Histology Chapter 8

1. Overview of Bone

1.1 Definition and Function

  • Bone is a specialized type of dense connective tissue with a mineralized extracellular matrix.

  • Characteristic features:

    • Provides support and protection.

    • Serves as a storage site for calcium and phosphate.

    • Plays a role in homeostatic regulation of blood calcium levels.

1.2 Composition of Bone Matrix

  • Major components:

    • Type I collagen: Approximately 90% of bone matrix proteins.

    • Other collagen types in trace amounts: Type V, III, XI, and XIII.

    • Non-collagenous proteins: 10% of bone matrix proteins, including:

      • Proteoglycans: Contribute to compressive strength.

      • Glycoproteins: Include osteonectin, podoplanin, and sialoproteins that facilitate cell attachment and matrix mineralization.

      • Bone-specific proteins: Vitamin K dependent proteins like osteocalcin and matrix Gla-protein (MGP).

      • Growth factors and cytokines: IGFs, BMPs, and cytokines play crucial roles in bone formation and maintaining bone health.

1.3 Cell Types in Bone

  • Osteoprogenitor cells: Stem cells that develop into osteoblasts.

  • Osteoblasts: Responsible for bone formation and matrix secretion, transforming into osteocytes when encased by matrix.

  • Osteocytes: Mature bone cells that maintain bone tissue, located in lacunae, communicate through canaliculi and gap junctions.

  • Bone-lining cells: Derived from osteoblasts; cover non-remodeling surfaces of bone.

  • Osteoclasts: Multinucleated cells responsible for bone resorption.

2. Structure of Bone

2.1 Types of Bone Tissue

  • Compact Bone: Dense layer forming the outer surface (about 80% of bone mass).

  • Spongy Bone: Composed of trabeculae, filled with marrow (about 20% of bone mass).

2.2 General Structure

  • Compact Bone: Organized in osteons (Haversian systems) with concentric lamellae around Haversian canals.

  • Spongy Bone: Forms a meshwork with spaces filled with marrow, critical for hematopoiesis.

  • Endosteum and Periosteum: Lining of cavities (endosteum) and outer sheath (periosteum), involved in growth and repair.

2.3 Classification of Bones by Shape

  • Long Bones: Diaphysis and epiphyses (e.g., femur).

  • Short Bones: Nearly equal in length and width (e.g., carpal bones).

  • Flat Bones: Thin and plate-like (e.g., skull).

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

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

3. Bone Development and Remodeling

3.1 Bone Formation

  • Intramembranous Ossification: Bone develops directly from mesenchymal tissue (e.g., flat bones).

  • Endochondral Ossification: Bone replaces cartilage precursor during development (e.g., long bones).

3.2 Growth and Remodeling

  • Epiphyseal Plate: Zones responsible for longitudinal growth.

  • Appositional Growth: Increasing bone diameter by layering from periosteum.

3.3 Bone Repair

  1. Fracture hematoma formation: Initial inflammatory response.

  2. Soft callus formation: Fibrocartilage stabilizes the fracture.

  3. Hard callus formation: Osteoblasts replace soft callus with bone.

  4. Bone remodeling: Osteoclasts and osteoblasts restore original structure.

4. Bone Physiology

4.1 Calcium Homeostasis

  • The skeleton stores approximately 99% of the body's calcium.

  • Blood calcium levels are regulated by parathyroid hormone (PTH) and calcitonin.

4.2 Joint Anatomy

  • Joints allow movement between bones and categorized into:

    • Fibrous joints: Immovable, connected by dense tissue.

    • Cartilaginous joints: Limited movement, connected by cartilage.

    • Synovial joints: Freely movable, characterized by joint cavities.