Overview
This lecture covers the structure of bone tissue, the process of bone remodeling, and the development and treatment of osteoporosis.
Bone Structure
- Bone is made up of two main layers:
- Compact bone: a dense, organized outer shell that provides strength and protection.
- Cancellous (spongy) bone: an inner structure made of thin, interlocking plates called trabeculae, which create a supportive network.
- Bone tissue is composed of bone cells, known as osteocytes, which include:
- Osteoblasts: cells that build new bone.
- Osteoclasts: cells that break down bone tissue.
Bone Remodeling Process
- Bone constantly renews its structure and strength through a process called remodeling.
- Remodeling is triggered by physical stress on the bone and changes in blood calcium levels, which prompt osteocytes to start the process.
- The steps of bone remodeling include:
- Resorption: Osteoblasts release cytokines that cause immature osteoclasts to mature. Mature osteoclasts use enzymes to break down and remove old bone tissue.
- After resorption, osteoclasts release chemicals that stimulate immature osteoblasts to mature. These mature osteoblasts then release osteoprotegerin, a protein that deactivates osteoclasts and stops further bone breakdown.
- Bone formation: Mature osteoblasts deposit osteoid, a matrix containing minerals (calcium and phosphorus) and collagen, a strong and flexible protein.
- Calcium and phosphorus mineralize the osteoid, forming hydroxyapatite crystals, which complete the process of creating strong bone.
Hormonal Regulation and Bone Health
- In both men and women before midlife, bone resorption and bone formation are balanced, maintaining bone strength.
- Hormones, especially estrogen, play a key role in bone remodeling:
- Estrogen suppresses osteoclast activity, reducing bone resorption and helping to preserve bone mass.
- After midlife, estrogen levels decrease, leading to increased osteoclast activity and greater bone resorption, which can result in bone loss.
Osteoporosis
- Osteoporosis is a progressive disease where bone resorption outpaces bone formation, causing bones to weaken.
- As osteoporosis develops:
- The trabeculae in cancellous bone become fewer and more widely spaced.
- The compact bone layer becomes thinner.
- Overall, bones become weak, fragile, and more likely to fracture.
Osteoporosis Treatments
- Several approaches are used to treat osteoporosis and maintain bone health:
- Calcium and vitamin D supplements: Help maintain blood calcium levels, supporting bone strength.
- Anti-resorptive medications: Prevent activation of osteoclasts, reducing excessive bone breakdown and promoting bone strength.
- Anabolic medications: Increase osteoblast activity, encouraging new bone formation.
- Exercise: Weight-bearing activities add physical stress to bones, stimulating remodeling and strengthening bone tissue.
Key Terms & Definitions
- Compact bone: Dense, organized outer shell of bone tissue.
- Cancellous bone (trabecular bone): Spongy inner network of thin plates called trabeculae.
- Osteoblast: Bone cell responsible for building new bone tissue.
- Osteoclast: Bone cell responsible for breaking down and resorbing old bone tissue.
- Osteocyte: Mature bone cell that regulates the bone remodeling process.
- Osteoid: Unmineralized bone matrix produced by osteoblasts, containing minerals and collagen.
- Hydroxyapatite: Mineral crystals formed from calcium and phosphorus that give bone its strength.
- Resorption: The process of breaking down and removing bone tissue.
- Osteoprotegerin: Protein released by osteoblasts that inhibits osteoclasts and stops bone resorption.
- Osteoporosis: Disease in which bone loss exceeds bone formation, resulting in weak and fragile bones.
Action Items / Next Steps
- Review the differences between osteoblasts, osteoclasts, and osteocytes, and their specific roles in bone remodeling.
- Study the factors that influence bone remodeling, including hormonal changes and physical stress.
- Learn more about risk factors for osteoporosis and strategies for prevention and treatment.