94 - Histology of Bones and Joints Flashcards
Most common type of cartilage
Hyaline
Components of hyaline cartilage ECM
1-4
– Collagen type II (mainly)
– aggrecans (large GAGs including chondroitin
sulphate and heparan sulphate)
– hyaluronic acid
– chondronectin (glycoprotein, binds collagen, aggrecans and integrins)
Surrounded by perichondrium
Cellular component of hyaline cartilage
Chondrocytes
Components of elastic cartilage
Hyaline cartilage with elastin.
Surrounded by perichondrium.
Where is elastic cartilage found?
In the ear, ear canals, epiglottis, larynx.
Role of fibrocartilage
Binds solid joints, forms menisci and IV discs
Composition of fibrocartilage
Mixture of dense connective tissue and isolated islands of cartilage
Location of fibrocartilage
In joints, in the form of menisci, in IV discs, where bones fuse
Composition of fibrocartilage
No perichondrium.
Type 1 collagen
Chondrocytes (that have differentiated from fibroblasts)
Composition of IV discs
External ring of fibrocartilage (annulus fibrosis) Nucleus pulposis (form gelatinous centre with collagen type II, replaced with fibrocartilage around age 20)
Structure of cortical bone
Concentric layers (cylindrical modules of bone, concentrically surrounding artery). These are called Haversian systems.
Outermost part of bone is made up of layers of cortical bone, not Haversian systems
Structure of trabecular bone
1-4
1) Open structure braces joints
2) Spaces are continuous and full of marrow and blood vessels
3) Made up of lamellae
4) Osteocytes are in contract with the marrow cavity and vessels via their processes
Differences between yellow and red marrow
Yellow marrow is mostly fat cells, red marrow is mostly haemopoietic cells.
In early life, red marrow predominates, in later life yellow marrow predominates.
Yellow marrow retains some haemopoietic cells, can become actively haemopoietic if required.
Blood supply in bone marrow that is unusual
Blood vessels include sinusoids (large, open capillaries with pores in the walls).
Periosteum
Thin layer of connective tissue that surrounds bony surface
Layers of periosteum
1
2
Outer layer is fibrous (fibroblasts, blood vessels, more collagen)
Inner layer is more cellular (osteoprogenitor cells, which can give rise to osteoblasts)
Components of endosteum
Bone lining inner marrow cavity covered by endosteum
thinner than periosteum, still contains osteoprogenitor cells
Components of where tendons and ligaments attach to bone
Collagen is parallel to surface of compact bone.
At tendon connection, bone collagen is continuous with collagen of ligament or tendon.
Collagen fibres penetrate bone surface (these are Sharpey’s fibres)
Blood supply to bone
1-4
1) Arteries supply bones at discrete points
2) Branch in marrow cavity
3) Arteries supply shaft (diaphysis) and ends (epiphysis_ separately
4) Periosteum is separately supplied
How are bones innervated?
Nerves follow blood vessels
Components of articular cartilage
Hyaline cartilage
Slippery, smooth, resistant to compression
No perichondrium in joints
Problems with having no perichondrium in articular cartilage
Perichondrium contains cells to repair cartilage, so lacking it makes articular cartilage repair very slowly.
Type of surface that synovial membrane is
A connective tissue structure that is very porous (it is not epithelium)
Structure of synovial membrane
1-4
1) Not epithelial
2) Surface layer (intima) is two-three cells thick, a mix of fibroblast-like cells and macrophage-like cells
3) Sub-intimal layer is fibrous connective tissue
4) Synovial fluid is an ultrafiltrate of synovial blood vessels plus proteoglycans
Cells embedded in Haversian systems
Osteocytes.
Cells of bone
1-4
Make bone:
Osteoprogenitor cells
Osteoblasts
Osteocytes
Degrade bone:
Osteoclasts
Osteoprogenitor cell location
In periosteum, endosteum
Features of osteoprogenitor cells
1-3
Normally resting or quiescent
Can give rise to new osteoblasts to grow or repair bone.
Renewed from stem cells in bone marrow
Features of osteoblasts
1
2
3
Works on bone surface to make bone.
Makes osteoid (organic ECM of bone, which is type I and bone matrix proteins).
Inactive osteoblasts are flattened cells that look like osteoprogenitor cells
Markers of osteoblast activity
Osteocalcin (Ca2+ binding protein) and alkaline phosphatase
Osteocyte location
Present within bony matrix (not on surface like osteoprogenitor cells and osteoblasts).
Role of osteocytes
Maintain bone in response to loading.
Can destroy local bone to free Ca2+
Effect of loss of osteocytes
Bone resorption
Morphology of osteoclasrts
Giant multinuclear cells (different progenitors to osteoprogenitor cells/osteoblasts/osteocytes)
Role of osteoclasts
Destroy bones in growth, repair and normal turnover.
How do osteoclasts resorb bone?
Seals itself to bone around edge.
Secretes HCl, to degrade mineral component.
Secretes proteases to degrade organic component.
Marker of osteoclast activity
Tartrate-resistance acid phosphatase
Progenitors of osteoclasts
Related to granulocytes (macrophages, neutrophils, etc).
What regulates osteoclast activity?
Parathyroid hormone increases activity, calcitonin (form thyroid) decreases activity.
Bone development
During bone development, bone is laid down as either membrane bone or endochondral bone.
Membrane bone forms directly from mesenchyme (EG: skull, flat bones of the face, mandible, clavicles form this way)
Endochondral bone (EG: weight-bearing bones, bones of the extremities) forms in hyaline cartilage. A cartilage structure of skeleton is replaced by bone.
How does endochondral bone form?
It is not cartilage converted to bone (calcified cartilage is a pathology).
Bone first forms in diaphysis (shaft) of hyaline cartilage. Things can’t diffuse across bone, so cartilage around bone is starved of nutrients and starts dying off. Bone forms where the cartilage dies off.
Then bone starts forming within epiphyses.
Growth plate is remnant of cartilage between diaphysis bone formation and epiphysis bone formations.
Number of centres of ossification in endochondral bone
Three normally.
One in diaphysis, one in each epiphysis.
When do growth plates in humans fuse?
Around 21-22 years
Structure of growth plate 1 2 3 4 5
Layers.
Distally is normal hyaline cartilage (resting zone)
Line of dividing chondrocytes (proliferation zone)
Region of mature chondrocytes (maturation zone)
Zone of hypertrophic (dying) chondrocytes (hypertrophy zone)
Degenerating cartilage is degraded, bone is laid down on its surface
Type of bone that is newly-made
Woven bone (a special type of bone).
Features of woven bone
1-4
No Haversian systems.
More collagen.
More cellular.
Woven bone is soon remodelled by osteoblasts and osteoclasts.
How is woven bone remodelled?
Osteoclasts focally degrade woven bone (form tunnels).
Tunnels formed along stress axis of bone.
Blood vessels and osteoprogenitor cells (endosteum) invade tunnels.
These form Haversian systems
Haversian system formation
Form in tunnels formed by osteoclasts in woven bone.
Osteoblasts lay layers of bone with the collagen alternating in direction (one layer is clockwise, other is anticlockwise)
Final layer leaves a final space for blood vessel.
