Bones and Bone Tissue: Structure, Function, and Physiology

Bones and Bone Tissue

Overview of 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 osseous tissue, dense regular and irregular connective tissue, and bone marrow.

• Bones: Main organs of the skeletal system, providing structure and support.

• Joints: Sites where two or more bones meet, allowing for movement and flexibility.

• Supporting Tissues: Include ligaments, tendons, and cartilage.

Functions of the Skeletal System

The skeletal system performs several critical functions necessary for survival and homeostasis.

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

• 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 (formation 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 the structural framework for the body.

Bone Structure and Classification

Classification of Bones by Shape

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

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

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

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

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

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

Structure of a Long Bone

Long bones have a unique structure that supports their function in movement and weight-bearing.

• Periosteum: Outer dense irregular connective tissue membrane 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 epiphyseal (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, and some skull bones contain sinuses to reduce weight.

Blood and Nerve Supply to Bone

Bones are highly vascularized and innervated. Blood supply to short, flat, irregular, and sesamoid bones comes from vessels in the periosteum. Long bones receive blood from the periosteum and nutrient arteries entering through the nutrient foramen.

Red and Yellow Marrow

Bone marrow exists in two forms:

• Red Bone Marrow: Site of hematopoiesis, abundant in children and in certain adult bones (pelvis, vertebrae, ribs, sternum, clavicles, scapulae, proximal femur and humerus).

• Yellow Bone Marrow: Contains adipocytes and blood vessels, more prevalent in adults.

Bone Marrow Transplantation

Clinical Application: Bone Marrow Transplantation

Bone marrow transplantation is used to treat diseases such as leukemia, sickle-cell anemia, and aplastic anemia. The procedure involves harvesting red marrow from a donor and transplanting it into a recipient whose marrow has been destroyed. Peripheral blood stem cell donation is an alternative method.

The Extracellular Matrix of Bone

Composition of Bone Matrix

The bone matrix is composed of inorganic and organic components, each contributing to bone's unique properties.

• Inorganic Matrix: About 65% of bone's weight, mainly hydroxyapatite crystals (calcium and phosphate), providing strength and resistance to compression.

• Organic Matrix (Osteoid): About 35% of bone's weight, mainly collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and osteocalcin, providing flexibility and resistance to tension.

Bone Cells

Types of Bone Cells

Bone is a dynamic tissue maintained by three main cell types:

• Osteoblasts: Derived from osteogenic cells, responsible for bone deposition by secreting organic matrix and aiding in inorganic matrix formation.

• Osteocytes: Mature osteoblasts trapped in lacunae, maintaining the ECM and signaling for bone remodeling.

• Osteoclasts: Large, multinucleated cells derived from bone marrow, responsible for bone resorption by secreting acids and enzymes.

Bone Diseases: Osteopetrosis

Osteopetrosis is a genetic disorder where defective osteoclasts fail to resorb bone, resulting in increased bone mass but brittle, weak bones. Symptoms include nerve entrapment, fractures, and decreased marrow space.

Histology of Bone

Compact Bone

Compact bone is organized into osteons (Haversian systems), which are cylindrical structures that resist stress.

• Lamellae: Concentric rings of bone matrix.

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

• Lacunae: Small spaces housing osteocytes.

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

• Perforating (Volkmann) Canals: Connect central canals of neighboring osteons.

Spongy Bone

Spongy bone consists of trabeculae, which are oriented along lines of stress and contain lamellae, lacunae, and canaliculi but lack central canals.

Bone Formation: Ossification

Ossification (Osteogenesis)

Ossification is the process of bone formation, which occurs through two main mechanisms:

• Intramembranous Ossification: Bone develops from a mesenchymal membrane, forming flat bones like the skull and clavicles.

• Endochondral Ossification: Bone develops from a hyaline cartilage model, forming most bones below the head except the 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 the periosteum lay down early compact bone.

Steps of Endochondral Ossification

1. Chondroblasts in the perichondrium differentiate into osteoblasts.

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

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

4. Remaining cartilage is replaced by bone; epiphyses finish ossifying. Cartilage remains in epiphyseal plates and as articular cartilage.

Comparison of Ossification Types

Intramembranous ossification forms flat bones from a fibrous membrane, while endochondral ossification forms long and short bones from a cartilage model. The order of bone formation also differs: compact bone forms first in endochondral ossification, while spongy bone forms first in intramembranous ossification.

Bone Disorders

Osteoporosis

Osteoporosis is a disease characterized by inadequate inorganic matrix, leading to brittle bones and increased fracture risk. Causes include dietary deficiencies, hormonal changes, age, lack of exercise, and genetic factors. Prevention includes adequate calcium and vitamin D intake, exercise, and medications.

Achondroplasia

Achondroplasia is the most common cause of dwarfism, resulting from abnormal growth factor receptors on cartilage. It leads to decreased endochondral ossification, shortened limbs, and characteristic facial features.

Bone Growth

Longitudinal Growth

Long bones grow in length at the epiphyseal plate, which consists of five zones:

1. Zone of Reserve Cartilage

2. Zone of Proliferation

3. Zone of Hypertrophy and Maturation

4. Zone of Calcification

5. Zone of Ossification

Appositional Growth

Appositional growth increases bone width. Osteoblasts in the periosteum lay down new circumferential lamellae, thickening the compact bone. Osteoclasts enlarge the medullary cavity as the bone widens.

Hormonal Regulation of Bone Growth

Hormones play a significant role in bone growth:

• Growth Hormone: Increases mitosis of chondrocytes and osteogenic cells, stimulates osteoblasts.

• Testosterone: Increases appositional growth and mitosis at the epiphyseal plate, accelerates plate closure.

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

Gigantism and Acromegaly

Excess growth hormone before epiphyseal plate closure causes gigantism (excessive height), while excess after closure causes acromegaly (enlarged bones of the face, hands, and feet).

Bone Remodeling

Bone Remodeling Process

Bone remodeling is a continuous process involving bone deposition by osteoblasts and bone resorption by osteoclasts. It maintains calcium homeostasis, repairs bone, and adapts bone to stress.

Bone Deposition and Resorption

• Bone Deposition: Osteoblasts secrete organic and inorganic matrix components, leading to calcification.

• Bone Resorption: Osteoclasts secrete acids and enzymes to dissolve bone matrix, releasing minerals into the blood.

Factors Influencing Bone Remodeling

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

• Hormones: Testosterone promotes deposition; estrogen inhibits osteoclasts.

• Age: Hormone levels decline with age, reducing bone formation.

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

Calcium Ion Homeostasis

Calcium levels are regulated by negative feedback involving parathyroid hormone (PTH) and calcitonin.

• 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. A hematoma forms between bone fragments.

2. Fibroblasts and chondroblasts form a soft callus.

3. Osteoblasts build a bone callus of primary bone.

4. The bone callus is remodeled into secondary bone.

Types of Fractures

Fractures are classified based on the nature of the break and the involvement of surrounding tissues.

Type of Fracture	Description
Simple (Closed)	Skin and surrounding tissue remain intact
Compound (Open)	Bone pierces the skin; risk of infection
Transverse	Fracture line is perpendicular to the bone's long axis
Spiral	Fracture spirals around the bone
Comminuted	Bone is shattered into multiple pieces
Compression	Bone is crushed (often in vertebrae)
Greenstick	Bone breaks on one side and bends on the other (common in children)
Epiphyseal Plate	Fracture involves the growth plate

Treatment of Fractures

• Closed Reduction: Bone ends are aligned without surgery.

• Open Reduction: Surgical fixation with plates, wires, or screws.

• Immobilization for about 6 weeks is standard for healing.