Bones and Bone Structure

Overview and Objectives

This chapter introduces the structure and function of bones within the human skeletal system. It covers the classification of bones, bone markings, the anatomy of long bones, bone tissue types, bone growth and development, mineral homeostasis, and clinical aspects such as bone disorders and fracture repair.

• Identify divisions of the skeleton and major functions of the skeletal system.

• Classify bones by shape and describe bone markings.

• Describe the anatomy and internal structure of long bones.

• Explain the types and functions of bone cells.

• Compare compact and spongy bone.

• Outline bone growth and ossification processes.

• Discuss bone abnormalities and mineral homeostasis.

• Describe fracture types and healing mechanisms.

Skeletal System Organization

Axial vs. Appendicular Skeleton

The human skeleton is divided into two main regions: the axial and appendicular skeletons. Each division has distinct components and functions.

• Axial Skeleton: Consists of 80 bones, including the skull, vertebral column, and thoracic cage. Provides support and protection for the brain, spinal cord, and organs in the thorax.

• Appendicular Skeleton: Composed of 126 bones, including the limbs and girdles (pectoral and pelvic). Facilitates movement and interaction with the environment.

• Associated Cartilages and Ligaments: Support and connect bones within both divisions.

Functions of the Skeletal System

Major Functions

The skeletal system performs several essential functions for the body:

• Support: Provides structural framework for the body.

• Protection: Shields vital organs (e.g., brain, heart, lungs).

• Movement / Leverage: Acts as levers for muscles to produce movement.

• Mineral and Lipid Homeostasis: Stores minerals (e.g., calcium, phosphate) and lipids in bone marrow.

• Blood Cell Production: Houses red bone marrow, which produces blood cells (hematopoiesis).

Bone Classification

Types of Bones by Shape

Bones are classified according to their shapes, which relate to their functions and locations in the body.

Bone Markings

General and Specific Bone Markings

Bones have surface features called markings, which serve as sites for muscle attachment, articulation, and passage of nerves and blood vessels.

• General Bone Markings: Head, Diaphysis (shaft), Neck.

• Specific Bone Markings: Related to particular functions, including elevations/projections and depressions/grooves/tunnels.

Elevations or Projections

• Process: Any projection or bump.

• Tubercle: Small, rounded projection.

• Tuberosity: Large, rough projection.

• Trochlea: Smooth, grooved articular process.

• Condyle: Smooth, rounded articular process.

• Trochanter: Large, rough projection (found only on femur).

• Facet: Small, flat articular surface.

Depressions, Grooves, and Tunnels

• Canal or Meatus: Tube-like passageway.

• Sinus: Chamber, normally filled with air.

• Foramen: Hole for blood vessels or nerves.

• Fissure: Narrow slit between adjacent bony parts.

• Sulcus: Deep, narrow groove.

• Fossa: Shallow depression.

Long Bone Anatomy

Parts and Internal Structure

Long bones have a distinct anatomy that supports their function in movement and support.

• Epiphysis: Expanded ends of the bone, mostly spongy bone.

• Diaphysis: Shaft of the bone, composed of compact bone.

• Metaphysis: Region where diaphysis and epiphysis meet.

• Articular Cartilage: Covers joint surfaces, reduces friction.

• Medullary Cavity: Central cavity containing bone marrow.

• Red Bone Marrow: Site of blood cell production.

• Yellow Bone Marrow: Stores fat.

Blood Supply and Innervation

• Nutrient Artery & Vein: Enter through nutrient foramen to supply bone.

• Metaphyseal Artery & Vein: Supply metaphysis and epiphysis.

• Periosteum: Contains smaller blood vessels, lymphatics, and sensory nerves.

Bone Tissue

Cell Types and Functions

Bones are dynamic tissues composed of several cell types, each with specialized functions.

• Osteogenic Cells: Stem cells that differentiate into osteoblasts. Located in periosteum and endosteum.

• Osteoblasts: Produce new bone matrix (osteogenesis/ossification). Become osteocytes when trapped in matrix.

• Osteocytes: Mature bone cells in lacunae; maintain bone tissue.

• Osteoclasts: Break down and remodel bone matrix.

Bone Matrix Composition

• Organic Component: ~33% protein fibers (mainly collagen) for flexibility.

• Inorganic Component: ~66% mineral salts (hydroxyapatite, calcium phosphate, calcium carbonate, ions like Na+, Mg2+, F-) for strength.

Compact and Spongy Bone Structure

Compact Bone (Cortical Bone)

Compact bone is dense and forms the outer layer of bones, providing strength and protection.

• Osteon (Haversian System): Structural unit with concentric lamellae around a central canal.

• Central Canal: Contains blood vessels and nerves.

• Canaliculi: Tiny channels connecting lacunae and central canal for nutrient/waste exchange.

Spongy Bone (Trabecular Bone)

Spongy bone is found at the ends of long bones and inside flat bones, providing lightweight support and housing marrow.

• Trabeculae: Struts and plates forming a network.

• No blood vessels in matrix: Nutrients diffuse through canaliculi.

• Red Bone Marrow: Fills spaces between trabeculae.

Bone Growth and Development

Appositional Growth

Appositional growth increases bone diameter by adding new layers to the surface.

• Osteogenic Cells: Differentiate into osteoblasts, which deposit new bone.

• Osteoclasts: Remove matrix from inner surface, enlarging the medullary cavity.

• Periosteum: Outer fibrous layer and inner cellular layer support growth and repair.

Endochondral Ossification

Most bones form by replacing hyaline cartilage with bone through endochondral ossification.

1. Cartilage model enlarges; chondrocytes die and leave cavities.

2. Blood vessels grow around cartilage; perichondrium cells become osteoblasts and form superficial bone.

3. Blood vessels penetrate cartilage; fibroblasts become osteoblasts, forming spongy bone at the primary ossification center.

4. Medullary cavity develops; bone grows in length and diameter.

5. Secondary ossification centers form in epiphyses; remaining cartilage forms articular cartilage and epiphyseal plate.

6. Bone grows in length at epiphyseal plate until puberty, when epiphyseal closure occurs.

Intramembranous Ossification

Some bones (e.g., cranial vault, mandible, clavicle, sesamoid bones) form directly from mesenchymal tissue.

1. Mesenchymal cells cluster and differentiate into osteoblasts, forming bone matrix at ossification centers.

2. Bone grows out in spicules; osteoblasts mature into osteocytes.

3. Blood vessels enter, spicules fuse, and spongy bone forms.

4. Remodeling creates compact bone and periosteum.

Bone Growth Abnormalities

Disorders Affecting Bone Length and Structure

• Pituitary Growth Failure: Reduced growth hormone leads to shortened bones.

• Achondroplasia: Slow growth of epiphyseal cartilage; normal trunk, short limbs.

• Marfan Syndrome: Excessive cartilage formation; tall stature, long limbs, connective tissue defects.

• Congenital Talipes Equinovarus (Clubfoot): Inherited abnormality causing distorted bone growth.

• Gigantism: Overproduction of growth hormone before puberty; excessive bone lengthening.

• Acromegaly: Overproduction of growth hormone after epiphyseal plates close; bones thicken, especially in face and hands.

• Fibrodysplasia Ossificans Progressiva (FOP): Gene mutation causes bone formation in soft tissues (heterotopic/ectopic ossification).

• Osteoporosis: Excessive loss of calcium weakens bones.

• Rickets/Osteomalacia: Failure of bones to calcify; bones become soft.

Bones as Mineral Reservoirs

Mineral Storage and Homeostasis

Bones store essential minerals, especially calcium and phosphate, which are critical for physiological processes.

Calcium is vital for muscle contraction, blood coagulation, and nerve impulse generation.

Hormonal Regulation of Calcium Metabolism

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating osteoclasts, enhancing intestinal absorption, and increasing kidney reabsorption.

• Calcitonin (Thyroid): Decreases blood calcium by inhibiting osteoclasts, reducing intestinal absorption, and decreasing kidney reabsorption.

Bone Fractures and Repair

Types of Fractures

• Closed (Simple) Fracture: Bone breaks but does not penetrate skin.

• Open (Compound) Fracture: Bone breaks and penetrates skin.

• Greenstick Fracture: Incomplete break, common in children.

• Impacted Fracture: One bone fragment is driven into another.

Steps in Fracture Repair

1. Fracture Hematoma Formation: Large clot forms to close damaged vessels within hours.

2. Callus Formation: Internal callus (spongy bone) and external callus (cartilage and bone) stabilize the fracture.

3. Spongy Bone Formation: Cartilage of external callus replaced by spongy bone; dead bone removed.

4. Compact Bone Formation: Spongy bone is remodeled into compact bone.

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