Bone and Joint Biology

Bone and Joint Biology

Prepared by Dr. Alper DUNKI

• Bone Functions: Support, mineral homeostasis, marrow housing
• Bone Types: Long (endochondral), flat (intramembranous)
• Macrostructure:
  Cortical → dense, load-bearing
  Trabecular → porous, marrow-rich, weak in osteoporosis
• Cells:
  Osteoblasts (matrix formation, Runx2, osterix)
  Osteocytes (mechanosensors, RANKL secretion)
  Osteoclasts (resorption, RANKL/M-CSF, inhibited by OPG)
• Matrix: Mineral (HA, TCP) + collagen (type I) + growth factors (BMP, TGF-β, IGF)
• Bone Homeostasis: Balance of formation/resorption → disrupted in osteoporosis, osteopetrosis
• Fracture Healing:
  Primary → direct, stable fixation
  Secondary → hematoma, callus, endochondral ossification, remodelling
• Therapies: Bisphosphonates, PTH (intermittent), anti-RANKL, calcitonin
• Synovial Joint: Cavity, capsule, cartilage, synovium (Type A & B cells), synovial fluid (HA, lubricin); proprioception (A fibers), pain (C fibers).
• Non-Synovial Joints:
  Symphysis (fibrocartilage, e.g. pubic symphysis)
  Synchondrosis (cartilage-only, e.g. costal, cranial base)
  Syndesmosis (fibrous, e.g. distal tibiofibular)

  Bone

Functions: Provides mechanical support, regulates mineral homeostasis, harbors bone marrow elements.

Types:

• Long      bones: Formed via endochondral ossification from a cartilage model.
• Flat      bones: Formed via intramembranous ossification directly from mesenchymal tissue.

Anatomy:

• Diaphysis:     Cortical bone tube enclosing the medullary canal with trabecular bone;      surfaces consist of periosteum and endosteum.
• Metaphysis:     Transition zone between epiphysis and diaphysis; composed of loose      trabecular bone.
• Epiphysis:     Articular end containing subchondral bone and the growth plate.

Vascular and neural supply: Neurovascular bundles enter through the periosteum and run within Haversian and Volkmann canals. Inner two-thirds of cortical bone are supplied by the nutrient artery, while the outer one-third is nourished by periosteal vessels.

Macrostructure:

• Cortical bone: Dense, load-bearing; serves as boundary in metaphysis/epiphysis.
• Trabecular bone: Porous, marrow-containing; architecture compromised in osteoporosis.

Microstructure:

• Woven bone: Primary bone, irregular collagen alignment.
• Lamella bone: Secondary bone, organized structure.
• Lacunar–canalicular system: Provides osteocyte interconnections.

Extracellular matrix:

• Mineral      (60–70%): Hydroxyapatite, tricalcium phosphate; provides compressive      strength and mineral reservoir.
• Organic      (20–25%): 90% type I collagen, other collagens, adhesive proteins      (fibronectin, vitronectin), matrix proteins, proteoglycans, growth factors      (BMP, TGF-β, IGF).

Cells:

• Osteoblasts:     Synthesize bone matrix, regulate osteoclasts; differentiation via Runx2      and osterix.
• Osteocytes:     Mechanosensors, secrete RANKL, maintain bone homeostasis.
• Osteoclasts:     Multinucleated, perform bone resorption; activated by RANKL and M-CSF,      inhibited by OPG.

Bone homeostasis: Maintained by the balance of osteoblast and osteoclast activity. Renewal occurs via Howship’s lacunae in trabecular bone and osteons in cortical bone.

Disease and treatment:

• Remodeling      impaired in conditions like osteoporosis and osteopetrosis.
• Therapies:     Bisphosphonates, intermittent PTH, anti-RANKL agents, calcitonin.

Fracture healing:

• Primary      healing: Direct bone formation under stable fixation.
• Secondary      healing: Hematoma, inflammation, cartilage callus, endochondral      ossification, and remodeling.
• Growth      factors (BMP, TGF-β, IGF) and angiogenesis play critical roles.

2. Synovial Joint

Structure: Joint cavity, articular cartilage, capsule, ligaments, tendons.

Development: Arises from mesenchymal condensation; apoptosis within interzone forms the cavity.

Components:

• Articular      cartilage: Provides low-friction motion.
• Ligaments:     Provide stability.
• Capsule:     Encloses the joint.
• Synovium:     Contains type A cells (macrophage-like) and type B cells (fibroblast-like,      producing hyaluronan); provides nutrition and synovial fluid.
• Synovial      fluid: Plasma ultrafiltrate rich in hyaluronic acid and lubricin.

Innervation:

• Type      A fibers (proprioception).
• Type      C fibers (pain).

Function: Enables wide range of motion between bones with minimal friction.

3. Non-Synovial Joints

Types:

• Symphysis:     Fibrocartilaginous disc between bones (e.g., intervertebral disc, pubic      symphysis); stability, load transfer, limited mobility.
• Synchondrosis:     Cartilage-covered joint surfaces without synovium; limited motion (e.g.,      sternomanubrial, costal cartilage, cranial base).
• Syndesmosis:     Fibrous connection without cartilage interface; limited movement (e.g.,      distal tibiofibular joint).

References

1. Parini P, Canalis E, Schilling T. Bone remodeling: an operational process ensuring survival and function. Bone Res. 2022;10:8. doi:10.1038/s41413-022-00219-8

2. Sims NA, Gooi JH. Current perspectives on the multiple roles of osteoclasts. J Mol Endocrinol. 2024