Bone Formation and Remodelling

Bone Formation

• Osteoblasts are responsible for the formation of new bone matrix.
• Osteocytes are mature bone cells, formed from osteoblasts, that reside within lacunae and maintain bone tissue.
• Osteoclasts are multinucleated cells that resorb bone.
• Bone formation occurs through two processes:
  • Endochondral ossification: Cartilage is replaced by bone during embryonic development.
  • Intramembranous ossification: Bone forms directly from mesenchymal tissue.

Bone Modelling

• The characteristic microstructure of bone is created by the coordinated action of osteoblasts and osteoclasts.
• This process is known as modelling.

Bone Remodelling

• After the bone has reached maturity, osteoclast and osteoblast activity continually maintains the bone, removing micro-cracks and damage. This process is called remodelling.
• Basic multicellular units (BMUs) are responsible for remodelling, consisting of a group of osteoclasts and osteoblasts that work together.
• In cortical bone, a BMU forms a cylindrical canal called an osteon, where:
  • Cutting cone: Osteoclasts dig a circular tunnel.
  • Closing cone: Osteoblasts fill the tunnel with new bone.
• In cancellous bone, the same cellular processes occur, but the bone is laid down in pancake-like packets.

Bone Tissue

• The adult human body has 206 bones, although this varies by individual.
• Bones are light and strong, weighing no more than 9kg and capable of withstanding a ton of compression.
• Bones have several functions:
  • Provide shape and support
  • Protect organs
  • Produce blood cells
  • Store fat
  • Store and release minerals
  • Produce hormones (e.g., osteocalcin).
• Bones are composite materials, consisting of an organic component (mainly collagen) and a mineral component (hydroxyapatite).

Basic Structure of Bone

• Long bones have distinct regions:
  • Epiphyses: The ends of the bone
  • Metaphysis: The region where the epiphysis meets the diaphysis
  • Diaphysis: The shaft of the bone
  • Articular cartilage: Covers the ends of the bone
  • Marrow cavity: Contains red marrow (blood cells) and yellow marrow (fat).
• There are two main types of bone:
  • Cancellous Bone: Spongy and trabecular, with a lower density and greater elasticity than cortical bone.
  • Cortical Bone: Dense and compact, with a strong and rigid structure.

Cortical Bone

• Osteons are the basic structural units of cortical bone.
• Each osteon contains a central canal (Haversian canal), which houses blood vessels and nerve fibers.
• Concentric rings of matrix called lamellae surround the central canal.
• Canaliculi connect lacunae to each other and to Haversian canals, providing a network for nutrient transport.
• Interstitial lamellae are remnants of old osteons located between osteons.

Cancellous Bone

• Trabeculae are the microscopic plates and rods that make up cancellous bone.
• Cancellous bone is highly porous, representing 20% of skeletal mass but 80% of bone surface.
• Trabeculae are aligned along lines of stress, providing maximum strength in response to compressive loads.

Rate of Formation

• Bone can be classified by its rate of formation:
  • Woven Bone: Fast formation, irregular collagen fibers, abundant osteocytes.
  • Lamellar Bone: Slow formation, regular collagen fibers, fewer osteocytes.

Collagen and Mineral

• Collagen is a structural protein that provides flexibility and strength to bone:
  • Type 1 collagen is the primary collagen in bones and forms triple helices.
  • Collagen fibrils assemble into layers, with mineral crystals deposited between them.
• Mineral (hydroxyapatite) provides hardness and rigidity to bone.
• Mineral crystals deposit on collagen fibrils soon after osteoid formation.

Bone Cells

• Osteoprogenitor cells differentiate into osteoblasts.
• Osteoblasts synthesize new bone matrix.
• Osteocytes are mature bone cells, maintaining bone tissue.
• Osteoclasts dissolve bone matrix.

Osteocytes

• Osteocytes reside within lacunae and are the most abundant bone cell type.
• Cell processes connect with each other and with bone lining cells to facilitate communication.
• Osteocytes secrete growth factors that can activate lining cells or stimulate osteoblasts and osteoclasts.

Osteoblasts

• Osteoblasts are responsible for bone formation, synthesizing proteins that form the bone matrix and control mineralization.

Wolff's Law and Optimum Design

• Wolff's Law states that bone adapts to the stresses placed upon it.
• Trabecular trajectories align with the direction of maximum principal stress.
• Three rules govern bone adaptation:
  1. The Dynamic Stimulus: Dynamic strain rather than static strain induces bone adaptation.
  2. A Case of Diminishing Returns: Bone cells become desensitized to prolonged loading.
  3. Bone Cells Accommodate to Routine Loading: Bone cells retain memory of previous mechanical environments.

Disuse and Bone Loss

• Disuse of bone due to inactivity (e.g., bedridden, cast immobilization, astronaut) leads to reduced stress and bone loss.
• Disuse results in decreased bone formation and increased bone turnover/resorption.