Skeletal System: Structure, Function, Development, and Homeostasis

Skeletal System Overview

Introduction

The skeletal system provides the framework for the human body, supporting movement, protecting organs, and serving as a reservoir for minerals. Understanding its structure, function, and development is essential in Anatomy & Physiology.

Cartilage and Bone Tissue

Types and Growth of Cartilage

• Types of Cartilage: The three main types are hyaline cartilage, elastic cartilage, and fibrocartilage.

• Most Abundant: Hyaline cartilage is the most prevalent in the skeletal system.

• Growth Mechanisms: Cartilage grows by appositional growth (addition of new layers on the surface) and interstitial growth (expansion from within).

Example: Articular cartilage at the ends of long bones is hyaline cartilage.

Functions of Bones

• Support: Provides structural support for the body.

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

• Movement: Acts as levers for muscles.

• Mineral Storage: Stores calcium and phosphate.

• Blood Cell Formation: Houses red bone marrow for hematopoiesis.

• Triglyceride Storage: Stores fat in yellow marrow.

• Hormone Production: Produces osteocalcin, which regulates bone formation and energy metabolism.

Classification and Structure of Bones

Bone Classification

• Axial Skeleton: Skull, vertebral column, and thoracic cage.

• Appendicular Skeleton: Limbs and girdles (shoulder and pelvic).

• Shape-Based Classification: Long bones, short bones, flat bones, irregular bones.

• Sesamoid Bones: Bones embedded within tendons (e.g., patella).

Bone Structure

• Types of Bone Tissue: Compact (dense) bone and spongy (cancellous) bone.

• Long Bone Anatomy:

  • Diaphysis: Shaft of the bone, contains the medullary cavity.

  • Medullary (Marrow) Cavity: Central cavity containing yellow marrow in adults.

  • Epiphyses: Ends of the bone, contain spongy bone and red marrow.

  • Articular Cartilage: Covers joint surfaces, reduces friction.

  • Epiphyseal Plate/Line: Growth plate in children; becomes epiphyseal line after growth.

  • Metaphysis: Region between diaphysis and epiphysis.

  • Periosteum: Double-layered membrane covering bone; contains osteogenic cells.

  • Endosteum: Membrane lining internal bone surfaces.

  • Nutrient Foramen: Opening for blood vessels to enter bone.

Bone Cells

• Osteogenic (Osteoprogenitor) Cells: Stem cells that differentiate into osteoblasts.

• Osteoblasts: Bone-forming cells; secrete bone matrix.

• Osteocytes: Mature bone cells; maintain bone matrix.

• Bone Lining Cells: Flat cells on bone surfaces; help maintain matrix.

• Osteoclasts: Bone-resorbing cells; break down bone tissue.

Compact (Lamellar) Bone Structure

• Osteon (Haversian System): Structural unit of compact bone.

• Lamella: Concentric rings of bone matrix.

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

• Perforating (Volkmann's) Canals: Connect blood vessels of periosteum to central canal.

• Lacunae: Small spaces housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae for nutrient/waste exchange.

Spongy Bone Structure

• Trabeculae: Lattice-like network providing strength with minimal weight.

• Spaces: Filled with red or yellow marrow.

Bone Matrix Components

• Organic Components: Collagen fibers and ground substance (osteoid).

• Inorganic Components: Crystalline mineral salts, primarily hydroxyapatite ($Ca_{10}(PO_4)_6(OH)_2$).

• Osteoid: Unmineralized bone matrix; provides flexibility and tensile strength.

Properties: Collagen fibers provide flexibility; mineral salts provide hardness.

Bone Development (Ossification)

Types of Ossification

• Endochondral Ossification: Bone forms by replacing hyaline cartilage; most bones below the skull.

• Intramembranous Ossification: Bone develops from fibrous membrane; forms flat bones (e.g., skull, clavicle).

Endochondral Ossification Steps

1. Bone collar forms around diaphysis of cartilage model.

2. Cavitation of the hyaline cartilage within the model.

3. Invasion of internal cavities by periosteal bud and spongy bone formation.

4. Formation of medullary cavity; appearance of secondary ossification centers in epiphyses.

5. Ossification of epiphyses; hyaline cartilage remains only in epiphyseal plates and articular cartilages.

Primary vs. Secondary Ossification: Primary occurs in diaphysis; secondary in epiphyses. Primary retains less spongy bone.

Postnatal Bone Growth

• Lengthening: Occurs at epiphyseal plates via interstitial growth of cartilage.

• Thickening: Appositional growth beneath periosteum.

• Epiphyseal Plate Zones: Proliferation, hypertrophic, calcification, ossification.

• Closure: Plate replaced by bone, forming epiphyseal line.

Bone Homeostasis: Remodeling & Repair

Bone Remodeling

• Definition: Continuous process of bone resorption and deposition.

• Importance: Maintains bone strength and mineral homeostasis.

• Location: Occurs at endosteal and periosteal surfaces.

• Cells Involved: Osteoclasts (resorption), osteoblasts (deposition).

• Control Loops: Hormonal (e.g., parathyroid hormone, calcitonin) and mechanical (Wolff's law: bone adapts to stress).

Calcium Homeostasis

• Hypercalcemia: High blood calcium levels.

• Hypocalcemia: Low blood calcium levels.

• Hormones: Parathyroid hormone increases blood Ca2+; calcitonin lowers it.

Fractures and Repair

• Fracture Types: Nondisplaced vs. displaced, complete vs. incomplete, open (compound) vs. closed (simple).

• Reduction: Closed (external) vs. open (internal) realignment of bone ends.

Fracture Type	Description
Nondisplaced	Bone ends retain normal position
Displaced	Bone ends out of alignment
Complete	Bone broken through
Incomplete	Not broken through
Open (Compound)	Bone ends penetrate skin
Closed (Simple)	Bone ends do not penetrate skin

• Stages of Fracture Repair:

  1. Hematoma formation

  2. Fibrocartilaginous callus formation

  3. Bony callus formation

  4. Bone remodeling

Clinical Correlations

Rickets and Osteomalacia

• Rickets: Occurs in children; bones are soft and deformed due to vitamin D deficiency.

• Osteomalacia: Adult form; bones inadequately mineralized.

• Common Cause: Vitamin D or calcium deficiency.

Osteoporosis

• Definition: Condition of decreased bone mass and increased fracture risk.

• Risk Factors: Age, gender (female), low calcium/vitamin D intake, sedentary lifestyle, hormonal changes.

Additional info: This study guide covers the essential aspects of bone structure, function, development, and clinical relevance, suitable for exam preparation in a college-level Anatomy & Physiology course.