Structural Cells: Bone & Cartilage - Summary

Connective Tissue

  • Connective tissue supports the human body.
  • Composition:
    • Specialized cells (chondrocytes, osteocytes, adipocytes, fibroblasts).
    • Extracellular matrix (fibers and ground substance).
    • Fibers: collagen, elastin, reticular.
    • Ground substance: macromolecules, proteoglycans, water.
  • Extracellular matrix is the dominant component.

Cartilage

  • Composed of chondrocytes embedded in an extracellular matrix (collagen, proteoglycans, glycosaminoglycans).
  • Avascular and aneural.
  • Types:
    • Hyaline: strong, resists compression; collagen type II.
    • Elastic: strong, flexible, elastic; collagen type II, elastic fibers.
    • Fibrous: strong, tough, resists compression; collagen type II, collagen type I.
  • Locations:
    • Hyaline: respiratory tract, articular surfaces, costal cartilage, developing skeletal structures.
    • Elastic: external ear, epiglottis.
    • Fibrous: intervertebral discs, pubic symphysis.

Bone

  • Composed of osteocytes embedded in a calcified extracellular matrix.
  • Rich neurovascular supply.
  • Extracellular matrix organized into osteons with a central canal.
  • Matrix components: collagen, unique bone proteins, proteoglycans, hydroxyapatite crystals (calcium phosphate).
  • Specialized cells:
    • Osteoblasts: form new bone, secrete uncalcified matrix (osteoid), regulate calcification by releasing Ca2+Ca^{2+} and P3−P^{3-}.
    • Osteocytes: encased in matrix, maintain mineral/protein composition, connect via cytoplasmic processes.
    • Osteoclasts: break down and resorb bone, secrete acids and enzymes.
  • Functions: structural support, protection, muscle attachment, mineral storage.
  • Locations: axial and appendicular skeleton.

Developmental Origins

  • Mesenchymal cells: multipotent stem cells giving rise to connective tissues.
  • Lineages:
    • Paraxial mesoderm: axial skeleton (vertebral column, ribs, some craniofacial).
    • Lateral plate mesoderm: appendicular skeleton (limbs).
    • Cranial neural crest cells: some craniofacial structures.

Chondrogenesis

  • Process of cartilage development from mesenchymal stem cells.
  • Steps: condensation, proliferation, aggregation, changes in gene expression.

Ossification

  • Bone tissue formation, replacing pre-existing connective tissue.
  • Types:
    • Intramembranous ossification: mesenchyme replaced by bone (some craniofacial bones, clavicle).
    • Endochondral ossification: hyaline cartilage replaced by bone (most axial and appendicular skeleton).
  • Endochondral Ossification Steps:
    • Chondrocyte hypertrophy and apoptosis.
    • Blood vessel invasion and primary ossification center formation.
    • Secondary ossification center formation.
    • Cartilage matrix acts as scaffold for bone deposition.
  • Epiphyseal Growth Plates
    • Hyaline cartilage plates that persist through childhood/adolescence.
    • Chondrocyte proliferation drives longitudinal bone growth until adulthood.
    • Zones: resting, proliferating, hypertrophic, calcified, ossification.
  • Disorders: Achondroplasia (reduced chondrocyte proliferation), limb length discrepancies.

Regenerative Capacity

  • Bone: remarkable repair potential via hematoma formation, callus formation, and remodeling.
  • Cartilage: limited repair potential; damaged hyaline cartilage replaced by fibrocartilage (scar tissue) due to limited chondrocyte migration and lack of neurovascular supply.