Untitled Quiz

Cartilage

• A specialized form of connective tissue with a firm extracellular matrix
• Able to withstand mechanical stress without permanent distortion
• Provides support for soft tissues and acts as a shock absorber due to its resilience
• Plays a crucial role in movable joints, providing a smooth and lubricated surface for sliding movements
• Guides the development and growth of long bones
• Contains high concentrations of Glucosaminoglycans (GAGs) & proteoglycans that interact with collagen and elastic fibers
• The extracellular matrix composition and cells determine the three types of cartilage: hyaline, elastic, and fibrocartilage
• Chondrocytes are responsible for producing and maintaining the extracellular matrix components
• Chondrocytes reside in matrix cavities called lacunae
• All cartilage types contain collagen, hyaluronic acid, proteoglycans, and glycoproteins
• Hyaline cartilage is the most common type and contains collagen type II
• Elastic cartilage contains elastin, providing pliability and distensibility
• Fibrocartilage possesses a dense network of coarse type I collagen fibers, making it suitable for regions subjected to pulling forces
• Cartilage is avascular, relying on diffusion from the perichondrium and synovial fluid in areas lacking perichondrium
• Perichondrium is a dense connective sheath surrounding cartilage
• Chondrocytes exhibit low metabolic activity
• Cartilage lacks lymphatics and nerves, but they are present in the perichondrium
• Fibrocartilage resists compression and tension
• Fibrocartilage consists of rows of thick collagen fibers alternating with rows of chondrocytes (in matrix)
• Examples include knee menisci and the annulus fibrosus of intervertebral discs

Growth Of Cartilage

• Appositional growth: "Growth from outside"
  • Chondroblasts in the perichondrium (external covering of cartilage) secrete matrix
• Interstitial growth: "Growth from within"
  • Chondrocytes within the cartilage divide and secrete new matrix
• Cartilage growth ceases in late teens (chondrocytes stop dividing)
• Regenerates poorly in adults

Bone

• A specialized connective tissue composed of cells and a predominantly collagenous extracellular matrix called osteoid, which becomes mineralized by the deposition of calcium hydroxyapatite

Functions of Bone

• Support
• Movement: muscles attach by tendons and use bones as levers to move the body
• Protection: skull protects the brain, vertebrae protect the spinal cord, rib cage protects thoracic organs
• Mineral storage: stores calcium and phosphate, which are released as ions into the blood as needed
• Blood cell formation and energy storage: bone marrow produces red blood cells and stores fat

Bone Cells

• Osteoblasts: synthesize osteoid and mediate its mineralization, found on bone surfaces
• Osteocytes: inactive osteoblasts trapped within formed bone in cavities called lacunae between bone matrix layers (lamellae), with cytoplasmic extensions into small canaliculi between lamellae
• Osteoclasts: phagocytic multinucleated cells that erode bone and are essential for resorption, remodeling, and refashioning of bone, found in depressions or cavities known as Howship's lacunae
• Osteoclastic activity is regulated by hormones and signaling factors
• Osteoclasts respond to parathyroid hormone and calcitonin
• Parathyroid hormone stimulates osteoclastic resorption and calcium ion release from the bone
• Calcitonin inhibits osteoclastic activity

Disorders Of Bone

• Osteopetrosis: defective osteoclasts, resulting in heavy bones

Bone Surfaces

• Both the internal and external surfaces of bones are lined by connective tissue layers: endosteum and periosteum, respectively
• Both contain osteogenic cells

Extracellular Matrix Components

• Organic components: collagen type I, proteoglycans, and several glycoproteins
• Inorganic components: calcium hydroxyapatite, phosphate, bicarbonate, citrate, magnesium, potassium, and sodium ions
• The ions of hydroxyapatite are hydrated, facilitating ion exchange between the mineral and body fluids
• The association of minerals with collagen fibers during calcification gives bone its hardness and resistance

Classification Of Bones

• Long bones
• Short bones
• Flat bones
• Irregular bones
• Pneumatized bones
• Sesamoid bones (included in short bones)

Gross Anatomy of Bones

• Compact bone (cortical): represents about 80% of the total bone mass
• Spongy (cancellous or medullary or trabecular) bone: represents about 20% of the total bone mass
• Expanded ends are called the epiphysis
• The shaft is known as the diaphysis
• At the junction of the diaphysis and epiphysis in long bones is the epiphyseal or growth plate (occupied with hyaline cartilage during the growing stage of life)
• Blood vessels: nutrient arteries and veins pass through nutrient foramen
• Membranes: periosteum and endosteum

Compact Bone

• Consists of osteons (pillars)
• Lamellae: concentric tubes surrounding a Haversian canal
• Haversian canals contain blood vessels and nerves
• Osteocytes reside in lacunae between lamellae

Spongy Bone

• Consists of layers of lamellae and osteocytes
• Osteocytes seem to align along stress lines

Chemical Composition of Bones

• Organic component (35%): cells, matrix of collagen fibers and ground substance, contributing to flexibility and tensile strength
• Inorganic component (65%): primarily calcium phosphate, located in and around collagen fibrils, contributing to bone hardness
• Small amount of water

Bone Development

• Osteogenesis: "formation of bone"
  • Occurs from osteoblasts
  • Bone tissue first appears in week 8 of embryonic development
• Ossification: "to turn into bone"
  • Intramembranous ossification (also called "dermal" since it occurs deep in the dermis): forms directly from mesenchyme, not modeled first in cartilage
    • Examples: most skull bones except a few at the base, clavicles, sesamoid bones
  • Endochondral ossification: modeled in hyaline cartilage, then replaced by bone tissue
    • Examples: all other bones

Intramembranous Ossification

• Forms directly from mesenchyme
• Osteoblasts secrete osteoid, which becomes mineralized, forming bone

Endochondral Ossification

• Modeled in hyaline cartilage, called the cartilage model
• Gradually replaced by bone, beginning late in the second month of development
• Perichondrium is invaded by vessels and becomes periosteum
• Osteoblasts in the periosteum lay down a collar of bone around the diaphysis
• Calcification occurs in the center of the diaphysis
• Primary ossification centers form
• Secondary ossification occurs in epiphyses
• Epiphyseal growth plates close at the end of adolescence, fusing the diaphysis and epiphysis, ending bone lengthening

Factors Regulating Bone Growth

• Vitamin D: increases calcium absorption from the gut
• Parathyroid hormone (PTH): increases blood calcium levels (some from bone)
• Calcitonin: decreases blood calcium levels (opposes PTH)
• Growth hormone and thyroid hormone: modulate bone growth
• Sex hormones: growth spurt at adolescence and closure of epiphyses

Bone Remodeling

• Osteoclasts: responsible for bone resorption
• Osteoblasts: responsible for bone deposition
• Triggers: hormonal (parathyroid hormone) and mechanical stress
• Osteocytes are transformed osteoblasts

Repair Of Bone Fractures

• Simple and compound fractures
• Closed and open reduction

Disorders Of Cartilage And Bone

• Defective collagen: numerous genetic disorders, such as Marfan syndrome, Ehlers-Danlos syndrome, and osteogenesis imperfecta