Bone Structure and Cell Types

Cell Types of Bone

• Osteoprogenitor cells: undifferentiated cells that can divide to replace themselves and become osteoblasts; found in the inner layer of periosteum and endosteum.
• Osteoblasts: form bone matrix and collagen fibers, but cannot divide.
• Osteocytes: mature cells that no longer produce bone matrix.
• Osteoclasts: huge cells formed from fused monocytes (WBC) that function in bone resorption at surfaces such as endosteum.

Bone Matrix

• Compact bone: looks solid and hard, making up the shaft of long bones and the external layer of all bones; resists stresses produced by weight and movement.
• Osteon parts:
  • Central canal: a tube that blood vessels go through.
  • Lamella: concentric rings of calcified bone matrix.
  • Lacunae: osteocytes found in spaces.
  • Canaliculi: canals that transport materials to and from osteocytes.
  • Interstitial lamellae: remainders of old osteons.

Bone Remodeling

• Osteoclasts carve out small tunnels, and osteoblasts rebuild osteons.
• Fracture repair:
  • Formation of fracture hematoma to stop bleeding (6-8 hours).
  • Inflammation brings in phagocytic cells for cleanup duty.
  • New capillaries grow into the damaged area.
  • Formation of bony callus: osteoblasts secrete spongy bone that joins the two broken ends of bone (lasts 3-4 months).
  • Bone remodeling: osteoclasts burrow out, and osteoblasts rebuild.

Calcium Homeostasis

• Parathyroid hormone (PTH) is secreted if calcium blood levels are too low.
• Calcitonin is secreted if calcium blood levels get too high.

Bones/Locations/Functions

• Cranial bones:
  • Frontal bone: forms the forehead and anterior part of the cranial floor.
  • Supraorbital foramen: passage for blood vessels for the eyebrow and eyelids.
  • Ethmoid bone: forms the inferior, central portion of the anterior cranial floor.
  • Cribriform plate: roof of the nasal cavity through which olfactory nerves pass to the brain.

Classification of Joints

• Functional classification:
  • Synarthrosis: immovable joints.
  • Amphiarthrosis: slightly movable joints.
  • Diarthrosis: freely movable joints.
• Structural classification:
  • Fibrous joints: collagen fibers.
  • Sutures (no movement; synarthrosis): examples include sutures.
  • Syndesmosis (some movement; amphiarthrosis): examples include ulna/radius and tib/fib.
  • Gomphoses (some movement; amphiarthrosis): examples include teeth.
  • Cartilaginous joints: made of cartilage, with little to no movement.