Connective Tissue - Bone, Osteo & Cartilage

• Cartilage

  • Specialised connective tissue
  • Chondrocytes found in lacunae
  • Cartilage is poorly vascularised and aneural
  • May or may not be surrounded by perichondrium
• Extracellular matrix (ECM) of cartilage

  • Characterised by type II collagen
  • Large ratio of glycosaminoglycans (GAGs) to type II collagen fibres
  • Permits diffusion of substances between blood vessels and chondrocytes
  • Chondrocytes produce and maintain the ECM
• Cartilage ECM - GAGs

  Highly negatively charged, attract cations and water, inflexible and strongly hydrophilic
• Types of cartilage

  • Hyaline cartilage
  • Elastic cartilage
  • Fibrocartilage
• Hyaline cartilage

  • Matrix contains type II collagen, GAGs, proteoglycans, and multiadhesive glycoproteins
• Elastic cartilage

  • Contains elastic fibres and elastic lamellae in addition to hyaline cartilage matrix
• Fibrocartilage

  • Contains abundant type I collagen fibres as well as hyaline cartilage matrix
• Hyaline cartilage ECM

  • Capsular (pericellular) matrix: ring of more densely staining matrix located immediately around the chondrocyte
  • Territorial matrix: region that is more removed from the immediate vicinity of the chondrocytes. Surrounds the isogenous group and stains less intensely than the capsular matrix
  • Interterritorial matrix: region that surrounds the territorial matrix and occupies the space between groups of chondrocytes
• Perichondrium

  Firmly attached dense connective tissue surrounding some cartilage, source of new cartilage cells

  
• Fibrocartilage ECM

  • Combination of dense regular connective tissue and hyaline cartilage, chondrocytes dispersed among collagen fibres
• Chondrogenesis

  Development of new cartilage from mesenchyme

  • Mesenchymal chondroprogenitor cells differentiate into chondroblasts which secrete cartilage matrix

  • When they are completely surrounded by matrix material, the cells are called chondrocytes.

  • The mesenchymal tissue immediately surrounding the chondrogenic nodule gives rise to the perichondrium.

  
• Cartilage growth- Expansion of existing cartilage


  1. Appositional growth (new cartilage at surface)

  2. Interstitial growth (new cartilage within existing cartilage)
• Cartilage Repair

  • Cartilage has limited ability for repair
  • Lack of response to injury is attributable to the avascularity of cartilage, the immobility of the chondrocytes, and the limited ability of mature chondrocytes to proliferate.
  • Some repair can occur but only if the defect involves the perichondrium.
  • Repair mostly involves the production of dense connective tissue
• Bone

  Provides support and protection, and regulates blood calcium levels
• Constituents of bone

  • Bone tissue (structural component)
  • Hemopoietic tissue
  • Fat tissue
  • Blood vessels
  • Nerves
  • Hyaline cartilage at synovial joints

  
• Bone tissue

  • Compact bone (dense layer)
  • Spongy bone (thin, anastomosing spicules)

  
• Extracellular matrix of bone

  • Mineralised matrix=hard tissue
  • Calcium phosphate forming hydroxyapatite crystals
  • Storage site for calcium and phosphate
  • Collagen proteins (mainly Type I collagen)=90% bone matrix
  • Ground substance (~10% bone matrix) containing non-collagenous proteins
  • Lacunae connected by canaliculi

  
• Bone cells

  • Osteoprogenitor cells
  • Osteoblasts
  • Bone-lining cells
  • Osteocytes
  • Osteoclasts

  
• Bone cell types - Osteoprogenitor cells

  • Precursors for osteoblasts
  • Derived from mesenchymal stem cells (MSC) in the bone marrow
  • Differentiate into bone-forming cells in response to molecular stimuli
  • Located in the innermost layer of periosteum as periosteal cells, lining of marrow & osteonal cavity & volkman's canal

  
• Bone cell types - Osteoblasts

  • Cuboidal or polygonal cells with basophilic cytoplasm
  • Synthesize and secrete type I collagen fibers
  • Synthesize bone matrix proteins (BMPs) which form the unmineralized bone matrix called OSTEOID
  • Responsible for calcification of bone matrix
  • Located at the surface of newly synthesized bone

  
• Bone cell types - Bone lining cells

  • Derived from osteoblasts
  • Flat cells with few organelles in cytoplasm
  • Cover non-remodelling bone surfaces
  • Periosteal cells line outer surface of bone; endosteal cells line internal bone surface

  
• Functions of bone lining cells

  • Maintenance and nutritional support of underlying osteocytes
  • Regulate influx/efflux of calcium and phosphate ions

  
• Osteocytes

  • Derived from osteoblasts
  • Mature bone cells enclosed by bone matrix
  • Maintain bone matrix
  • Mechanotransduction
  • Can synthesize and degrade bone matrix

  
• Osteoclasts

  • Large, multinucleated cells derived from mononuclear hemopoietic progenitor cells
  • Found at the site of active bone resorption within Howship's lacuna
  • Resorption of the bone matrix by releasing protons and lysosomal hydrolases
  • Phagocytoses degrading bone

  
• Osteoclast structure

  • Ruffled border
  • Clear (Sealing) zone
  • Basolateral region
• Periosteum

  Sheath of dense fibrous CT containing osteoprogenitor cells
• Periosteum in active growing bone

  • Outer fibrous layer: composed of dense CT arranged parallel to bone surface
  • Inner cellular layer: containing osteoprogenitors and periosteal cells

  
• Periosteum in bone formation not in progress

  Fibrous layer predominant

  
• Endosteum

  Single layer of osteoprogenitor cells that can differentiate into bone-secreting cells

  
• Bone marrow

  • Red bone marrow: consist of developing blood cells
  • Yellow marrow: filled with fat cells

  
• Bone classification

  • Cancellous (spongy) bone
  • Compact bone

  
• Compact bone

  • Composed of structural units called osteons (Haversian systems)
  • Also called lamellar bone due to organization of lamellar in the ECM

  
• Lamellae in compact bone

  • Concentric lamellae: located within each osteon
  • Interstitial lamellae: between osteons; remnants of previous concentric lamellae
  • Circumferential lamellae: follows the entire inner of outer circumference of the shaft of long bone

  
• Volkmann's canals(AKA perforating canals)

  Channels in lamellar bone through which blood vessels and nerves travel between osteonal canals
• Haversian system/Osteon

  • Central Haversian/osteonic canal
  • Contains blood vessels and nerves
  • Lined by endosteal cells
  • Osteocytes in lacunae
  • Canaliculi containing processes of osteocytes
• Cement line

  • Dark ring around the circumference of the osteons
  • Region of collagen-poor matrix that defines the borders between neighbouring osteons
• Cancellous bone

  • Tissue arranged as trabeculae and spicules
  • Numerous interconnecting marrow spaces of various sizes between bone tissue lined by endosteum
  • No Harvesian systems
  • The matrix is lamellated

  
• Immature bone

  • No lamellar =woven / bundle bone
  • Interlacing arrangement of collagen fibers
  • More cellular than mature bone
  • Contains more ground substance and less minerals than mature bones

  
• In response to appropriate stimuli, such as extreme blood loss, yellow marrow can revert to red marrow. In the adult, red marrow is normally restricted to the spaces of spongy bone in a few locations such as the sternum and the iliac crest. Diagnostic bone marrow samples and marrow for transplantation are obtained from these sites.
• Intramembranous ossification

  • Develops directly from mesenchymal cells E.g. Flat bones of the skull & face
  • Mesenchymal cells -> Osteoprogenitor cells -> Osteoblasts -> Osteocytes
  • Osteoblasts secrete osteoid -> Osteoid calcified = bone -> Bone remodelled (Osteoclast?) -> Mature bone