Lecture on Bone Cell Types and Bone Structure

Major Bone Cell Types

• Osteogenic Cells

  • Stem cells found in periosteum and endosteum.
  • Also known as osteoprogenitor cells.
  • Mitotically active and differentiate into osteoblasts or bone lining cells.
  • Activated by bone damage, aiding in bone repair.

• Osteoblasts

  • Bone-forming cells derived from osteogenic cells.
  • Secrete osteoid (collagen and calcium-binding proteins).
  • Actively mitotic and responsible for bone growth.

• Osteocytes

  • Mature bone cells trapped in lacunae.
  • Maintain the bone matrix and act as stress sensors.
  • Help in bone remodeling by communicating with other cells.

• Bone Lining Cells

  • Flat cells on the bone surface, maintaining the outer matrix.
  • Known as periosteal cells externally and as endosteal cells internally.

• Osteoclasts

  • Multinucleated cells derived from hematopoietic stem cells.
  • Involved in bone resorption by breaking down bone matrix.
  • Form resorption bays and increase surface area for enzyme action.

Structure of Compact Bone

• Osteons (Haversian Systems)

  • Structural unit of compact bone, consisting of elongated cylinders.
  • Made of concentric rings of lamellae with collagen fibers.
  • Resists stress and is stronger than steel in tensile strength.

• Components of Osteons

  • Central Canal (Haversian Canal): Contains blood vessels and nerve fibers.
  • Canaliculi: Small canals that transport nutrients and connect lacunae.
  • Perforating Canals (Volkmann's Canals): Connect blood and nerves of central canals.

• Lamellae Types

  • Interstitial Lamellae: Fill spaces between osteons.
  • Circumferential Lamellae: Surround the entire bone.

Spongy Bone

• Appears disorganized but aligns along lines of stress.
• Trabeculae confer strength and are interspersed with lamellae and osteocytes.

Bone Composition

• Organic Components

  • Include cells and osteoid, which is one-third of the bone matrix.
  • Osteoid contains collagen and calcium-binding proteins, providing tensile strength and flexibility.
  • Sacrificial bonds between collagen molecules absorb shock and prevent fractures.

• Inorganic Components

  • Hydroxyapatites make up 65% of bone mass.
  • Consist of calcium phosphate crystals surrounding collagen fibers.
  • Responsible for bone hardness and compression resistance.
  • Allow bones to endure and provide insight into ancient diets and environments.