Module 6.1: Introduction to Bones as Organs

The Skeletal System

The skeletal system consists of bones, joints, and supporting tissues. Bones are made of osseous tissue and other connective tissues, including bone marrow.

• Osseous tissue: The primary tissue of bone, providing strength and structure.

• Bone marrow: Soft tissue inside bones responsible for blood cell production.

Functions of the Skeletal System

Bones perform several essential functions in the body:

• Protection: Shields vital organs (e.g., skull protects the brain).

• Mineral Storage and Acid-base Homeostasis: Stores minerals such as calcium and phosphorus; helps regulate blood pH.

• Blood Cell Formation: Occurs in red bone marrow (hematopoiesis).

• Fat Storage: Yellow bone marrow stores lipids.

• Movement: Bones act as levers for muscles.

• Support: Provides structural framework for the body.

Classification of Bones by Shape

Bones are classified into five types based on their shapes:

• Long bones: Longer than they are wide (e.g., femur, humerus).

• Short bones: Nearly equal length and width (e.g., carpals).

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

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

• Sesamoid bones: Embedded in tendons (e.g., patella).

Structure of a Long Bone

Long bones have several distinct features:

• Periosteum: Dense connective tissue covering the bone's surface.

• Perforating fibers (Sharpey's fibers): Anchor periosteum to bone.

• Diaphysis: Shaft of the bone.

• Epiphyses: Ends of the bone.

• Marrow cavity: Central cavity containing bone marrow.

• Compact bone: Dense outer layer.

• Spongy bone: Porous inner layer.

• Endosteum: Membrane lining the marrow cavity.

• Epiphyseal lines/plates: Sites of bone growth.

Module 6.2: Microscopic Structure of Bone Tissue

Bone Matrix

The bone matrix is composed of inorganic and organic components:

• Inorganic matrix: Mainly calcium salts (about 65% of bone's weight), including hydroxyapatite crystals ().

• Organic matrix: Collagen fibers, proteoglycans, glycosaminoglycans, glycoproteins, and bone-specific proteins (about 35% of bone's weight).

Bone Cells

Three main types of bone cells regulate bone dynamics:

• Osteoblasts: Build bone by depositing bone matrix.

• Osteocytes: Mature bone cells that maintain bone matrix, residing in lacunae.

• Osteoclasts: Break down bone matrix (bone resorption).

Histology of Bone Tissue

Bone tissue is organized into two main types:

• Compact bone: Dense, organized into osteons (Haversian systems).

• Spongy bone: Porous, composed of trabeculae.

Osteon Structure

• Osteon: Cylindrical unit of compact bone.

• Lamellae: Concentric rings of bone matrix.

• Central canal: Contains blood vessels and nerves.

• Canaliculi: Tiny channels connecting osteocytes.

Other Features of Compact Bone

• Interstitial lamellae: Fill spaces between osteons.

• Circumferential lamellae: Encircle the bone beneath the periosteum.

• Perforating (Volkmann's) canals: Connect central canals of osteons.

Spongy Bone Structure

• Trabeculae: Lattice-like network providing strength and housing bone marrow.

Module 6.3: Bone Formation (Ossification)

Ossification

Bone formation, or ossification (osteogenesis), occurs during embryonic development and childhood.

• Intramembranous ossification: Forms flat bones (e.g., skull, clavicles) from mesenchymal tissue.

• Endochondral ossification: Forms most bones by replacing hyaline cartilage with bone.

Process of Ossification

• Primary ossification center forms first, followed by secondary centers.

• Bone growth occurs at the epiphyseal plate (growth plate).

Module 6.4: Bone Growth in Length and Width

Growth in Length

Long bones grow in length by longitudinal growth at the epiphyseal plate.

• Epiphyseal plate 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 is ossified (usually by age 18-21).

Growth in Width (Appositional Growth)

• Osteoblasts in the periosteum lay down new bone on the surface.

• Bones thicken even after lengthwise growth has stopped.

Hormonal Regulation of Bone Growth

• Growth hormone: Stimulates cell division and osteogenic activity.

• Testosterone: Increases bone growth and appositional growth (thicker bones in males).

• Estrogen: Increases longitudinal bone growth and accelerates closure of the epiphyseal plate.

Module 6.5: Bone Remodeling and Repair

Bone Remodeling

Bone is continuously remodeled through bone deposition (by osteoblasts) and bone resorption (by osteoclasts).

• Remodeling adapts bone to stress and repairs microdamage.

• Factors influencing remodeling:

  • Hormones

  • Age

  • Calcium ion intake

  • Vitamin D, C, and K intake

  • Protein intake

Bone Remodeling and Calcium Ion Homeostasis

Bone stores most of the body's calcium, which is essential for many physiological processes. Homeostasis is maintained by a negative feedback loop:

• Low blood calcium is detected by parathyroid glands.

• Parathyroid hormone (PTH) is secreted, stimulating osteoclasts to resorb bone and release calcium.

• When calcium returns to normal, PTH secretion decreases.

• Calcitonin (from the thyroid) lowers blood calcium by inhibiting osteoclasts.

• Vitamin D increases calcium absorption from the gut.

Bone Repair

Bones can be injured, most commonly by fractures. The repair process involves several steps:

1. A hematoma fills the gap between bone fragments.

2. Fibroblasts and chondroblasts infiltrate the hematoma and form a soft callus.

3. Osteoblasts build a bone callus.

4. The bone callus is remodeled and replaced with secondary bone.

5. Bone repair generally takes 6-8 weeks.

Classification of Fractures

• Simple fractures: Bone breaks cleanly without piercing the skin.

• Compound fractures: Bone breaks and pierces the skin.

Summary Table: Bone Cell Functions

Key Equations

• Hydroxyapatite formula:

Additional info:

• Bone remodeling is essential for maintaining bone strength and mineral homeostasis throughout life.

• Hormonal regulation is critical for proper bone growth and development, especially during puberty.