Bones and Bone Tissue: Structure, Function, and Clinical Relevance

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 primary organs, with adults typically having 206 bones. Each bone is composed of 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 from injury.

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

• 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.

Bone Structure

Classification of Bones by Shape

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

• Short Bones: About as long as wide (e.g., carpals, tarsals).

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

• 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 anchors that attach periosteum to bone matrix.

• Diaphysis: Shaft of the bone, 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 cartilage (hyaline cartilage).

• Compact Bone: Dense outer layer that resists compression and twisting.

• Spongy (Cancellous) Bone: Inner honeycomb-like structure that houses bone marrow.

• Epiphyseal Lines: Remnants of the growth plate, indicating where bone growth occurred in children.

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.

Blood and Nerve Supply to Bone

• Short, flat, irregular, and sesamoid bones receive blood from periosteal vessels.

• Long bones are supplied by periosteal vessels and nutrient arteries entering through the nutrient foramen.

Red and Yellow Marrow

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

• Red Marrow: Site of hematopoiesis; abundant in children, restricted to certain bones in adults (pelvis, vertebrae, sternum, etc.).

Bone Marrow Transplantation

Clinical Application

• Used to treat diseases like leukemia, sickle-cell anemia, and aplastic anemia.

• Donor marrow is harvested from the pelvic bone and transplanted after recipient’s marrow is destroyed.

• Peripheral Blood Stem Cell (PBSC) donation is an alternative, where stem cells are collected from blood after stimulation.

The Extracellular Matrix of Bone

Inorganic Matrix

• Comprises ~65% of bone weight.

• Mainly hydroxyapatite crystals (calcium and phosphate), providing strength and resistance to compression.

• Other ions: bicarbonate, potassium, magnesium, sodium.

Organic Matrix (Osteoid)

• Comprises ~35% of bone weight.

• Contains collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and osteocalcin.

• Collagen resists torsion and tensile forces; osteocalcin organizes the inorganic matrix.

Bone Cells

Types of 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 that resorb bone by secreting acids and enzymes; derived from bone marrow cell fusion.

Bone Diseases

• Osteopetrosis: Defective osteoclasts cause increased bone mass but weak, brittle bones; can lead to nerve entrapment and marrow failure.

• Osteoporosis: Inadequate inorganic matrix leads to brittle bones and increased fracture risk; prevention includes calcium/vitamin D intake, exercise, and medications.

• Achondroplasia: Most common cause of dwarfism; abnormal cartilage growth affects endochondral ossification, resulting in short limbs and characteristic features.

Histology of Bone

Compact Bone

• Composed of osteons (Haversian systems) with concentric lamellae, central canals, lacunae, and canaliculi.

• Interstitial and circumferential lamellae strengthen bone; perforating (Volkmann) canals connect osteons.

Spongy Bone

• Consists of trabeculae with lamellae, lacunae, and canaliculi but no central canals; nourished by marrow blood vessels.

Bone Formation: Ossification

Types of Ossification

• Intramembranous Ossification: Bone develops from a mesenchymal membrane; forms flat bones (skull, clavicles).

• Endochondral Ossification: Bone develops from a hyaline cartilage model; forms most bones below the head except clavicles.

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; larger bones require fusion of ossification centers.

Steps of Endochondral Ossification

1. Chondroblasts in perichondrium differentiate into osteoblasts.

2. Osteoblasts build a 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. Remaining cartilage is replaced by bone; epiphyses finish ossifying; cartilage remains in epiphyseal plates and articular cartilage.

Bone Growth

Longitudinal Growth (Length)

Occurs at the epiphyseal plate, which has five zones:

• Zone of Reserve Cartilage

• Zone of Proliferation

• Zone of Hypertrophy and Maturation

• Zone of Calcification

• Zone of Ossification

Growth continues until the epiphyseal plate closes (ossifies), forming the epiphyseal line (typically between ages 13–21).

Appositional Growth (Width)

• Osteoblasts in the periosteum lay down new circumferential lamellae, thickening the bone.

• Osteoclasts enlarge the medullary cavity as bone thickens.

Hormonal Regulation of Bone Growth

• Growth Hormone: Increases mitosis of chondrocytes, 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.

Growth Disorders

• Gigantism: Excess growth hormone before epiphyseal plate closure; results in excessive height and bone thickness.

• Acromegaly: Excess growth hormone after plate closure; causes enlarged bones and soft tissues.

Bone Repair

Steps of Fracture Healing

1. Hematoma forms at the fracture site, cutting off blood supply and causing cell death.

2. Fibroblasts and chondroblasts form a soft callus of connective tissue and cartilage.

3. Osteoblasts build a bone callus of primary bone.

4. The bone callus is remodeled into secondary bone over several months.

Types of Fractures and Treatment

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

• Compound (Open) Fracture: Bone breaks through the skin, increasing risk of infection.

• Treatment: Stabilization and immobilization; closed or open reduction as needed.

Classification of Fractures

Type	Description
Transverse	Fracture line is perpendicular to the bone's long axis.
Oblique	Fracture line is at an angle to the bone's long axis.
Spiral	Fracture line spirals around the bone, often due to twisting force.
Comminuted	Bone is broken into several pieces.
Greenstick	Bone bends and cracks, common in children.
Compression	Bone is crushed, often in vertebrae.
Avulsion	Fragment of bone is pulled off by a tendon or ligament.
Impacted	One bone fragment is driven into another.
Fissure	Crack extending into but not through the bone.
Segmental	Bone is fractured in two places, leaving a segment detached.

Additional info: This guide covers the essential structure, function, and clinical relevance of bones and bone tissue, suitable for introductory college-level anatomy and physiology courses.