.

Bone Growth in Length

Growth in the length of a bone is the major source of increased height in an individual and occurs in the epiphyseal plate through endochondral ossification.

  • Epiphyseal Side: Chondrocytes divide, form stacks, and increase in number.
  • Diaphyseal Side: Chondrocytes enlarge and die, and the cartilage matrix becomes calcified. Osteoclasts remove much of the cartilage surrounding the enlarged cells, and osteoblasts replace the dying chondrocytes by depositing bone lamellae on the surface of the calcified cartilage.
  • Overall Process: Bone is continuously added to the diaphysis while the overall thickness of the epiphyseal plate remains unchanged.

Endochondral Bone Growth Diagram

Endochondral Ossification

  • A cartilage model surrounded by perichondrium provides the general shape of mature bone.
  • Chondrocytes enlarge and cartilage calcifies, causing the perichondrium of the diaphysis to become periosteum.
  • Primary Ossification Center: Blood vessels and osteoblasts invade calcified cartilage, where osteoblasts lay down bone matrix to form trabeculae.
  • Secondary Ossification Centers: Form in the epiphyses of long bones.

Endochondral Ossification Stages

Bone Remodeling

Bone remodeling occurs in all bones and involves two key actions:

  • Removal of existing bone by osteoclasts.
  • Deposition of new bone by osteoblasts.

This process is responsible for changes in bone shape, bone repair, adjustment of bone to stress, and calcium ion regulation.

Bone Repair

Bone repair occurs through four sequential steps:

  1. Hematoma Formation: A broken bone causes bleeding, leading to the formation of a blood clot (hematoma).
  2. Callus Formation: An internal callus forms between the ends of the bone fragments, and an external callus forms a collar around the break, creating a fibrous network of cartilage and fibers.
  3. Callus Ossification: Osteoblasts enter the callus to form cancellous bone. Woven, spongy bone replaces the internal and external calluses over a period of 4 to 6 weeks after injury.
  4. Bone Remodeling: Compact bone replaces woven bone, and part of the internal callus is removed to restore the medullary cavity.

Bone Repair Stages

Bone and Calcium Homeostasis

  • Bone serves as a major storage site for calcium.
  • The movement of calcium into and out of bone helps determine blood levels of calcium.
  • Calcium moves into bone as osteoblasts build new bone matrix.