AHE: BLOCK 2: MSK SYSTEM: INTRO & HISTOLOGY

Compact Bone - Microscopic Anatomy

• Osteons (Haversian systems): The basic structural unit of compact bone.
  • Central canal: Contains blood vessels and nerves to nourish the bone.
  • Concentric lamellae: Layers of bone matrix surrounding the central canal, giving the osteon its characteristic appearance.
  • Lacunae: Spaces between the lamellae that house osteocytes.
    • Canaliculi: Tiny channels that connect lacunae and allow for communication between osteocytes via cellular extensions.
  • Perforating (Volkmann's) canals: Connect central canals, providing a pathway for blood vessels and nerves to travel throughout the bone.
  • Circumferential lamellae: Located adjacent to the periosteum and endosteum, these lamellae help to strengthen the overall bone structure.
• Interstitial lamellae: Fill in the gaps left over from remodeled osteons, representing remnants of previous bone structures.

Trabecular Bone

• Located deep to compact bone, within the marrow cavity.
• Named for the needle-like projections called trabeculae, which make up the spongy structure.
• Lamellae are not arranged around central canals like in compact bone.
• Nutrients diffuse through canaliculi connected to capillaries in the endosteum, which surrounds the trabeculae.

Osteoid

• Related tissue to bone.

Periosteum

• Double-layered connective tissue membrane covering the outer surface of the bone.
  • Fibrous outer layer: Dense irregular connective tissue, anchoring tendons and ligaments to the bone.
  • Osteogenic inner layer: Contains osteogenic cells that create new bone cells.
    • Sharpey's fibers: Connect the periosteum to the bone.

Endosteum

• Internal connective tissue membrane lining the medullary cavity of the bone.

Bone Cells

• Osteogenic (osteoprogenitor) cells: Mitotically active stem cells located within the periosteum and endosteum.

  • They differentiate into osteoblasts and bone lining cells.

• Osteoblasts: Bone-building cells located on bone surfaces.

  • Synthesize and secrete osteoid (unmineralized organic matrix of bone).
  • Become osteocytes when surrounded by the matrix they create.

• Osteocytes: Mature bone cells responsible for bone maintenance.

  • React to strain or stress through deformation, loading, and weightlessness.
  • Stimulate osteoblasts and osteoclasts.
  • Located within lacunae.
  • Connect and communicate with other osteocytes, osteoblasts, and bone lining cells through protoplasmic projections.

• Bone lining cells: Flat cells on bone surfaces where remodeling is not occurring.

  • Help maintain bone matrix.
  • Called periosteal cells on the external bone surface and endosteal cells on the internal surface.

• Osteoclasts (osteophages): Bone-resorbing cells responsible for bone breakdown (osteolysis).

  • Large, motile, multinucleated cells that originate from bone marrow through fusion of monocytes.
  • Enzymes secreted by osteoclasts break down bone matrix.

Cartilage

• Tough, durable form of supporting connective tissue essential for mechanical and protective roles in the body.

Functions of Cartilage:

• Provides flexibility.
• Provides smooth, low-friction, gliding surfaces on articular surfaces (joints).

Components of Cartilage:

• Gelatinous extracellular matrix (ECM) with abundant fibers.
• Chondrocytes: Cartilage cells found within the ECM in spaces called lacunae.
• Perichondrium: Dense connective tissue covering that supplies cartilage with nutrients through diffusion.

Characteristics of Cartilage:

• Avascular: Cartilage lacks blood vessels and relies on diffusion for nutrient delivery.
• Lacks nerves: Cartilage is insensitive to pain.

Cartilage Types:

• Hyaline cartilage: Most common type, providing movement and support.

  • Sparse chondrocytes and collagen fibers.
  • Juvenile: Forms scaffolding for developing bone, allowing for longitudinal bone growth.
  • Adults: Found at the ends of bones in many joints, in the respiratory tract, and connecting ribs to the sternum.

• Elastic cartilage: Provides flexibility and stretching abilities.

  • Contains more elastin fibers than hyaline cartilage.
  • Found in the ear and epiglottis of the upper respiratory tract.

Test your knowledge on the microscopic structure of compact bone with this quiz. You'll explore terms like osteons, lacunae, and canaliculi, uncovering the intricate details of bone anatomy and physiology. Perfect for students studying biology or medicine!