Cartilage, Bones and Joints Flashcards
What are the 4 components of the ECM of hyaline cartilage?
Collagen type II (mainly)
Aggrecans (large GAGs including chondroitin sulphate and heparan sulphate)
Hyaluronic acid
Chrondronectin
What is chondronectin and what is its role?
A glycoprotein that binds collagen, aggrecans and integrans
What is the role of aggrecans and hyaluronic acid in hyaline cartilage?
Attracts water to provide stiffness
What is elastic cartilage?
Hyaline cartilage with elastin
Where is hyaline cartilage located?
Most common type of cartilage, located in ribs, trachea and joints
Where is elastic cartilage located?
Ears and ear canals
Epiglottis
Larynx
Where is fibrocartilage found?
Binds solid joints
Menisci
Intervertebral joints
Describe the structure of fibrocartilage
Dense connective tissue with type I collagen and isolated islands of cartilage Chondrocytes (differentiated from fibroblasts) No perichondrium (cf hyaline cartilage)
What is the perichondrium?
Tight wrapping of vascular tissue that surrounds the cartilage of developing bones
Describe the structure of intervertebral discs
Annulus fibrosis: external ring of fibrocartilage
Nucleus pulposus: gelatinous centre with type II collagen
What happens to the nucleus pulposus with age?
It disintegrates by age 20 and is replaced by fibrocartilage
Describe the structure of trabecular bone
Continuous spaces full of marrow and blood vessels, constructed of lamellae
How do osteocytes (trapped within the trabecular bone) receive nutrition?
Processes extend from the medullary cavity and run in the spaces in the trabecular bone to the outer surface where they can access O2 and nutrients from neighbouring blood vessels
Which bones in the adult skeleton retain red marrow?
Flat bones
Pelvic bones
Describe the blood supply of the medullary cavity
Blood vessels include sinusoids instead of capillaries
What is the role of sinusoids in the medullary cavity?
They have a large diameter and can form pores, allowing newly formed white cells to enter the systemic circulation easily
What is the periosteum?
Thin outer layer of connective tissue covering the bone surface
Describe the structure of the periosteum
Outer layer: fibrous with fibroblasts, blood vessels and collagen
Inner layer: more cellular, osteoprogenitor cells (can give rise to osteoblasts)
Describe the structure of the endosteum
Thinner than periosteum
Still has osteoprogenitor cells
What are Sharpey’s fibres?
At the connection of tendon and ligaments to bone, the bone collagen is continuous with the collagen of the tendon or ligament; the collagen fibres penetrating the bone surface are called Sharpey’s fibres
Describe the blood flow to bones
Arteries supply bones by entering at discrete points and branching in the marrow cavity
Diaphysis and epiphysis are supplied separately (metaphysis can also have its own blood supply)
Periosteum is supplied separately
Describe the nerve supply to bones
Nerves are abundant and follow blood vessels
Describe the characteristics of articular cartilage
Hyaline cartilage forms the joint surface
Slippery, smooth and resistant to compression
Lacks perichondrium/periosteum
What is the significance of the absence of perichondrium at the articular surface?
Poorly vascularised - if damaged occurs, repair is slow and difficult
Describe the composition of synovial fluid
Ultrafiltrate of plasma from synovial blood vessels with proteoglycans (long CHO molecules) acting as a lubricant
Describe the structure of the synovial joint
Synovial space created by connecting bones outside the articular cartilage
Space full of synovial fluid (lubricates, provides nutrients)
Lined by synovial membrane
Describe the structure of synovial membrane
Surface layer (intima) 2-3 cells thick: mix of fibroblast-like cells and macrophage-like cells Sub-intimal layer: fibrous connective tissue
What are some of the differences between epithelium and synovial membrane?
Synovial membrane lacks basement membrane, tight junctions, desmosomes
Synovial membrane is leaky
What are osteons/Haversian systems?
Cylindrical modules of dense cortical bone
What is an osteoprogenitor cell? Where is it found and what is its role?
A flattened cell found in the periosteum and endosteum, responsible for giving rise to new osteoblasts; usually in the quiescent state
How are osteoprogenitor cells renewed?
From stem cell in bone marrow
What is the role of osteoblasts?
To make osteoid
What is osteoid?
The organic ECM of bone
List 4 products of osteoblasts
Osteocalcin/osteonectin (Ca2+ binding proteins)
Adhesive proteins (sialoproteins, osteopontin)
Proteoglycans
Alkaline phosphatase
What products are measured as markers of osteoblast activity?
Osteocalcin
Alkaline phosphatase
What do inactive osteoblasts look like?
Flattened like osteoprogenitor cells
What is the role of osteocytes?
Maintains bone in response to loading
What are the consequences of loss of osteocytes?
Bone resorption
What is the role of osteocytes in calcium homeostasis?
Can destroy local bone to free Ca2+
What is the role of osteoclasts?
Destroys bone in growth, repair and normal turnover
What is the histological hallmark of an osteoclast?
Giant, multinuclear cells
How do osteoclasts promote bone resoprtion?
Seals to bone around edge
Secretes H+ and Cl- (produced from H2CO3 breakdown), and proteases
What is used as a marker of osteoclast activity?
Tartrate-resistant acid phosphatase
Where do osteoclasts originate? What cells are they related to?
Bone marrow
Related to granulocytes and macrophages (not to osteoblasts)
How do osteoclasts respond to PTH?
Increase activity
How do osteoclasts respond to calcitonin?
Decrease activity
What is the effect of calcitonin on osteoblast activity?
Promotes
What is the effect of PTH on osteocyte activity?
Promotes
How is membrane bone formed? Which bones develop in this way?
Membrane bone forms directly from the mesenchyme
Skull and flat bones of the face, as well as the mandible and clavicles, develop in this way
How is endochondral bone formed? Which bones develop in this way?
A cartilage model of bone is produced and the cartilage is replaced by bone
Weight-bearing bones and bones of the extremities develop in this way
Does the process of endochondral bone development involve the transformation of cartilage into bone?
No; cartilage is destroyed and replaced with bone
Calcification of cartilage is pathological
Describe the process of endochondral bone development
Bone collar forms around diaphysis
Cartilage beneath the collar is denied nutrients and degenerates
Blood vessels invade, bringing in bone cell progenitors
Secondary centres of ossification appear in each epiphysis
Zones of ossification grow together, leaving a thin zone of cartilage between them called the epiphyseal growth plate
Epiphyseal growth plate fuses at 21-22 y.o.
Is the growth plate largely symmetrical or asymmetrical?
Asymmetrical; more growth occurs on the diaphyseal side
Describe the structure of the growth plate
Resting zone: normal hyaline cartilage distally
Proliferation zone: dividing chondrocytes
Maturation zone: mature chondrocytes
Hypertrophic zone: hypertrophic (dying) chondrocytes
Ossification zone: cartilage destroyed and bone laid down on its surface
What is woven bone and when is it produced?
New bone formed during development or repair (later remodelled by osteoclasts and osteoblasts)
What are the features of woven bone?
More cellular
More collagen
No Haversian systems
How does bone remodelling occur?
Osteoclasts make new cavities along the stress axis of the bone; this determines the size and shape of Haversian systems
Blood vessels and endosteum invade these cavities
Osteoblasts line the new spaces and beginning laying down lamellae
How are Haversian systems produced?
Osteoblasts lay down layers of bone with collagen alternating in direction
Final layer leaves a space around the blood vessel, called the Haversian canal
