Bones and Bone Tissue

Introduction to the Skeletal System

The skeletal system is a complex organ system that includes bones, joints, and supporting tissues. Bones are the main organs, with adults typically having 206 bones. Each bone is composed of bone (osseous) tissue, dense regular and irregular connective tissue, and bone marrow.

Functions of the Skeletal System

• Protection: Bones such as the skull, sternum, and ribs protect vital organs like the brain, heart, and lungs.

• Mineral Storage and Acid-Base Homeostasis: Bones store minerals (calcium, phosphorus, magnesium) essential for electrolyte and acid-base balance.

• Blood Cell Formation: Red bone marrow is the site of hematopoiesis, the production of blood cells.

• Fat Storage: Yellow bone marrow stores triglycerides in adipocytes.

• Movement: Bones serve as levers for muscle action, enabling movement at joints.

• Support: The skeleton provides structural support for the body.

Classification and Structure of Bones

Classification by Shape

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

• Short Bones: Roughly cube-shaped (e.g., wrist and ankle bones).

• Flat Bones: Thin and broad (e.g., skull, pelvis).

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

• Sesamoid Bones: Small, oval-shaped, within tendons (e.g., patella).

Structure of a Long Bone

• Periosteum: Outer membrane of dense irregular connective tissue with blood vessels and nerves.

• Perforating Fibers: Collagen fibers anchoring periosteum to bone matrix.

• Diaphysis: Shaft containing the medullary (marrow) cavity lined by endosteum and filled with marrow.

• Epiphyses: Ends of the bone, filled with red marrow and covered with articular (hyaline) cartilage.

• Compact Bone: Dense outer layer resisting compression and twisting.

• Spongy (Cancellous) Bone: Inner honeycomb-like structure housing marrow.

• Epiphyseal Lines: Remnants of growth plates in adults.

Structure of Short, Flat, Irregular, and Sesamoid Bones

These bones share similarities with long bones but have fewer structures. In flat bones, the spongy bone is called diploë. Some skull bones contain sinuses to reduce weight.

Bone Marrow

• Red Bone Marrow: Site of hematopoiesis; abundant in children, limited to certain bones in adults.

• Yellow Bone Marrow: Contains adipocytes and blood vessels; increases with age.

The Extracellular Matrix of Bone

Inorganic Matrix

• Comprises about 65% of bone weight.

• Mainly calcium and phosphorus in the form of hydroxyapatite crystals, providing strength and resistance to compression.

• Other ions include bicarbonate, potassium, magnesium, and sodium.

Organic Matrix (Osteoid)

• About 35% of bone weight.

• Composed of collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins (e.g., osteocalcin).

• Collagen resists torsion and tensile forces; aligns with hydroxyapatite for hardness.

Bone Cells

• Osteoblasts: Derived from osteogenic cells; build bone by secreting organic matrix and aiding inorganic matrix formation.

• Osteocytes: Mature osteoblasts trapped in lacunae; maintain ECM and signal bone remodeling.

• Osteoclasts: Large, multinucleated cells from bone marrow; break down bone ECM via bone resorption.

Histology of Bone

Compact Bone

• Osteons (Haversian Systems): Structural units with concentric lamellae, central canal, lacunae, and canaliculi.

• Lamellae: Rings of bone matrix; collagen fibers in alternating directions resist twisting.

• Central Canal: Contains blood vessels and nerves.

• Lacunae: Small cavities housing osteocytes.

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

• Perforating (Volkmann) Canals: Connect central canals and carry blood from periosteum.

Spongy Bone

• Trabeculae: Branching bone "ribs" covered with endosteum; contain lamellae, lacunae, and canaliculi but lack central canals.

Bone Formation: Ossification

Primary vs. Secondary Bone

• Primary (Woven) Bone: Immature, irregular collagen, little inorganic matrix; replaced by secondary bone.

• Secondary (Lamellar) Bone: Mature, organized lamellae, parallel collagen, more inorganic matrix.

Types of Ossification

• Intramembranous Ossification: Forms flat bones (e.g., skull, clavicles) from mesenchymal membrane; spongy bone forms first.

• Endochondral Ossification: Forms long and short bones from hyaline cartilage model; compact bone forms first.

Steps of Intramembranous Ossification

1. Osteoblasts develop in the primary ossification center from mesenchymal cells.

2. Osteoblasts secrete organic matrix, which calcifies; trapped osteoblasts become osteocytes.

3. Osteoblasts lay down trabeculae of early spongy bone; some mesenchyme becomes periosteum.

4. Osteoblasts in periosteum lay down early compact bone; matrix is remodeled to mature bone.

Steps of Endochondral Ossification

1. Chondroblasts in perichondrium differentiate into osteoblasts.

2. Bone begins to ossify from outside: osteoblasts build bone collar; internal cartilage calcifies and chondrocytes die.

3. Osteoblasts replace calcified cartilage with early spongy bone; secondary ossification centers and medullary cavity develop.

4. Medullary cavity enlarges; remaining cartilage replaced by bone; epiphyses finish ossifying; cartilage remains in epiphyseal plates and articular cartilage.

Bone Growth

Longitudinal Growth (Length)

Occurs at the epiphyseal plate through chondrocyte division and ossification. The plate has five zones:

• Zone of Reserve Cartilage

• Zone of Proliferation

• Zone of Hypertrophy and Maturation

• Zone of Calcification

• Zone of Ossification

Appositional Growth (Width)

• Osteoblasts between periosteum and bone surface lay down new bone, thickening the diaphysis.

• Osteoclasts enlarge the medullary cavity as bone widens.

Hormonal Regulation of Bone Growth

• Growth Hormone: Increases chondrocyte mitosis, osteogenic cell activity, and osteoblast activity.

• Testosterone: Increases appositional growth and mitosis; accelerates epiphyseal plate closure.

• Estrogen: Similar effects as testosterone but less pronounced; earlier epiphyseal plate closure in females.

Bone Remodeling

Overview

Bone remodeling is the continual process of bone deposition (by osteoblasts) and resorption (by osteoclasts). It maintains calcium homeostasis, repairs bone, replaces old bone, and adapts to stress.

Bone Deposition

• Osteoblasts secrete organic matrix and facilitate inorganic matrix formation.

• Proteoglycans and glycoproteins bind calcium; vesicles initiate calcification.

Bone Resorption

• Osteoclasts secrete hydrogen ions (dissolve inorganic matrix) and enzymes (break down organic matrix).

• Minerals and organic components are released into the blood.

Factors Influencing Bone Remodeling

• Mechanical Stress: Compression and tension stimulate deposition; pressure stimulates resorption.

• Hormones: Testosterone promotes deposition; estrogen inhibits osteoclasts.

• Age: Hormone levels decline, reducing bone formation.

• Nutrient Intake: Adequate calcium, vitamin D, K, C, and protein are essential for healthy bone remodeling.

Calcium Homeostasis

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption, increasing intestinal absorption, and reducing urinary excretion.

• Calcitonin: Decreases blood calcium by stimulating bone deposition (less potent in adults).

Bone Repair

Steps of Fracture Healing

1. Hematoma forms at the fracture site.

2. Fibroblasts and chondroblasts form a soft callus.

3. Osteoblasts build a bone callus (primary bone).

4. Bone callus is remodeled into secondary bone.

Types of Fractures

Clinical Correlations

Osteopetrosis

"Marble bone disease" caused by defective osteoclasts, leading to increased but brittle bone mass. Infantile form is severe and can be fatal; adult form causes pain and fractures.

Osteoporosis

Bone disease due to inadequate inorganic matrix, resulting in brittle bones and increased fracture risk. Prevention includes adequate calcium/vitamin D, exercise, and medications.

Achondroplasia

Most common cause of dwarfism, due to abnormal growth factor receptor on cartilage, affecting endochondral ossification and resulting in short limbs and characteristic features.

Gigantism and Acromegaly

Both result from excess growth hormone. Gigantism occurs before epiphyseal plate closure (excessive growth), while acromegaly occurs after closure (enlarged bones and soft tissues).

Additional info: This guide covers the essential structure, function, and physiology of bones and bone tissue, including clinical correlations relevant to Anatomy & Physiology students.