Hyaline Cartilage, Types and Characteristics

Cartilage

• Tough and resilient, cartilage supports soft tissues
• Cushioned, low-friction surfaces in joints provided by cartilage
• Cartilage cells comprised of chondroblasts and chondrocytes
• The cartilage extracellular matrix (ECM) includes collagen and proteoglycans
• Aggrecan binds large amounts of water

Cartilage Characteristics

• Cartilage lacks blood vessels, lymphatics, and nerves
• Perichondrium, a dense connective tissue, usually surrounds cartilage and is vascularized

Forms of Cartilage

• There are 3 forms of cartilage: hyaline, elastic, and fibrocartilage

Hyaline Cartilage

• The most common type of cartilage, it is homogeneous and semitransparent in the fresh state
• In adults, it's located in articular surfaces of movable joints
• Hyaline cartilage is also located in the walls of larger respiratory passages (nose, larynx, trachea, bronchi)
• Hyaline cartilage is located in the ventral ends of the ribs, epiphyseal plates of long bones, and as a temporary skeleton in the embryo
• The ECM has less collagen and more proteoglycan immediately around the lacunae -This produces slight staining differences in this territorial matrix.
• Chondrocytes occur singly or in small, mitotically derived isogenous groups
• Perichondrium is usually present, except at the hyaline cartilage of articular surfaces or the epiphyses of growing long bones.

Hyaline Cartilage Matrix

• The matrix is 40% collagen embedded in a firm, hydrated gel of proteoglycans and glycoproteins
• Type II collagen is present
• Aggrecan, the most abundant proteoglycan in HC, is present
• Chondronectin, a multiadhesive glycoprotein, is present
• Chondronectin binds specifically to GAGs, collagen, integrins to mediate chondrocyte adherence to the ECM

Perichondrium

• Essential for growth and maintenance of cartilage
• Outer region consists largely of collagen
• Type I fibers and fibroblasts
• The inner layer adjoining the cartilage matrix contains mesenchymal stem cells
• Mesenchymal stem cells provide a source for new chondroblasts that divide and differentiate into chondrocytes.

Elastic Cartilage

• Resembles hyaline cartilage in chondrocytes and major ECM components
• Matrix includes abundant fibers, visible with special stains, which increase flexibility
• Possesses a perichondrium.
• Elastic cartilage provides flexible support for the external ear
• It also supports structures of the middle ear, the epiglottis and the larynx

Fibrocartilage

• Contains combinations of hyaline cartilage in small amounts of dense connective tissue
• Provides strong support at tendon insertions, in intervertebral discs and certain joints
• Consists of small chondrocytes in a scant matrix with thick, parallel bundles of type I collagen fibers
• Found in areas requiring support and tensile strength
• Lacks a perichondrium

Cartilage Formation

• All forms of cartilage form from embryonic mesenchyme
• Cartilaginous structures grow by mitosis of existing chondroblasts in lacunae
• This is known as "interstitial growth" and is important in increasing length of long bones
• New chondroblasts form peripherally from progenitor cells in the perichondrium
• This is "appositional growth" and is important in postnatal development
• Repair or replacement of injured cartilage is very slow and ineffective
• This is due in part to the tissue's avascularity and low metabolic rate

Bone

• Bone/Osseous tissue, a specialized type of connective tissue
• Characterized by a calcified extracellular matrix
• Bone supports the body and protects many internal organs
• It acts as a reservoir of calcium and phosphate

Bone Cells: Osteoblasts

• They originate from mesenchymal stem cells
• Growing cells which synthesize and secrete the organic components of the matrix
• These components are collagen, proteoglycans, glycoproteins (osteonectin)
• Osteoblasts secrete components of the initial matrix called osteoid, which allow matrix mineralization to occur

Osteoid

• A layer of unique collagen material is between the osteoblast layer and preexisting bone surface
• Key components include type I collagen and osteocalcin, the protein, which binds Ca2+
• Matrix vesicles that contain enzymes generating PO4- are also present in osteoid
• High concentrations of Ca2+ and PO4− ions cause formation of hydroxyapatite crystals, -Their growth gradually calcifies the matrix

Osteoblast Activity

• Secrete type I collagen, several glycoproteins, and proteoglycans
• Secrete osteocalcin and certain glycoproteins
• These glycoproteins bind Ca2+ with high affinity and raising local concentration
• Release very small membrane-enclosed matrix vesicles which contain alkaline phosphatase and other enzymes
• The included enzymes remove PO4− ions from various matrix macromolecules
• This process creates a high concentration of these ions locally

Bone Matrix Mineralization

• High Ca2+ and PO4− ion concentrations will cause calcified nanocrystals to form in and around the matrix vesicles
• The crystals grow and mineralize further; Forming small growing masses of calcium hydroxyapatite
• These masses surround the collagen fibers and all other macromolecules
• Masses of hydroxyapatite merge as a confluent solid bony matrix as calcification of the matrix is completed

Bone Cells: Osteocytes

• Osteoblasts gradually surround themselves, and their own secretion differentiates into osteocytes
• Enclosed singly within spaces (lacunae)
• Occurs throughout the mineralized matrix
• Osteocytes maintain the calcified matrix and detect mechanical stresses on bone
• Osteocytes maintain communication with adjacent cells via gap junctions at the ends of their processes

Bone Cells: Osteoclasts

• Giant, multinucleated cells
• Involved in removing calcified bone matrix and remodeling bone tissue
• Formed by fusion of several blood monocytes
• Activity controlled by hormones and local factors

Bone Matrix Composition

• Inorganic materials consist of:
• Hydroxyapatite, bicarbonate, citrate, magnesium, potassium and sodium ions
• Organic materials consist of:
• Type I Collagen
• Ground Substance
• Proteoglycans
• Glycoproteins (osteonectin, osteocalcin)

Periosteum

• A dense connective tissue layer on the outer surface of bone
• Bound to bone matrix by bundles of type I collagen
• These bundles are called perforating (or Sharpey) fibers
• Regions of periosteum adjacent to bone are rich in osteoprogenitor cells and osteoblasts
• These cells mediate increases in bone thickness by appositional growth

Endosteum

• A thin layer of active and inactive osteoblasts lining all the internal surfaces within bone
• Osteoblasts are required for bone growth

Woven Bone

• Exhibits random distribution of type I collagen fibers
• Woven Bone features a lower mineral content
• First bone tissue to appear in embryonic development and fracture repair.
• Temporary and replaced by lamellar bone in adults
• Except in very few places in the body

Lamellar Bone

• Both compact and cancellous bone are lamellar bone
• Lamellae are organized concentrically around small central canals
• Canals containing blood vessels and nerves (osteon or haversian system)

Osteon/Haversian System

• Refers to the complex of concentric lamellae surrounding a central canal
• Includes small blood vessels, nerves, and endosteum
• Between each lamellae are lacunae with osteocyte, all interconnected by the canaliculi

Perforating Canals/Volkmann Canals

• Allow canals to communicate with one another
• Features only a few concentric lamellae

Interstitial Lamellae

• Numerous irregularly shaped groups of parallel lamellae
• Can arise from remaining osteons
• Partially destroyed by osteoclasts during growth and remodeling of bone

Compact bone

• Dense area near the surface
• Accounts for 80% of the total bone mass

Cancellous Bone

• Deeper areas with numerous interconnecting cavities
• Accounts for 20% of total bone mass

Osteogenesis

• Bone formation occurs via intramembranous and endochondral ossification

Intramembranous Ossification

• Osteoblasts differentiate directly from progenitor cells in condensed "membranes” of mesenchyme
• Key examples of Intramembranous Ossification include the bones of the skull and jaw

Endochondral Ossification

• A preexisting matrix of hyaline cartilage is eroded and invaded by osteoblasts
• The osteoblasts then begin osteoid production
• Most bones are formed this way

Intramembranous Ossification Characteristics

• Takes place within condensed sheets (“membranes”) of embryonic mesenchymal tissue
• Examples: bones of the skull and jaw, scapula, clavicle

Endochondral Ossification Characteristics

• Occurs within hyaline cartilage and forms most bones of the body

Endochondral Ossification Phases

• Cartilage Model Formation
• Bone Collar Formation
• Primary Ossification Center
• Secondary Ossification Center
• Bone replace cartilage except the articular cartilage and epiphyseal plates.
• Epiphyseal plates ossify and form epiphyseal lines

Processes During Bone Collar Formation

• Perichondrium becomes periosteum
• Osteoblasts produce compact bone on the surface
• There is an increased size of cartilage through Interstitial growth and appositional growth
• Further increased size from Hypertrophy of chondrocytes
• Chondrocytes initiate formation of hydroxyapatite crystals, also known as calcified matrix

Further Clarification on Endochondral Ossification

• Blood vessels grow into the enlarged lacunae of the calcified cartilage
• The osteoclasts remove any remaining cartilage matrix
• Osteoblasts replace the dying chondrocytes and secrete its component in the calcified matrix to form the trabeculae

Cartilage Remaining

• Articular cartilage remains within the joints between long bones; Normally persists through adult life
• Epiphyseal cartilage (epiphyseal plate or growth plate) is specially organized
• It connects each epiphysis to the diaphysis and allows longitudinal bone growth

Epiphyseal Plate

• Contains a zone of reserve cartilage composed of typical hyaline cartilage
• The zone of proliferation is where chondrocytes undergo mitosis and appear stacked within elongated lacunae
• In the Zone of Hypertrophy mature chondrocytes in these lacunae swell up
• They compress the matrix, and undergo apoptosis
• In the Zone of Calcified Cartilage chondrocytes about to undergo apoptosis release matrix vesicles and osteocalcin to begin matrix calcification by the formation of hydroxyapatite crystals
• The Zone of Ossification is where bone tissue first appears
• Further, woven bone is laid down initially by osteoblasts and remodeled into lamellae bone

Appositional Bone Growth

• Increases the circumference of a bone by osteoblast activity at the periosteum
• The process is accompanied by enlargement of the medullary marrow cavity

Bone Growth

• Occurs throughout life
• Cells & matrix turn over continuously
• Accomplished through activities of osteoblasts and osteoclasts
• Lamellae and osteons are temporary structures replaced and rebuilt continuously
• This is bone remodeling, in which bones can change size and shape according to mechanical stress

Bone Repair

• Soft, fibrocartilage-like callus – initial bone formation which involves the activation of periosteal fibroblasts after fracture or other injury
• Hard callus of woven bone – replaces the soft callus and this is soon remodeled to produce stronger lamellar bone

Calcium

• A key ion for all cells
• Stored in bone when dietary calcium is adequate
• Mobilized from bone when dietary calcium is deficient

Calcium Importance

• Maintenance of proper blood calcium levels involves the activity of all three major bone cells
• Regulated by subtle paracrine interaction among these and other cells
• Hormones affecting calcium deposition and removal from bone include:
• Parathyroid hormone (PTH)
  • Indirectly stimulates osteoclasts to elevate levels of calcium in blood
• Calcitonin
  • Serves to inhibit osteoclast activity, lowering blood calcium levels