Bone Tissue

Bone Tissue: Comprehensive Study Notes

Skeletal System Functions

  • Provides essential structural support for the body.

  • Protects delicate internal organs (e.g., brain, heart, lungs).

  • Assists with body movements through articulation with muscles.

  • Serves as a vital reservoir for storing and releasing calcium and phosphorus, critical minerals for many physiological processes.

  • Participates in hematopoiesis, the production of blood cells (red blood cells, white blood cells, and platelets) in the red bone marrow.

  • Stores triglycerides (fats) in yellow bone marrow, serving as a chemical energy reserve.

Bone Structure

  • Diaphysis: The main shaft or body of a long bone.

  • Epiphysis: The distal and proximal ends of a long bone.

  • Metaphysis: The region where the epiphyses and the diaphysis join. In growing bones, it contains the epiphyseal growth plate.

  • Articular cartilage: A thin layer of hyaline cartilage covering the epiphysis where bones form joints, reducing friction and absorbing shock.

Periosteum

  • A tough connective tissue membrane that surrounds the external bone surface wherever it is not covered by articular cartilage.

  • Composed of two layers:

    • Outer fibrous layer: Contains dense irregular connective tissue.

    • Inner osteogenic layer: Abundant in osteogenic cells.

  • Functions:

    • Contains osteogenic cells vital for bone growth in diameter (appositional growth).

    • Assists significantly with fracture repair by providing new bone-forming cells.

    • Serves as an important attachment point for tendons (connecting muscle to bone) and ligaments (connecting bone to bone).

Critical Thinking: The Importance of the Periosteum

  • Historically, it was common practice in surgical fracture repair to remove the periosteum.

  • Modern surgical approach: Surgeons now take great care to avoid damaging the periosteum.

  • Explanation: The periosteum is crucial due to its osteogenic cells, which are necessary for bone healing and appositional growth. Damaging it compromises the bone's ability to repair itself effectively and grow in thickness, potentially leading to slower healing or poorer outcomes.

Internal Bone Structures

  • Medullary cavity (marrow cavity): The hollow, cylindrical space located within the diaphysis of a long bone.

    • In adults, it contains fatty yellow bone marrow.

  • Endosteum: A thin membrane that lines the medullary cavity.

    • Composed of osteogenic cells and a small amount of connective tissue.

    • Contains osteoblasts and osteoclasts which are involved in bone remodeling (resorption and deposition) from the inside.

Bone Tissue Characteristics

  • Bone tissue (osseous tissue) is a highly vascularized connective tissue.

  • It possesses a hard, mineralized extracellular matrix (ECM).

  • Two main types of bone tissue:

    • Compact bone: Found on the outside of bones, providing protection and support.

    • Spongy bone: Found on the inside of bones (especially epiphyses), providing lightweight support.

Comparison of Compact and Spongy Bone

  • Compact Bone:

    • Provides protection and structural support.

    • Forms the outer layer of the diaphysis and the external layer of all bones.

    • Very dense with few spaces.

  • Spongy Bone (Trabecular or Cancellous Bone):

    • Provides lightweight structure while still offering support.

    • Forms the core of the epiphysis and fills the internal cavity of short, flat, and irregular bones.

    • Less dense, consisting of a network of bone spicules called trabeculae.

Blood and Nerve Supply to Bone

  • Bone is highly vascularized:

    • Periosteal arteries: Supply blood to the periosteum and the outermost compact bone tissue.

    • Nerves: Accompany arteries into the bone, providing sensory innervation, which explains why bone injuries are painful.

    • Nutrient artery (one or more): Enters the diaphysis through a nutrient foramen, branching to supply the inner compact bone, spongy bone, and bone marrow.

    • Metaphyseal arteries: Supply the metaphysis region and the epiphyseal plate.

    • Epiphyseal arteries: Supply the epiphysis.

Cartilage Tissues

  • Cartilage is a connective tissue that is poorly vascularized, meaning it has limited blood supply.

  • Types of cartilage:

    • Hyaline cartilage: Most abundant type, found at the ends of long bones (articular cartilage), and in the nose, trachea, and fetal skeleton.

    • Fibrocartilage: Strongest type, containing visible collagen fibers; found in intervertebral discs, menisci of the knee, and pubic symphysis.

    • Elastic cartilage: Contains elastic fibers, providing flexibility; found in the epiglottis and external ear.

  • Perichondrium: A connective tissue membrane that surrounds the surface of most cartilage.

    • Contains chondrocytes (mature cartilage cells), which are responsible for maintaining the cartilage matrix.

    • Many major bones (especially long bones) initially form from a hyaline cartilage model before being replaced by bone (endochondral ossification).

Cytology: Cells in Osseous Tissues

  • Osteogenic (Osteoprogenitor) Cells:

    • Unspecialized bone stem cells.

    • Derived from mesenchymal stem cells (bone cell lineage).

    • Undergo cell division and differentiate into osteoblasts.

    • Found in the inner layer of the periosteum, the endosteum, and along blood vessels in osteons.

  • Osteoblasts: