Lecture 4.1: Cartilage and Bone Flashcards
What is Cartilage?
A form of connective tissue
Where is Cartilage found?
In nearly all joints
In structures that must be deformable and strong
Cartilage forms a template for the development of many bones in utero
General characteristics of Cartilage
Pliant
Resists compression
Avascular
Not innervated
Comprised of two cell types: chondroblasts and chondrocytes which produce
ECM
Types of Cartilage (3)
1) Hyaline Cartilage
2) Elastic Cartilage
3) Fibrocartilage
Hyaline Cartilage
ECM contains:
- Proteoglycans
- Hyaluronic acid
- Type II collagen
Elastic Cartilage
Similar to hyaline cartilage
But it also contains many elastic fibres
Fibrocartilage
Similar to hyaline cartilage
But matrix contains abundant type I collagen fibres
Comprised of dense regular connective tissue and hyaline cartilage
Contains fibroblasts and chondrocytes
Location of Hyaline Cartilage
Articular cartilage covers the ends of most bones and movable joints
Costal cartilages connect ribs to sternum
Larynx and trachea
Features of Hyaline Cartilage
It is is covered by a fibrous perichondrium (except at the articular surfaces of synovial joints)
Large amounts of hyaluronic acid means that hyaline cartilage is well hydrated
It is pliable and resilient under pressure
Large ratio of GAGs to collagen in the ECM facilitates diffusion of substances between chondrocytes and blood vessels surrounding the cartilage
Articular Cartilage has no perichondrium. Where does it get its nutrients from?
The surrounding joint fluid
The Extracellular Matrix (ECM) of Hyaline Cartilage
Comprised of type II collagen, water and ground substance (GAGs such as
hyaluronic acid and proteoglycans)
Hyaluronate proteoglycan aggregates are resistant to deformation
Negative charges on surface of GAGs strongly attract polarised H2O molecules forming a hydrated gel.
Appositional Growth
The increase in the diameter of bones by the addition of bony tissue at the surface of bones
Interstitial Growth
Interstitial growth is a bone growth which results in the lengthening of the bone
This growth occurs within the lacunae
Perichondrium
A dense layer of fibrous connective tissue that covers cartilage
Osteoarthritis
The most common form of arthritis, prevalence increasing with age
Results from focal and progressive hyaline articular cartilage loss with changes in underlying bone; Soft tissue structures in and around the joint also affected
Severe joint injury has a high likelihood of eventual osteoarthritis, likewise obesity increases the risk.
Growth Plates
Hyaline cartilage forms epiphyseal growth plates at the metaphysis of bones
This is where chondrocytes undergo a sequence of cell division and hypertrophy, followed by death and ossification by invading osteoblasts
Locations of Elastic Cartilage
External ear (pinna)
External acoustic meatus
Auditory tube
Epiglottis
Locations of Fibrocartilage
• Intervertebral discs
• Articular discs of the sternoclavicular and
temporomandibular joints
• Menisci of the knee
• Pubic symphysis
• Entheses (between tendon and bone)
Fibrocartilage in an Intervertebral Disc
Annulus fibrosus (tough circular exterior of the intervertebral disc), chondrocyes in lacunae are embedded in large bundles of type I collagen fibres
Meniscal Damage
The menisci of the knee are formed of fibrocartilage discs separating femur and tibia.
Menisci prevent degeneration of articular cartilage underneath
Meniscal Damage: Most Common Causes
Sports-related injuries are most common cause of meniscal lesions in young people
Long-term degeneration from use is most common cause of meniscal lesions from middle-age
Features of Bones
• Strength and rigidity enables forceful muscle
contractions to result in movement
• Provides protection for internal structures
• Highly vascular and innervated
• Adapts to changing mechanical demands
• Regenerates following injury
• Mineral storage
• Blood cell formation
Divisions and Classifications of Bones
1) Axial skeleton
2) Appendicular skeleton
3) Shapes of Bones
Divisions and Classifications of Bones: Axial Skeleton
Skull
Vertebral column
Ribs
Sternum
Hyoid
Divisions and Classifications of Bones: Appendicular Skeleton
Everything else except Skull, Vertebral column, Ribs, Sternum, Hyoid
Divisions and Classifications of Bones: Shapes of Bones
Long
Short
Flat
Irregular
Sesamoid
Pneumatic
Bone Composition
Inorganic: calcium hydroxyapatite crystals give strength
Organic: type I collagen confers flexibility and resistance to stress
Types of Mature Bone Structure
Cancellous (spongy) bone is light and provides spaces for bone marrow
Compact (cortical) bone forms external surfaces and constitutes 80% of skeletal mass.
Organisation of Cortical Bone
Osteon (lamellar structure provides slippage panes, allowing a degree of deformation)
Haversian (central) and Volkmann’s (perforating) canals carry blood vessels, lymph vessels and nerves
Cortical bone is surrounded the periosteum
Structures within an Osteon
Osteocyte in lacuna
Canaliculi (Osteocytes have very slender cytoplasmic processes that reach out to adjacent osteocytes)
Haversian (or central) canal
Lamella of adjacent osteon
Cancellous (Spongy) Bone
Osteocytes reside in lacuna between lamellae (which are more irregular)
Arranged in trabeculae
Space for Adipose (yellow marrow) and haemopoietic cells (red marrow) between trabeculae
Type 1 Collagen
Internal Trabeculae Structure
Osteocytes
Osteoclasts
Interstitial Lamellae
Osteoblasts aligned along trabeculae of new bone
How much bone is turned annually?
5-10% of adult bone turns-over annually
Bone Remodelling Unit Components
A cutting cone of osteoclasts
A reversal zone containing osteoprogenitors
A closing cone where osteoblasts secrete the organic components of bone (osteoid)
This process is accompanied by angiogenesis (formation of new blood vessels)
What factors affect bone remodelling?
Largely dictated by the mechanical loading applied to bone
Bone resorption increases when gravitational forces are reduced
Age (rate of remodelling decreases)
Increased sport activity (can cause bone hypertrophy)
How can bones resist fracture?
Due to balance between flexibility and rigidity
Lamellae are able to “slip” relative to each other to help disperse forces
Fractures occur when the forces applied are too strong
Stages of Fracture Repair (4)
1) Haematoma: Torn vessels bleed, forming a blood clot
2) Soft (fibrocartilage) callus: Clot is removed by macrophages and replaced
by a mass of procallus tissue comprised of fibroblasts and collagen
3) Hard (bony) callus: Callus is invaded by blood vessels and osteoblasts.
Fibrocartilage is gradually replaced by woven bone.
4) Remodelling: Woven bone remodelled as compact and spongy bone
Osteogenesis/ Ossification
It is when new bone is formed
Endochondral Ossification
A pre-existing hyaline cartilage template is replaced by bone
Osteoblasts from circulating osteoprogenitors invade the cartilage, depositing
osteoid (protein matrix) as a substrate for calcification
Most of the bones of the body develop in this way
What is the Osteoid?
The organic component of bone
Largely comprised of type I collagen (secreted by osteoblasts)
Undergoes calcification as hydroxyapatite crystals become embedded between collagen fibres
Intramembranous Ossification
Bone formation takes place within condensations or “membranes” of mesenchymal tissue
Osteoblasts differentiate from local mesenchymal stem cells and deposit osteoid
Most flat bones develop in this way
Bone formation in a 14 week foetus: what stain?
Alizarin Red
Diaphysis
Shaft or central part of a long bone
Epiphysis
The expanded end of the long bones in animals
Metaphysis
The wide portions of long bones and the regions of the bone where growth occurs
Medullary Cavity
The hollow part of bone that contains bone marrow
Epiphysis of an adult long bone
Epiphyseal growth plates of long bones are no longer visible after the cessation of growth
Endochondral Ossification in Utero (1ary)
1) Skeletal elements are initially composed of hyaline cartilage
2) Late in the 1st trimester, a bone collar develops around the diaphysis long bone, with degeneration of underlying cartilage
3) Invasion of capillaries and osteoprogenitors create a
primary ossification centre; Osteoid undergoes calcification to form woven bone
Endochondral Ossification in Adolescence (2ndary)
4) Secondary ossification centres appear in the epiphyses around the time of birth
5) In childhood, primary and secondary ossification centres are separated by the epiphyseal growth plate that allows elongation
6) No further elongation can occur when the growth plates close, but an osteoblast reservoir in the periosteum allows thickening
Appositional Growth
Occurs when chondroblasts secrete new matrix along existing surfaces and this causes the cartilage to expand and widen
Interstitial Growth
Chondrocytes secrete new matrix within the cartilage and this causes it to grow in length
