Lecture 6: Cartilage, Bone, and Synovial Joints Flashcards
Cartilage composition
60-80% water, 15% collagens, 9% proteoglycans, 5% multiadhesive glycoproteins, 3-5% cells (mostly chondrocytes)
Functions of cartilage
-Semi-rigid, flexible but sturdy
-Compression resilient/shock absorbing (due to fluid)
-Avascular, no nerves
-Supportive
-Articular surface for joints
-Bone development forerunner
Types of cartilage
- Hyaline (“glassy”)
- Articular (joints)
- Fetal
- Elastic (elastin fibers)
- Fibrocartilage (Type 1 collagen)
Hyaline cartilage
“Default” cartilage. Primarily Type II collagen, glassy appearance due to same refraction between fibers and ground substance. Calcifies with age and has a perichondrium. Chondrocytes in isogenous groups with TM and IM.
Elastic cartilage
Hyaline cartilage with added elastin fibers for extra flexibility. Has a perichondrium. Chondrocytes arranged in singlets.
Fibrocartilage
Hyaline cartilage + added Type I collagen fibers. Less hydrated and stiffer, no perichondrium. Contains fibroblasts and sometimes columns of chondrocytes. Type I collagen fibers anchor into cartilage and connect with other CT.
Fetal cartilage
Early, developing cartilage. During chondrogenesis, mesenchyme condenses and differentiates central chondroblasts as cartilage matrix develops outwards. Superficial mesenchyme later becomes the perichondrion, with nearby cells retaining potency.
Perichondrium
Dense sheet-like CT that encloses hyaline and elastic cartilage compartments. Vascularized; provides nutrient supply for cartilage as nutrients diffuse through the fluid ground substance.
Chondrocytes
Mature chondroblasts that become trapped in lacunae in the matrix. Gather in isogenous groups (due to mitotic division). Primarily glycolytic with large, prominent Golgi and abundant RER (basophilic cytoplasm)
Chondroblasts
Chondrocyte precursors; squamous cells found near the perichondrium/margins. Divide and produce additional cartilage matrix
Territorial matrix (TM)
Cartilage matrix immediately surrounding cells; richer in ground substance and stains more basophilic due to this higher proteoglycan concentration.
Interterritorial matrix (IM)
Cartilage matrix further from chondrocytes; richer in Type II collagen and thus stains lighter than TM.
Capsular matrix
Not visible in LM. Region immediately around lacunae, very rich in Type VI collagen.
Regions of the perichondrium
-Outer fibrous region (more dense sheet like CT)
-Inner chondrogenic region (more cellular; contains chondroblasts)
Types of cartilage growth
- Appositional growth
- Interstitial growth
Appositional growth
Form of growth where new cartilage/bone is laid on an existing cartilage/bone surface. Occurs with differentiation of progenitor cells near the perichondrium/periosteum
Interstitial growth
Form of growth where new tissue is formed within existing tissue (bone/cartilage). Often comes with mitotic division of enclosed cells
How does damage affect cartilage? How does cartilage repair itself?
Cartilage has poor regenerative ability due to its slow metabolism and avascular nature. Perichondrial chondroblasts offer limited differential regeneration, perichondrial fibroblasts can generate scar tissue. Generally, injury can cause bone to replace cartilage and results in less resilient cartilage.
Sections of a long bone
Diaphysis: shaft, contains marrow cavity
Epiphysis: ends, often capped by articular cartilage
Metaphysis: connecting neck, area of growth plate
Composition of bone
Primarily mineralized, highly organized Type I collagen. Highly vascular with high regen capacity; crucial store of calcium and phosphate. Surrounded by periosteum and filled by endosteum/marrow.
Types of bone
Macro: compact (cortical) or spongy (cancellous, trabecular)
Micro: woven or lamellar
Bone cells and their origins
MSC: osteoprogenitors -> osteoblasts -> osteocytes, bone lining cells
HSC: monocytes -> inactive osteoclasts -> osteoclasts
Bone matrix composition
Inorganic component: hydroxyapatite mineral (calcium phosphate crystals)
Organic component: osteoid, secreted by osteoblasts
Osteoid
Organic component of bone secreted by osteoblasts. 90% Type I collagen; remainder is proteoglycans (hydration), glycoproteins (bind cells to fibers), osteocalcin and alkaline phosphatase (fix minerals) contained in matrix vesicles
Type I collagen in bone
Very eosinophilic; excreted as procollagen and assembles into fibrils. In bone, has periodicity, forming hole zones as spaces for hydroxyapatite crystal formation.
Matrix vesicles
Component of osteoid that contains alkaline phosphatase and osteocalcin to fix minerals into new bone. Calcium phosphate crystals populate hole zones and, at a critical point, the bone is mineralized.
Red marrow
Hematopoietic marrow
Yellow marrow
Fatty marrow; energy storage
Woven bone vs lamellar bone
Woven: new, immature bone. More cellular, less mineralized, more ground substance -> more basophilic
Lamellar: mature, remodeled bone. More mineralized/eosinophilic and arranged in organized lamellae
Osteons
AKA Haversian system. Lamellar structures only found in compact bone. Include central (Haversian) canals, concentric lamellae, osteocytes in lacunae oriented with lamellae, and canaliculi
Features of compact bone
Osteons, interstitial lamellae, perforating (Volkmann) canals, inner/outer circumferential lamellae
Features of trabecular bone
Trabeculae form according to stress/external forces and are lined by endosteum and often surrounded by thick layers of marrow. No osteons; lamellae follow the contours of the trabeculae. Provide a huge amount of surface area for calcium regulation. Constantly remodeled.
Composition of endosteum
Endosteum lines every inner bone surface and is a cellular layer of osteoprogenitors in a loose CT matrix that can differentiate into osteocytes/bone lining cells.
Features of periosteum
Periosteum lines all outer surfaces of bone. Composed of outer fibrous layer with dense sheet-like CT and Type I collagen Sharpey fibers perforating into the bone as anchors; as well as an inner cellular layer of loose CT, osteoprogenitors, osteoblasts, bone lining cells, and osteoclasts
Synovial joint components
-Articular cartilage
-Articular capsule
-Joint cavity
Articular cartilage
Specialized hyaline cartilage without a perichondrium that lines the epiphyseal ends of bones, facing the joint cavity. Type II collagen arches.
Articular capsule
CT capsule continuous with periosteum; composed of outer fibrous layer (dense CT) and inner synovial membrane
Joint cavity
Space between articular cartilage enclosed by articular capsule. Filled with synovial fluid for nutrition, cushioning, lubrication
Layers of articular cartilage
- Superficial zone (articular surface)
- Intermediate zone
- Deep zone
- Calcified cartilage
Superficial zone of articular cartilage
Lines joint cavity with elongated cells and fibers running parallel to the surface
Intermediate zone of articular cartilage
Contains rounded cells with arched fibers
Deep zone of articular cartilage
Cells are aligned in columns; fibers run perpendicular to the surface with some cell proliferation here.
Calcified cartilage of articular cartilage
Borders bone. Acellular or contains very small chondrocytes
Synovial membrane
Produces synovial fluid. Composed of intimal layer and subintimal layer
Intimal layer of synovial membrane
Contacts the joint cavity. Contains synoviocytes: CT cells without cell junctions or a basement membrane
Types of synoviocytes
Type A: macrophage-like, remove debris from synovial fluid
Type B (majority): fibroblast-like, make hyaluronic acid/synovial fluid
Subintimal layer of synovial membrane
Loose CT deep to the intimal layer; contains many fenestrated capillaries and some Type B synoviocytes
Nutrient foramen
Large channels running across bone for large vessels to access the marrow cavity
