Bones and Bone Tissue

Functions of the Skeletal System

The skeletal system is essential for multiple physiological functions and structural support in the human body. It consists of bones, joints, and supporting tissues.

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

• Mineral Storage and Acid-Base Homeostasis: Bone stores minerals (calcium, phosphorus, magnesium) critical for electrolyte and acid-base balance.

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

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

• Movement: Bones serve as attachment sites for muscles, enabling movement at joints.

• Support: The skeleton provides structural framework and supports body weight.

Classification of Bones by Shape

Bones are classified based on their shape, which relates to their function and location in the body.

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

• Short Bones: Roughly cube-shaped (e.g., wrist, 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

Long bones have a specialized structure to support their functions.

• Periosteum: Dense irregular connective tissue membrane covering the bone, containing blood vessels and nerves.

• Perforating Fibers: Collagen anchors that attach periosteum to bone matrix.

• Diaphysis: Shaft of the bone, containing the medullary cavity lined by endosteum and filled with marrow.

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

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

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

• Epiphyseal Lines: Remnants of growth plates (hyaline cartilage) in adults.

Structure of Short, Flat, Irregular, and Sesamoid Bones

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

Bone Marrow

Bone marrow is a vital tissue found within bones, responsible for blood cell production and fat storage.

• Red Bone Marrow: Contains hematopoietic cells; prevalent in children, limited to specific bones in adults.

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

Bone Marrow Transplantation

Bone marrow transplantation is used to treat diseases like leukemia and sickle-cell anemia. Donor marrow is harvested and transplanted after recipient marrow is destroyed. Peripheral Blood Stem Cell (PBSC) donation is an alternative method.

The Extracellular Matrix of Bone

Inorganic Matrix

The inorganic matrix makes up about 65% of bone weight and consists mainly of calcium and phosphorus salts, forming hydroxyapatite crystals:

• Formula: $ Ca_{10}(PO_4)_6(OH)_2 $

• Provides strength and resistance to compression.

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

Organic Matrix (Osteoid)

The organic matrix comprises about 35% of bone weight and includes:

• Collagen fibers: Resist torsion and tensile forces.

• Proteoglycans, glycosaminoglycans, glycoproteins: Draw water and bind matrix components.

• Osteocalcin: Binds calcium and organizes the matrix.

Bone Cells

Types of Bone Cells

Bone is a dynamic tissue, constantly remodeled by specialized cells:

• Osteoblasts: Build bone by secreting matrix; mature into osteocytes.

• Osteocytes: Maintain bone matrix; reside in lacunae.

• Osteoclasts: Break down bone matrix; large, multinucleated cells.

Osteoblasts and Osteocytes

Osteoblasts are derived from osteogenic cells and deposit bone matrix. When trapped in matrix, they become osteocytes, which maintain the ECM and recruit osteoblasts for repair.

Osteoclasts

Osteoclasts resorb bone by secreting hydrogen ions and enzymes, releasing minerals and organic components into the blood.

Osteopetrosis

Osteopetrosis is a disease caused by defective osteoclasts, resulting in increased bone mass but weak, brittle bones. Infantile and adult forms differ in severity and symptoms.

Histology of Bone

Compact Bone

Compact bone is organized into osteons (Haversian systems):

• Lamellae: Concentric rings of bone matrix; resist twisting.

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

• Lacunae: Small cavities housing osteocytes.

• Canaliculi: Tiny canals connecting lacunae for cell communication.

• Interstitial and Circumferential Lamellae: Strengthen bone.

• Perforating (Volkmann) Canals: Connect osteons and carry blood vessels.

Spongy Bone

Spongy bone consists of branching trabeculae, housing osteocytes and providing access to blood supply from bone marrow.

Bone Formation: Ossification

Ossification (Osteogenesis)

Ossification is the process of bone formation, occurring in two main forms:

• Intramembranous Ossification: Forms flat bones from mesenchymal membranes.

• Endochondral Ossification: Forms long and short bones from hyaline cartilage models.

Steps of Intramembranous Ossification

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

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

• Osteoblasts lay down trabeculae of early spongy bone; periosteum forms.

• Osteoblasts in periosteum lay down early compact bone; matrix is remodeled.

Steps of Endochondral Ossification

• Chondroblasts in perichondrium differentiate into osteoblasts.

• Osteoblasts build bone collar; internal cartilage calcifies and chondrocytes die.

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

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

Comparison: Intramembranous vs. Endochondral Ossification

Bone Diseases and Disorders

Osteoporosis

Osteoporosis is caused by inadequate inorganic matrix, making bones brittle and prone to fractures. Risk factors include diet, age, sex, exercise, hormones, genetics, and certain diseases. Prevention includes adequate calcium and vitamin D, exercise, and medications.

Achondroplasia

Achondroplasia is the most common cause of dwarfism, resulting from abnormal growth factor receptors affecting endochondral ossification and cell division in the epiphyseal plate.

Bone Growth

Longitudinal Growth

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

• Zone of Reserve Cartilage: Inactive cells, potential for division.

• Zone of Proliferation: Actively dividing chondrocytes.

• Zone of Hypertrophy and Maturation: Mature chondrocytes.

• Zone of Calcification: Dead, calcified chondrocytes.

• Zone of Ossification: Osteoblasts build bone.

Appositional Growth

Appositional growth increases bone width. Osteoblasts lay down new circumferential lamellae, thickening the compact bone, while osteoclasts enlarge the medullary cavity.

The Role of Hormones in Bone Growth

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

• Testosterone: Promotes appositional growth and closure of epiphyseal plates in males.

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

Gigantism and Acromegaly

Excess growth hormone before epiphyseal plate closure causes gigantism; after closure, it causes acromegaly, leading to enlarged bones and soft tissues.

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, replaces old bone, and adapts to stress.

Bone Deposition

Osteoblasts secrete matrix components and facilitate calcification by binding calcium ions to collagen fibers.

Bone Resorption

Osteoclasts secrete hydrogen ions and enzymes to break down the matrix, releasing minerals and organic molecules for reuse.

Bone Remodeling in Response to Tension and Stress

• Compression and Tension: Stimulate bone deposition.

• Pressure: Stimulates bone resorption.

Other Factors Influencing Bone Remodeling

• Hormones: Testosterone promotes deposition; estrogen inhibits osteoclasts.

• Age: Hormone levels decline, reducing bone remodeling.

• Nutrient Intake: Calcium, vitamin D, K, C, and protein are essential for bone health.

Calcium Ion Homeostasis

Calcium ions are vital for muscle contraction, nerve transmission, and blood clotting. Blood calcium is regulated by hormones:

• Parathyroid Hormone (PTH): Increases blood calcium by stimulating bone resorption.

• Calcitonin: Decreases blood calcium by promoting bone deposition.

Bone Repair

Steps of Fracture Healing

• Hematoma Formation: Blood fills the gap between bone fragments.

• Soft Callus Formation: Fibroblasts and chondroblasts produce connective tissue and cartilage.

• Bone Callus Formation: Osteoblasts lay down primary bone.

• Bone Remodeling: Primary bone is replaced with secondary bone.

Types of Fractures

Treatment of Fractures

• Simple (Closed) Fractures: Skin and tissue remain intact.

• Compound (Open) Fractures: Surrounding tissue is damaged.

• Closed Reduction: Bone ends are brought into contact.

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