Chapter 6: Bones & Skeletal Tissues

Overview

Bones are dynamic organs composed of several tissue types and perform essential functions in the human body. This chapter covers the structure, types, growth, and disorders of bones and skeletal tissues.

6.1: Hyaline, Elastic, and Fibrocartilage Help Form the Skeleton

A. Basic Structure, Types, and Locations

• Hyaline cartilage: Most abundant; provides support with flexibility. Found in nose, trachea, larynx, and at the ends of long bones.

• Elastic cartilage: Contains more elastic fibers; found in the ear and epiglottis.

• Fibrocartilage: Highly compressible and strong; found in intervertebral discs and menisci.

B. Growth of Cartilage

• Appositional growth: Cartilage grows by adding new layers to the surface.

• Interstitial growth: Cartilage expands from within as chondrocytes divide.

Cartilage vs. Bone: Cartilage is more flexible and avascular, while bone is rigid and vascularized.

6.2: Bone Functions

Major Functions of Bones

• Support: Framework for the body.

• Protection: Shields vital organs (e.g., skull protects brain).

• Anchorage: Attachment points for muscles.

• Mineral storage: Reservoir for calcium and phosphate.

• Blood cell formation: Hematopoiesis occurs in red marrow.

• Fat storage: Yellow marrow stores triglycerides.

• Hormone production: Osteocalcin regulates bone formation and energy metabolism.

6.3: Bone Classification by Location and Shape

A. Location

• Axial skeleton: Skull, vertebral column, rib cage.

• Appendicular skeleton: Limbs and girdles.

B. Shape

• Long bones: Longer than wide (e.g., femur, humerus).

• Short bones: Cube-shaped (e.g., carpals, tarsals). Includes sesamoid bones (e.g., patella) and sutural bones (Wormian bones).

• Flat bones: Thin, flattened, and usually curved (e.g., sternum, skull).

• Irregular bones: Complex shapes (e.g., vertebrae, hip bones).

6.4: Gross and Microscopic Anatomy of Bone

A. Gross Anatomy

• Compact bone: Dense outer layer; provides strength.

• Spongy bone: Internal network of trabeculae; reduces weight.

• Structure of long bones: Diaphysis (shaft), epiphyses (ends), metaphysis, medullary cavity (contains marrow), articular cartilage.

• Hematopoietic tissue: Red marrow in spongy bone.

B. Microscopic Anatomy

• Cells of bone tissue:

  • Osteoprogenitor cells: Stem cells that differentiate into osteoblasts.

  • Osteoblasts: Bone-forming cells.

  • Osteocytes: Mature bone cells trapped in lacunae, connected by canaliculi.

  • Osteoclasts: Bone-resorbing cells; perform osteolysis.

• Microscopic anatomy of compact bone:

  • Osteon (Haversian system): Structural unit; contains central canal, lamellae, osteocytes.

  • Lamellae: Concentric rings of bone matrix.

  • Central (Haversian) canal: Contains blood vessels and nerves.

  • Perforating (Volkmann's) canals: Connect osteons at right angles.

  • Interstitial and circumferential lamellae: Fill spaces between osteons and encircle the bone.

• Microscopic anatomy of spongy bone: Trabeculae align along lines of stress; contain marrow.

C. Chemical Composition of Bone

• Organic components: Osteoid (1/3 of matrix), ground substance, collagen fibers (secreted by osteoblasts).

• Inorganic components: Mineral salts (hydroxyapatite, calcium phosphate) provide hardness.

6.5: Bone Development: Intramembranous vs. Endochondral Ossification

A. Formation of the Bony Skeleton

• Ossification (osteogenesis): Process of bone formation.

• Calcification: Deposition of calcium salts.

• Begins during 6th-7th week of embryonic development.

• Two patterns:

  • Endochondral ossification: Bone forms by replacing hyaline cartilage (most bones).

  • Intramembranous ossification: Bone develops directly from fibrous connective tissue (flat bones).

Endochondral Ossification Steps

1. Bone collar forms around diaphysis.

2. Cartilage in diaphysis calcifies.

3. Periosteal bud invades and spongy bone forms.

4. Diaphysis elongates and medullary cavity forms.

5. Epiphyses ossify.

Intramembranous Ossification Steps

1. Ossification center develops in fibrous CT.

2. Osteoid is secreted and calcifies.

3. Immature spongy bone and periosteum form.

4. Compact bone replaces immature bone; red marrow develops.

6.5B: Postnatal Bone Growth

Longitudinal Growth

• Occurs at epiphyseal plates; maintains proper proportions between diaphysis and epiphysis.

• Zones: proliferation, hypertrophic, calcification, ossification.

• Epiphyses and diaphysis fuse when cartilage growth stops.

Appositional Growth

• Bone increases in thickness.

• Regulated by growth hormone (GH) and thyroid hormone.

• Estrogen also influences growth.

6.6: Bone Remodeling

Overview

• Bone architecture is continually changing.

• 20% of adult skeleton is recycled and replaced each year.

• Bone remodeling: Bone deposits and resorption occur at periosteal and endosteal surfaces.

• Some bones remodel more actively than others (e.g., distal part of femur is fully replaced every 4-5 months).

A. Bone Resorption

• Osteoclasts break down bone matrix, releasing calcium into the blood.

B. Bone Deposition

• Osteoblasts build new bone matrix.

• Maintains calcium homeostasis and bone strength.

• Mechanical and gravitational forces stimulate remodeling.

• About 5-7% of bone mass is recycled weekly.

C. Control of Remodeling

• Hormonal control:

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

  • Calcitonin: Lowers blood Ca2+ by inhibiting osteoclasts.

  • Calcitriol: Active form of vitamin D; increases Ca2+ absorption.

• Blood Ca2+ levels decrease: Must be increased (bone resorption).

• Blood Ca2+ levels increase: Must be decreased (bone deposition).

• Response to mechanical stress: Wolff's Law: Bone grows/remodels in response to demands placed on it.

6.7: Bone Repair

A. Fracture Classification

B. Fracture Treatment and Repair

1. Hematoma formation

2. Fibrocartilaginous callus formation

3. Bony callus formation

4. Bone remodeling

6.8: Bone Disorders

A. Osteomalacia and Rickets

• Osteomalacia: Softening of adult bones due to inadequate mineralization.

• Rickets: Childhood disorder; bones fail to calcify, leading to deformities.

B. Osteoporosis

• Characterized by decreased bone mass and increased fracture risk.

• Risk factors: Age, gender (female), low calcium intake, sedentary lifestyle.

• Prevention/treatment: Adequate calcium and vitamin D intake, weight-bearing exercise, medications.

C. Paget's Disease

• Disorganized bone remodeling; excessive bone formation and breakdown.

• Leads to weak, deformed bones.

Key Terms and Concepts

• Osteon: Structural unit of compact bone.

• Osteoblast: Bone-forming cell.

• Osteoclast: Bone-resorbing cell.

• Osteocyte: Mature bone cell.

• Hematopoiesis: Blood cell formation in bone marrow.

• Wolff's Law: Bone adapts to mechanical stress.

Important Equations

• Calcium Homeostasis:

Additional info: Some details, such as the full steps of ossification and the hormonal regulation of bone growth, were expanded for clarity and completeness.