Chapter6bstudents

Chapter 6: Bones and Skeletal Tissues Part B

Bone Development

• Osteogenesis (ossification): The process of bone tissue formation.

  • Stages of bone formation begin in the 2nd month of development.

  • Postnatal bone growth continues until early adulthood.

  • Bone remodeling and repair occur throughout life.

Two Types of Ossification

• Intramembranous Ossification:

  • Development of membrane bone from fibrous membrane.

  • Forms flat bones, including clavicles and cranial bones.

• Endochondral Ossification:

  • Bone forms by replacing hyaline cartilage.

  • This process forms most of the remaining skeleton.

Intramembranous Ossification Stages

1. Ossification Centers:

   • Central mesenchymal cells cluster and differentiate into osteoblasts, forming ossification centers.

2. Bone Matrix Formation:

   • Osteoblasts secrete osteoid, which calcifies over days.

   • Trapped osteoblasts become osteocytes.

3. Woven Bone and Periosteum Formation:

   • Accumulating osteoid creates woven bone in random orientation.

   • Vascularized mesenchyme becomes the periosteum.

4. Lamellar Bone Replacement:

   • Woven bone is replaced by mature lamellar bone and spongy bone contains red marrow.

Endochondral Ossification Process

• Begins in the second month of development using hyaline cartilage models.

• Requires breakdown of hyaline cartilage before ossification can proceed.

• A sequence of steps leads to the formation of the medullary cavity and secondary ossification centers.

Postnatal Bone Growth

• Appositional Growth: Increases thickness by producing new bone matrix by osteoblasts and osteoclasts.

• Interstitial Growth: Increases length of long bones through epiphyseal plate growth.

Functional Zones of Epiphyseal Plate Cartilage

1. Proliferation Zone: Cartilage cells undergo mitosis.

2. Hypertrophic Zone: Older cartilage cells enlarge.

3. Calcification Zone: Matrix becomes calcified; cartilage cells die.

4. Ossification Zone: New bone formation occurs.

Hormonal Regulation of Bone Growth

• Growth Hormone: Stimulates epiphyseal plate activity.

• Thyroid Hormone: Modulates growth hormone activity.

• Sex Hormones (estrogen & testosterone): Promote adolescent growth spurts and induce epiphyseal plate closure.

Bone Remodeling

• Remodeling Units: Osteoblasts and osteoclasts function at periosteal and endosteal surfaces.

• Bone remodeling occurs in response to hormonal mechanisms and mechanical stresses.

Bone Deposits and Resorption

• Bone Deposit: Occurs in response to injury or increased strength needs due to osteoblast activity, requiring a rich nutrient diet.

• Bone Resorption: Osteoclasts secrete enzymes and acids to digest organic matrix, releasing calcium into the bloodstream.

Control of Bone Remodeling

• Homeostasis is controlled by hormonal mechanisms maintaining blood calcium levels and mechanical/gravitational forces.

Hormonal Control of Blood Calcium Levels

• Essential for nerve impulses, muscle contraction, blood coagulation, secretion by glands, and cell division.

• Parathyroid Hormone (PTH): Increases blood Ca2+ levels by stimulating osteoclasts.

• Calcitonin: Lowers blood Ca2+ levels by stimulating osteoblast activity.

Response to Mechanical Stress

• Wolff's Law: Bones remodel based on the stress applied.

  • Evidence includes bone thickness in dominant limbs and thickened areas in curved bones.

Classification of Bone Fractures

• Classified by position of bone ends, completeness, orientation, and penetration through the skin:

  • Nondisplaced / Displaced

  • Complete / Incomplete

  • Linear / Transverse

  • Compound (open) / Simple (closed)

Common Types of Fractures

• Comminuted: Bone fragments into three or more pieces.

• Compression: Bone is crushed commonly in osteoporosis.

• Spiral: Ragged break occurs from twisting forces.

• Epiphyseal: Separation of epiphysis from diaphysis at the growth plate.

• Depressed: Broken bone portion is pressed inward, typical of skull fractures.

• Greenstick: Incomplete fracture typical in children.

Stages in Healing of Bone Fractures

1. Hematoma Formation: Blood vessel hemorrhage leading to clot formation.

2. Fibrocartilaginous Callus Formation: Phagocytes clear debris; osteoblasts form spongy bone.

3. Bony Callus Formation: New trabeculae form; hard callus develops.

4. Bone Remodeling: Final structure resembles original bone post mechanical stressors.

Homeostatic Imbalances

• Osteomalacia: Inadequately mineralized bones, usually due to vitamin D/calcium deficiency.

• Rickets: Inadequately mineralized bones in children, leading to deformities.

• Osteopenia to Osteoporosis: Loss of bone mass; spongy bone is most susceptible to fractures due to various risk factors.

Osteoporosis: Treatment and Prevention

• Include calcium/vitamin D supplementation, weight-bearing exercise, hormone replacement therapy, and certain medications to increase bone density.

Paget’s Disease

• Characterized by excessive bone formation and breakdown, usually affecting the spine, pelvis, femur, or skull; can be treated with calcitonin and biphosphonates.