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Alper DUNKI

Bone and Joint Biology

  • 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.

  • Lamellar      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).

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

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