Bone Remodeling and Growth Hormones

Skeletal System Components

• The skeleton provides body support through its various components.
• Cartilage functions as a shock absorber, reduces friction, and provides flexibility in joints.
• Ligaments connect bones together, stabilize joints, and provide support to the skeletal system.

Skeletal Component Functions

• Tendons connect muscles to bones, facilitating movement and providing stability.
• The rib cage (thoracic cage) protects the heart, lungs, and other thoracic organs.

Bone Structure

• Compact bone is dense and compact, while spongy bone is porous and lightweight.
• Osteocytes are located in the lacunae of spongy bone.
• In lamellar bone, collagen fibers are arranged in a spiral pattern.
• Central canals in lamellar bone contain blood vessels that supply osteocytes.
• In compact bone, osteocytes are arranged in a concentric pattern.

Bone Remodeling

• Osteoclasts remove old bone matrix, while osteoblasts replace it with new bone matrix during bone remodeling.
• A basic multicellular unit (BMU) involved in bone remodeling has an average life span of 4-6 months.

Hormonal Regulation

• Growth hormone is primarily responsible for the normal growth of all tissues, including cartilage.
• Estrogen is the primary female reproductive hormone that regulates bone growth.
• Excessive growth hormone secretion can lead to gigantism or acromegaly.

Bone Growth and Development

• In a long bone, the hollow cylinder with a medullary cavity provides a mechanical advantage, reducing weight while maintaining strength.
• Decreased levels of testosterone or estrogen during puberty can affect bone growth, leading to shorter stature.

Bone Strength

• Bone strength can be modified by factors such as exercise, diet, and mechanical stress.
• Mechanical stress can modify bone strength through the process of bone remodeling.

Bone Fractures

• Bone fractures are classified based on the direction of the fracture line.
• A non-displaced fracture is characterized by no visible damage to the skin at the injury site.
• A periprosthetic fracture may occur when a bone is weakened by the presence of a prosthetic implant.

Calcium Regulation

• Calcitonin decreases blood calcium levels by promoting calcium deposition in bones.
• Parathyroid hormone (PTH) increases blood calcium levels by stimulating osteoclasts and promoting bone resorption.
• Vitamin D3, derived from sunlight, increases blood calcium levels and is essential for bone growth.
• Vitamin D3 production is initiated when UV light from sunlight interacts with 7-dehydrocholesterol in the skin.
• PTH secretion is controlled by a negative feedback loop, where high blood calcium levels inhibit PTH secretion.
• Low blood calcium levels are the key signal for parathyroid hormone (PTH) secretion.