Section 7: Musculoskeletal System Flashcards
Bone (organ)
Organs are made up of diff types of tissue
Bone (tissue)
One of the tissues found in bones of skeleton
What is found in bone
CT Smooth muscle Nervous tissue Cartilage Bone tissue
Functions of skeletal system
Support Protection Movement Calcium and phosphorous reserve Haemopoiesis (red marrow) Fat storage (yellow marrow)
Tissues - soft or hard
Most tissues are soft and deformation, so need bone to hang and suspend the tissue
Muscle tissue
Soft tissue
Can shorten by ~1/3
Since they’re soft, they aren’t good at pulling on other tissues so attach to skeletal system –> allows movement
Functions of skeletal system: Calcium
Need to have a certain amount of Ca2+ in serum for organs to function properly
Determines muscle contraction
Important for APs
Where is Ca2+ found
~99% in body is skeleton, other 1% is dissolved in tissue fluid
Phosphorous is used a lot in…
Cellular structures
Functions of skeletal system: Haemopoiesis
Found inside bones that you make blood out of, e.g. RBC, WBC
Red
Functions of skeletal system: Fat storage
High fat content
Yellow
Adult skeleton =
Axial + Appendicular
Adult skeleton: Axial vs appendicular - no of bones
Axial: 80 (some paired)
Appendicular: 126 (all paired)
How many bones in total does the skeleton have
When born have ~270 centres of ossifications and eventually some fuse tgt
Adult skeleton ~206
As you get older (~30 years), some of the 206 bones will also fuse
Adult skeleton: Axial vs appendicular - found where
Axial: found on axis/core of body
Appendicular: upper and lower limbs
Adult skeleton: Axial vs appendicular - main regional differences in function
Axial:
Support / protection
Haemopoeisis
Appendicular:
Movement
Fat storage
Adult skeleton: Axial vs appendicular - bone marrow
Axial: most bone marrow is haemopoietic tissue (red)
Appendicular: most bone marrow is fat storage
Further from axial skeleton = more likely to find yellow marrow
Adult skeleton: All machinery needed to make body function is usually associated with…
The axial skeleton
Adult skeleton: Appendicular skeleton - environment
Sense environment
Manipulate environment
Move body through environment
Classic bone
Long bone
Means the bone is longer in one axis than it is in the other two
Ends of a long bone
Usually articulating with neighbouring bones at its ends
Parts of a long bone
Epiphysis = ends Diaphysis = length of bone Metaphysis = properties of epiphysis and diaphysis
Long bone - forces
Epiphysis: Since bone is in contact with bone, most forces are transmitted through joint itself
Diaphysis: As forces get down to shaft, aren’t perpendicular with surface and now are running parallel with surface –> don’t need plates but instead have thicker walls to resist the force
Long bone: Diaphysis - shape
Cylinder-shaped - one of the strongest shapes for its weight
Long bone: Diaphysis - weight
Quite light, but very strong
Long bone: Diaphysis - wall
Compact bone forms the wall
Long bone: Diaphysis - medullary cavity
Where (mainly yellow) bone marrow is found
Do all bones have marrow in them
No, some don’t
Long bone: Diaphysis - periosteum
Surrounds bones - covers most of its outer surface
Important for health of bone
Peri
Perimeter / outer layer
Long bone: Diaphysis - Sharpey’s / perforating fibres
Anchors periosteum to bone - strong
Bundles of collagen that infuse into matrix of bones
Usually small but can get big when there’s a tendon or ligament that needs to attach to bone
Long bone: Diaphysis - Endosteum
Thin, inner fibro-cellular layer lining medullary cavity
Long bone: Epiphysis - spongy bone
Made up of trabeculae to support outer layer of bone
Long bone: Epiphysis - trabeculae
Unit of spongy bone
Long bone: Epiphysis - medullary cavity
Spaces between trabeculae
Quite small
Usually red marrow
Long bone: Epiphysis - blood vessels
Inside compact bone and medullary cavity
Long bone: Epiphysis - articular cartilage
Usually only found where bone comes in contact with other bone
Bone rubbing directly against bone is painful
Biggest bone in body
Femur
Femur - holes
Nutrient foramen - how blood vessels get in
Quite small and lots of them
Femur - amount of trabeculae
As you move from diaphysis to metaphysis to epiphysis, amount of trabeculae increases
Inside of epiphysis = lots
Femur: Trabeculae - arrangement
Not randomly arranged
Radiate away from side of bone and go out for support
Since weight is slightly offset from centre, there’s a bending force on head of femur, so some trabecular move out in that plane
Bone - anaesthesia
Bone has poor hydration factor, so anaesthesia doesn’t get into the centre of the bone
Bone is a _____ CT
Specialised
CT - common?
The most common tissue in body
CT tends to be used for…
Packaging
CT - diverse?
Diverse range of physical properties because diverse functions
What is CT made of
Made up of cells which secrete material around them - called ECM
ECM components
Fibres Ground substance (quite a lot of water)
CT - hydration
Most CT is quite hydrated
Nerves often act on…
Blood vessels
What are the fibres in the bone
Collagen
Bone: ECM - organic?
Fibres = organic (C-based)
Ground substance = inorganic
Bone: ECM - made up of?
Fibres: collagen fibres (type I)
Ground substance: hydroxyapatite (calcium and phosphorous)
ECM: Ground substance - hydroxyapatite
Typically only found in bone
Good at resisting compression –> gives bone its unique properties
ECM - resists what
Fibres = resist tension (stretch/pull)
Ground substance = resist compression (squeeze/crush)
So combination of them allows to resist torsion
i.e. tension + compression = torsion
ECM - weight
Fibres = 1/3 of dry weight
Ground substance = 2/3 of dry weight
ECM: What determines tension
How loose the fibre was to begin with determines how far apart you can move the points of attachment before they start resisting
What is found wherever tension needs to be resisted
Collagen
Collagen arrangement
Ligaments and tendons that have lots of powerful tension - all in same orientation
Tissues where there are multiple tension forces - randomly arranged to resist as many forces possible
Types of bone cells
Osteogenic cell (osteoprogenitor cell) ↔
Osteoblast ↔
Osteocyte
Osteoclast
Osteogenic cells - precursor
Unspecialised stem cells - found in bone marrow, left from mesenchyme embryonic CT and overtime divided/specialised
Osteogenic cells - location
Surface of bone under peri/endosteal fibres and wait, but under right cues will start to divide –> osteoblast
Also in central canals of compact bone
Osteoblast - precursor
Osteogenic cell
Osteoblast - location
Usually in a layer under the peri/endosteum (now active!)
Wherever new bone is being formed
Osteoblast - structure
Quite fat because they have organelles inside them designed for secretion
Osteoblast - secretion
Secrete osteoids, which are rich in organic components of bone
Osteoblast - osteoid
The organic ECM (70% collagen, 30% proteoglycans, proteins, water) of bone, synthesised by osteoblasts prior to mineral deposition
Osteoblast - osteoid - calcification
Where the precursor matrix is infiltrated with bone salts (hydroxyapatite)
Can usually calcify osteoid up to 70-80% in 3-4 weeks
Makes bone strong and dense - nutritive fluids can’t diffuse freely through it
Osteoid weight
Before mature bone only forms ~25% of wet weight
In mature bone forms ~70%
Osteoblast - osteoid - calcification rate
Quite fast to begin with, but as time goes on, water is displaced (needed to bring nutrients in and take waste out), so rates start to drop off quite significantly
Can take years to fully calcify bones since removing water
Bone - nutrient diffusion
Bone is quite poor in nutrient diffusion because low water count
Osteocyte - precursor
Osteoblast
Osteocyte - location
Trapped within lacunae inside bone
Can communicate with neighbouring cells through their long cellular processes inside canaliculi - helps maintain contact with neighbours and cells on surface
Osteocyte - function
Bone tissue maintenance:
- live lattice inside bone that maintains microenvironment to make sure bones are healthy and can release signals
- localised minor repair
- rapid Ca exchange
What do osteocytes occupy
Occupy little spaces called lacunae
Osteoclast - precursor
Monocyte progenitor cells usually form WBCs, but can also move out of blood vessel (BV) and a collection of them can gather on surface of bone and fuse –> osteoclast
Osteoclast - location
At sites where bone resorption is occuring
Osteoclast - function
Secretes acid (which dissolves mineral/hydroxyapatite of bone, exposing collagen) and enzymes (which dissolves organic components/collagen of bone) These enzymes are inactive until they're exposed to the acid environment underneath the cell
Syncytium
A cell formed from fusion of other cells
Osteoclast - size
Big cell in comparison to others
Osteoclast - ruffled border
Very corrugated/convoluted membrane for absorption and secretion
Osteoclast - clear zone
Sucks cell onto surface and makes sure the acids and enzymes don’t get out and destroy other areas of body
Osteoclast - how can minerals / organic compounds get out of the clear zone
The only way is to be endocytosed into the cell and be neutralised
Then can get exocytosed out of cell
i.e. dissolves product and ejects it out the top of cell
Osteoclast - Howship’s lacunae
Like little pits
Osteoclasts are often found in…
Groups
Osteoclasts - nuclei
Multiple nuclei
Mineralised bone - structure
Lattice network
CT growth
A lot of CT undergoes interstitial growth, but bone can’t grow like this
CT - interstitial growth
Cells divide mitotically and secrete ECM which grows the tissue from within
How does bone grow
Via appositional growth
Bone: Appositional growth - where
Adds bone on outside
Bone: Bone resorption
Occurs in inner layer to decrease thickness
Bone remodelling
Overall mechanism of appositional growth and bone resorption
When is bone remodelling occuring
Constantly occurring throughout your life
Appositional growth - steps
Osteogenic cells get signals telling them to divide –> osteoblasts, some of which settle on surface where we want new bone –> secretes osteoid and calcifies it
Since layer has more than osteogenic cells, it’s now active
Some osteoblasts bury themselves and become trapped in lacunae, eventually becoming osteocytes
When growth stops, osteoblasts convert back into osteogenic cells or die
Osteoid is fully calcified and we are back to resting state (only osteogenic cells)
Appositional growth - net effect
We put down layers of bone on the outside and growth occurs outwards
Appositional growth - where
On existing surfaces
Mostly in periosteum, but can occur anywhere else
Lacunae - calcification
Walls of lacunae aren’t as calcified as central parts of tissue because osteocytes are exchanging with walls of lacunae
Osteocytes - how do they align
A lot of osteocytes tend to line up in rows
Bone resorption: Venules
Since blood is flowing slowly through them and the wall is thin, it’s easy for WBCs to wriggle through the wall
WBCs are ___ cells
CT
Bone resorption - steps
Messages from osteocytes cause monocyte precursor cells to leave BV and fuse on bone surface to form Howship’s lacunae (secretes acids and enzymes)
Once osteoclasts died, BVs grow into new area created by loss of bone - helps keeps cells alive
Osteoclasts - how long do they live
Relatively short-lived (2-3 months) and undergo apoptosis
Apoptosis
Self-destruction
Why can’t bone grow by interstitial growth
Bone tissue is too rigid; interstitial growth occurs in softer tissues that can deform
Bone is designed to resist deformation ,so can only grow by adding new bone onto existing surface (appositional growth)
Appositional growth and bone resorption occur ______ to each other
Independent
How do long bones grow in length
By a process called endochondral ossification
Long bones: Endochondral ossification
As cartilage plate gets thicker, the epiphysis moves away from the metaphysis
Cartilage in contact with metaphysis dies off –> gives osteoblasts the surface to put down bone and macrophages remove dead cartilage
Eventually rate at which cartilage grows is slower than rate of bone growth so epiphysis makes contact with metaphysis and the 2 surfaces fuse –> epiphyseal line
Is epiphysis fixed from bone to bone
No; a cartilage plate (made of hyaline cartilage) is found between it
Hyaline cartilage
Like a firm rubber, but can still undergo interstitial growth
Has chondrocytes in it that can divide and secrete more ECM
Why are males taller on average
During endochondral ossification, the fusion of the epiphysis and metaphysis usually ends earlier in females
Appositional growth and bone resorption - ratio
Baby: higher ratio of appositional growth
Our age: similar ratio
About age 30: rate of bone resorption starts to increase relative to appositional growth - this is why elderly have brittle bones
What affects how brittle your bones are when you’re older
How strong/dense they are in your younger years
2 main bone types
Woven/immature bone
Mature/lamellar bone
Woven bone - structure
Collagen fibres are wavy
Less densely packed and ECM is less dense
Not as strong as mature bone
Woven bone - babies
Born with woven bone because doesn’t need to be very strong when embryo
But when born and start crawling/walking, bone needs to strengthen
~3 years old, you’ve replaced all your woven bone with mature lamellar bone
When do we find woven bone in adults
When we break a bone
Mature/lamellar bone: What’s found between the fibres
Hydroxyapatite
Mature/lamellar bone: Bending
Inner surface is put under compression, whereas outer surface has tension
Mature/lamellar bone: Collagen arrangement
Typically put down in same direction within a layer, but can alternate up to 90° out of phase between layers
Enables bone to withstand forces from diff directions –> stronger
No matter which way you bend your bone, some fibres are going to be under tension
Types (subcategories) of mature/lamellar bone
Spongy bone
Compact bone
Spongy bone AKA…
Cancellous bone
Trabecular bone
How much of our skeleton is spongy bone
Usually 20% (less dominant)
But can change depending on where the bone is
e.g. long bone doesn’t have lots of compression; ~10%
Vertebrae ~40%
There is more spongy bone where there is more…
Compression
What are trabeculae covered in
Since they’re inside the bone, they’re covered in endosteum
What do you find on the surface of trabeculae
Osteoclasts
Spongy bone vs compact bone - SA
SA of spongy bone is significantly greater than SA of compact bone
Osteoporosis - females
One of the things that controls osteoclasts is oestrogen levels
Females go through menopause –> oestrogen levels drop –> decreased regulation of osteoclasts
Therefore females tend to be affected by this disease more
Osteoporosis - males
Males aren’t affected as much because testosterone and its derivatives help control osteoclasts
Osteoporosis - what happens / symptoms
Osteoclasts dissolve spongy bone in particular because high SA and high turnover
Makes bone look more porous/spongy
Spongy bone - direction of growth
Can only grow outwards, so newest lamellae is on the outer edge
Spongy bone - blood vessels
Blood vessels transport O2 and nutrients which are picked up by cells on surface of trabeculae
Spongy bone - narrowest dimension
0.4mm - can get quite a long/flat trabeculae as long as smallest dimension doesn’t exceed this length
Bone tissue is poorly hydrated so nutrients can’t move through tissue well
If trabeculae too thick, cells in centre won’t get enough nutrients
Compact bone AKA…
Cortical bone
Compact bone - thickness in diaphysis
Particularly thick
Spongy bone vs compact bone - thickness
Compact bone much thicker because it has blood vessels running through it which originate in periosteum
Compact bone - blood vessels
Originate in periosteum and send these branches through to Volkmann’s canals (perpendicular to surface)
Then link with other BVs that run parallel with surface (central/Haversian canal)
Compact bone: Haversian vs Volkmann’s canals
Haversian canals usually have concentric lamellae around them whereas Volkmann’s canals don’t
Compact bone: What do Haversian canals mark out
The centre of the unit that defines compact bone - the osteon
Osteon AKA…
Haversian system
Compact bone: Osteon - structure
Central canal with blood vessels running through
Concentric lamellae alternating between layers
Compact bone: Osteon - under force
If subjected to a common/predominant force that’s usually in one direction, collagen fibres between layers may be less extreme in alternation and line up better
If exposed to forces in diff directions, collagen fibres will become more at 90° to each other
Spongy bone vs compact bone - nutrient flow
Spongy bone: trabeculae had nutrient flow inwards
Compact bone: blood vessels are in centre so nutrient flow is outwards
Compact bone: Circumferential lamellae
Run around the perimeter of bone
Compact bone: Appositional growth in periosteum
Adds layers of circumferential lamellae
Compact bone: How are primary vs secondary osteons formed
Primary: Appositional growth
Secondary: Osteoclast activity
Compact bone: Formation of primary osteon - steps
- Osteoblasts in periosteum either side of a BV put down new bone, forming ridges
- As bone grows, the ridges come tgt and fuse –> tunnel around BV. Tunnel is now lined with endosteum
- Osteoblasts in endosteum build concentric lamellae onto walls of tunnel, which is slowly filled inward toward centre –> new osteon
- Bone continues to grow outward as osteoblasts in periosteum build new circumferential lamellae until you have a small hole just big enough to fit the BV and some soft tissue
Compact bone: Formation of primary osteon - how quickly is bone put down (in step 1)
Initially puts it down quite rapidly, but growth slows down when ridges form
Differences between periosteum and endosteum
Very similar
Main difference is periosteum is thicker because it’s needed for protection and attachment
Compact bone: Why do we need secondary osteons
Because there aren’t enough periosteal BVs to account for every osteon in compact bone
So, we need a way to develop an osteon in bone that’s already existing - secondary osteon does this
Compact bone: Where are secondary osteons created
Inside the existing bone
Compact bone: How do primary osteons differ from secondary osteons
Primary osteon: tunnel is created on surface of a bone it grows
Secondary osteon: tunnel is created inside the existing bone
Compact bone: Formation of secondary osteons - steps
- A group of osteoclasts bore a tunnel through existing bone - this area is called the ‘cutting cone’
- Osteoblasts move in behind the cutting cone, forming the new active endosteum, and start depositing osteoid onto wall of new tunnel. Osteoid layer is calcified –> new lamella. BV grows into newly formed tunnel to supply cells
- New lamellae slowly closes in tunnel - called the ‘closing cone’. Some osteoblasts are trapped in newly deposited lamellae –> osteocytes
- When tunnel is reduced to size of a typical Haversian canal, osteoblasts die, or form osteogenic cells –> resting endosteum
Compact bone: Formation of secondary osteons - the ‘cutting cone’ creates…
A tunnel inside the existing bone
Compact bone: Formation of secondary osteons - cement line
Sometimes at the end of the formation, a line can be seen at the junction between the outermost lamella of the new osteon and the pre-existing older bone
Compact bone: What happens if osteocytes detect damage in bone they can’t repair themselves
They release chemical cues that cause osteoclasts to move into the area
Compact bone: Formation of secondary osteons - what is the cutting cone
A collection of osteoclasts that act as a cellular drill
Compact bone: Formation of secondary osteons - what is the cutting cone under the control of
Under control of osteocytes already trapped in bone
Compact bone: Formation of secondary osteons - which area is most likely to be damaged
Cutting cone
Compact bone: Formation of secondary osteons - cutting cone speed
Quite slow, moves about 1mm every 20 days
Compact bone: Formation of secondary osteons - glycoproteins
The first layer the osteoblasts put down at the junction between old and new osteon quite often have lots of glycoproteins
Compact bone: Formation of secondary osteons - collagen arrangement
As osteoblasts put down new layers, they alternate the collagen orientation
Compact bone: Formation of secondary osteons - how are the trapped osteocytes connected to each other
Via canaliculi and lacunae
Compact bone: Formation of secondary osteons - closing cone; appositional growth in ______
Endosteum
Compact bone: Is primary or secondary osteon more common
Secondary
Compact bone: Formation of secondary osteons - maixmum size
Osteon can’t be bigger than 0.4mm
Compact bone: Interstitial lamellae
Not defined / old lamellae
Why do osteocytes tend to line up in rows
Tend to find osteocytes where the lamina is changing direction - rows
Spongy vs compact bone - osteons
By definition, osteons present = compact bone
No osteons = spongy bone
Osteons and age
Osteons harden with age –> imprint smaller than new bone
Bone: X-rays
Bone has to be at least ~50% calcified for X-rays not to pass through it
New vs older bone - osteon appearance
Newer bone: osteon is more complete (circle)
Arthro-
Joints
What is a joint / articulation
Any point where two/more bones interconnect
Joint - compromise
Compromise between need to provide support (stability) and need to remain mobile (movement)
Joints - functions
Movement
Force transmission
Growth
Functional classification of joints
Synarthrosis
Amphiarthrosis
Diarthrosis
Functional classification of joints: Synarthrosis
Immovable joint
Highly stable, low movement
Axial skeleton
Functional classification of joints: Amphiarthrosis
Slightly movable
Medium stability, medium movement
Axial skeleton
Functional classification of joints: Diarthrosis
Freely movable
Low stability, high movement
Appendicular skeleton
Can joints be trained
Yes
Joints: Function - what can affect movement
Soft tissue around each joint has big effect on flexibility - can stretch to get more movement
Bulk of tissue
Genetics
Age
Age and ability to repair tissue
As we get older, our ability to repair tissue is harder
What is the weakest part of the skeleton
Joints
Joints: Function - growth - why do we need joints
Since bones can’t undergo interstitial growth, need natural breaks in bones to create areas of soft tissue that can undergo interstitial growth
Functional classification of joints: Synarthrosis - ankylosis
Where joints disappear and bones fuse
Functional classification of joints: Amphiarthrosis - vertebral column
As you go down the vertebral column, you’re adding load, so amount of movement in each intervertebral disc reduces as you go down the column
Functional classification of joints: Which joint is damaged the most
Diarthrosis
What is the most common joint
Synovial joints
Synovial joints - restriction
Unlike other types of joints, they aren’t restricted by properties of a specific tissue
Apart from articular capsule, ends of articulating bones in a synovial joint are mostly free
What type of joint is a synovial joint
Diarthrosis
Synovial joints: Common features
Articular cartilage
Articular capsule
Joint cavity
Synovial fluid
Synovial joints: Articular cartilage - what is it
A specialised type of hyaline cartilage (type of CT)
Synovial joints: Articular cartilage - function
Protect ends of bones that come tgt to form a joint
Absorb shock
Support heavy loads for long periods
Provide a near frictionless surface when combined with synovial fluid
Synovial joints: Articular cartilage - structure
Thin layer - typically 1-7mm thick
Attached to bone
Synovial joints: Articular cartilage - degradation
Degradation of articular cartilage leads to arthritis
Synovial joints: Where is synovial fluid found
In the joint cavity
How much fluid do joints have
Just enough to lubricate them and keep the cartilage alive, but not excessive amounts (would cause problems)
Types of cartilage in body
Fibrocartilage
Hyaline cartilage
Elastic cartilage
Can bone absorb shock
No - it is hard so can’t absorb shock
Synovial joints: Articular cartilage - how is it different from other soft tissues
Unlike many other soft tissues, it can endure long periods of compression
Synovial joints: Articular cartilage - CoF
Coefficient of friction
A measure of how much friction 2 surfaces have when rubbed tgt
Joints v frictionless
Why are joints so frictionless
Due to design of cartilage and synovial fluid
Chondro-
Cartilage
Synovial joints: Articular cartilage - what is it composed of (%)
Cells ~5%
ECM ~95%
Synovial joints: Articular cartilage - cells
Chondrocytes
Synovial joints: Articular cartilage - where do cells live
In lacunae
Synovial joints: Articular cartilage - cells - function
Build, repair, maintain cartilage
Synovial joints: Articular cartilage - cells - found individually or in groups
Depending on zone, can occur by themselves or in groups called nests
Synovial joints: Articular cartilage - ECM - ground substance - fluid component
Water (and soluble ions) ~75% WW
Can move in and out of tissue
Synovial joints: Articular cartilage - ECM - ground substance - solid component
Glycosaminoglycans (GAG)
Proteoglycans (PG)
Fixed inside tissue
Provides swelling and hydrating mechanism
Glycosaminoglycans (GAG) - example(s)
Hyaluronic acid
Chondroitin sulphate
Keratin sulphate
Proteoglycans (PG) - example(s)
Aggrecan
Synovial joints: Articular cartilage - ECM - fibres
Collagen (type II) ~75% DW
Fixed inside tissue
Provides structural integrity to tissue
Specific zonation patterns
Glycosaminoglycans (GAG) are hydrophobic/hydrophilic
Hydrophilic
Why must cartilage be able to resist force
It has multiple forces subjected to it
Cartilage - types of forces
Expansion
Compression
Shear
Articular cartilage: Expansion
Lifts surface of cartilage away from bone
Articular cartilage: Shear forces
Under extreme load, one surface can slide against another in one or multiple planes
Articular cartilage - zones
Surface zone
Middle zone
Deep zone
Calcified cartilage
Articular cartilage: Surface zone - thickness
5-10% of total depth of functional cartilage
Articular cartilage: Surface zone - collagen fibres
Very fine and arranged parallel with surface - resists shear forces
Very tightly packed
Articular cartilage: Surface zone - chondrocytes
Don’t have lots of space –> flat
Articular cartilage: Surface zone - proteoglycans
Very few
Poke up through surface - help lubricate surface –> reduces friction
Articular cartilage: Middle zone - thickness
40-45% thickness
Articular cartilage: Middle zone - collagen fibres
Much thicker and less tightly packed
Orientated ~45 degrees to surface
Articular cartilage: Middle zone - chondrocytes
Have enough room to pump up –> chondrocytes slightly larger
Sit inside lacunae between bundles of collagen fibres
Articular cartilage: Middle zone - proteoglycan
From here is where we start to see proteoglycan content increase
Articular cartilage: Deep zone - collagen fibres
Strong bundles
Run perpendicular to surface
Articular cartilage: Deep zone - chondrocytes
Form stacks called nests
Likely to be undergoing division
Articular cartilage: Deep zone - interstitial growth
Chondrocytes divide and put more ground substance between each other –> gap between them separates and chondrocytes move up
Articular cartilage: Deep zone - proteoglycans
Highest PG content
Articular cartilage: Tide mark
Junction between functional cartilage and 4th zone (calcified cartilage)
i.e. mix of normal and calcified cartilage
Articular cartilage: Functional cartilage - zones
Top 3 zones; surface, middle, deep
Articular cartilage: Tide mark - collagen fibres
Collagen fibres continue through tide mark and calcified zone
Articular cartilage: When/where do collagen fibres anchor
Anchor themselves onto subchondral bone at osteochondral junction
Articular cartilage: Calcified cartilage - thickness
5-10% of thickness
Articular cartilage: Calcified cartilage - chondrocytes
Chondrocytes sit inside calcified lacunae - secrete hydroxyapatite
Articular cartilage: Calcified cartilage - why do we need it
If went straight from a deformable tissue to a non-deformable tissue, would put lots of strain on junction between them
Calcified cartilage has properties of both - helps distribute shear force over bigger surface
Articular cartilage: Calcified cartilage - proteoglycans
Low in PG, high in hydroxyapatite
Articular cartilage: Osteochondral junction
Boundary between cartilage and bone
Articular cartilage: Osteochondral junction - collagen fibres
Don’t go through osteochondral junction
Articular cartilage: Osteochondral junction - proteoglycans
Rich in cement-like proteoglycans –> cement line helps stick cartilage onto osteochondral junction
Articular cartilage: Osteochondral junction - structure
Very convoluted - increases SA for adhesion and makes it less likely to delaminate the cartilage off the surface
Articular cartilage - age
Functional layers get thinner as we get older
Healthy joint has ~5mm cartilage in a joint, but in elderly may be ~2mm
Articular cartilage: Middle and deep zone - proteoglycans
Rich in proteoglycans Causes swelling (water moves into these areas) --> hydration
Articular cartilage - numb
Cartilage is avascular and aneural –> numb tissue
Important because it’s getting compressed often
Articular cartilage - blood vessels
There’s occasionally subchondral BVs that come up from bone, but they don’t go further than the calcified zone
Articular cartilage: Chondrocytes are nourished by…
Diffusion only
Articular cartilage: What is a glycosaminoglycan (GAG) made up of
Repeating disaccharide units
Articular cartilage: Monosaccharide - charge
Often have carboxyl groups / sulphate groups on them, so when put in solution, end up with a -ve charge
Articular cartilage: Aggrecan
A common proteoglycan found in cartilage
Articular cartilage: GAG - charges
When GAG compresses = lots of resistance from -ve charges - fundamental trait of cartilage
If you remove the load, it acts like a molecular spring
Articular cartilage: Proteoglycan (PG)
Many GAGs attached to a protein core
-ve charges repel each other, so GAGs stand out like bristles on a bottle brush
Articular cartilage: Large proteoglycan complex
Proteoglycans attached to a long hyaluronic acid chain
Can attach to collagen fibres
Articular cartilage: What zones are the -ve charges found
Middle and deep zone
Attracts +ve ions
Articular cartilage: Loading cycle - steps
- Recently unloaded cartilage
- -ve charges on repeating disaccharide units attract +ve ions into cartilage from joint space
- Increased ionc conc creates an osmotic P/gradient –> draws water into matrix –> cartilage starts to swell –> surface zone moves away from subchondral bone
- As cartilage swells, collagen is placed under tension. Eventually swelling F = tension F, and cartilage stops swelling = unloaded equilibrium. Pre-stressed tissue
- When load is introduced, the fluid component is squeezed out of cartilage back into joint space - lubricates joint
- Loss of fluid reduces V of cartilage = creep. Pushes -ve charges close tgt. Eventually compressive load will be supported by solid component and repulsion of -ve charges. Cartilage stops shrinking = loaded equilibrium
- Back to start
Articular cartilage: Loading cycle - fixed solid component
Proteoglycan complex in matrix
Articular cartilage: Loading cycle - mobile fluid component
Ca2+
K+
Na+
H2O
Articular cartilage: What happens if you cut the collagen fibres in the deep zone
The cartilage will continue to swell
So, collagen is important for stopping it getting to its full V
Articular cartilage: Arthritic finger joint - osteophytes
Bone growing in ‘weird’ places with the aim to increase contact area to reduce loading
Articular capsule: All synovial joints are surrounded / enclosed by a…
Joint capsule, which forms a sleeve around the joint, connecting the ends of the bones
Articular capsule - tightness
Needs to be suitably loose to permit joint to function properly
Can become tight at extreme limits of natural range of joint movement - protects from damage by excessive movement
Articular capsule: Perforated by?
Vessels and nerves, and may be reinforced by ligaments
Ligaments
Dense regular CT connecting bone to bone
Poor blood supply –> takes a while to repair
Articular capsule - parts
Comprised of an outer fibrous layer and an inner synovial membrane
Articular capsule: Fibrous layer
Outer layer of dense CT (regular and irregular)
Variable in thickness
Articular capsule: Fibrous layer - collagen fibres
Made up of parallel and interlacing bundles of collagen fibres that are continuous with periosteum of bone
Regular vs irregular fibres
Regular = orientated in one direction Irregular = orientated in diff directions
Articular capsule: Fibrous layer - capsular ligaments
Thicker sections of the capsule
Resists predominant and tensional forces and check excessive joint movement
Articular capsule: Fibrous layer - function
Supports synovial membrane
Protects synovial membrane and whole joint
Articular capsule: Fibrous capsule - vascular?
Poorly vascularised but is richly innervated
This is why it hurts to sprain your joints
Articular capsule: Fibrous layer - what is it made of
Fibroblasts (secretes collagen)
Nerves (pain and proprioceptors)
Blood vessels (usually transitory)
Articular capsule: Synovial membrane
Inner layer of loose CT
Variable thickness
Articular capsule: Synovial membrane - where is it found
Lines all non-articular surfaces inside joint cavity, up to edge of articular cartilage
Articular capsule: Synovial membrane - layers
Intima
Subintima
Articular capsule: Synovial membrane - intima
Thin
Normally only 1-3 cells (synoviocytes) thick; completely absent in some joints
Articular capsule: Synovial membrane - intima - synoviocytes
Secrete some components found in synovial fluid
e.g. hyaluronic acid and glycoproteins - lubricate
Important for specialising synovial fluid
Articular capsule: Synovial membrane - subintima
Highly vascular
Helps maintain and protect articular capsule
Articular capsule: Synovial membrane - number of villi
Increases as we get older
Articular capsule: Synovial membrane - function
Makes it slippery
Articular capsule: Synovial membrane - intima vs subintima - density
Subintima not as densely packed
Articular capsule: Synovial membrane - subintima - blood vessels
Lots of BVs - important for health of cartilage because they’re the closest BVs to the avascular tissue
Leak out fluid and create synovial fluid
Constant exchange
Articular capsule: Synovial membrane - adipocytes
Can vary from not there to giant fat pads
Act like little cushions around joint to help reduce V of joint and cushion capsule
Synovial joints: Joint cavity
The small area between the articulating surfaces
Synovial joints: Joint cavity - peripheral margins
Filled by the collapsing and in-folding of synovial membrane (villi)
Contains a small amount of synovial fluid
Synovial joints: Joint cavity - amount of synovial fluid
In a healthy joint cavity rarely exceeds 2mL
Synovial joints: Synovial fluid
A clear / slightly yellow fluid that is an ultrafiltrate of blood plasma
Synovial joints: Synovial fluid - pathway
Leaks out of BVs into synovial membrane (subintima) into joint space
Synovial joints: Synovial fluid - free cells
Found in low conc
Monocytes, lymphocytes, macrophages, synoviocytes
Synovial joints: Synovial fluid - function
Joint lubrication
Shock absorption
Chondrocyte metabolism
Joint maintenance
Synovial joints: Joint cavity acts like…
Peritoneum
In any given joint, about ____ of the cartilage is in contact with the opposing cartilage
Half
Other half is likely to be in contact with synovial membrane of joint capsule
Synovial joints: Joint cavity - Why do we want to keep the amount of fluid between capsule and cartilage as low as possible
To aid exchange between BVs and synovial membrane
Synovial joints: What happens if there’s too much fluid
Nutrients become diluted
Synovial joints: What structures help fill in crevices of cavity to make sure there’s not too much fluid
Villi and fat pads
Bones and joints - passive
Can’t generate movement themselves
Muscle - push or pull
Muscle can only pull - doesn’t push
Does muscle always contract
No - it’s a contractile tissue but doesn’t always contract
Muscle - function
Convert chemical energy (ATP) into mechanical energy
Muscle: Function - stability
Stabilise joints and maintain posture
Muscle: Function - communication
Muscles are used for facial expression, body language, writing and speech
Muscle: Function - control of body openings and passages
Some sphincters help control admission of light, food and drink that enter our bodies
Elimination of waste
Sphincters
Ring-like muscles
Muscle: Function - heat production
Skeletal muscle can produce up to 85% of our body heat (biproduct)
Used to maintain body at 37 degrees
How much of our body mass is muscle
~40-50%
Origin vs insertion
Origin: attachment that moves the least during muscle contraction
Generally closer to axial skeleton and more proximal, but can change depending on action
Insertion: attachment that moves the most during muscle contraction
Generally closer to appendicular skeleton and more distal, but can change depending on action
Skeletal muscle: What is the contractile component
Muscle belly
Skeletal muscle: Muscle belly
An organ made up of multiple tissues, including muscle tissue
Pulls on bones via tendons
Tendons connect…
Muscle to bone
Skeletal muscle: Tendon
Dense regular CT
Poor blood supply
Skeletal muscle: Tendon - function
Strong and good at resisting tension
Skeletal muscle: Myotendinous junction (MTJ)
Between muscle belly and tendon
One of the weaker areas of muscle organ
Skeletal muscle: Which part is most often damaged when the muscle is strained
Myotendinous junction (MTJ) You do damage the muscle belly when you overwork it, but its highly vascular so it repairs itself quite quickly
Skeletal muscle: Osteotendinous junction (OTJ)
Between tendon and bone
Very strong because lots of collagen fibres in tendon blend with collagen matrix of bone (Sharpey’s fibres)
Skeletal muscle: Osteotendinous junction (OTJ) - under stress
If under extreme stress, it’s often not the junction that breaks, but the bone that comes away
Skeletal muscle: Biceps - primary function
Flex the arm
Skeletal muscle: How many attachments do muscles have
Most muscles have 2 attachments (origin and insertion) but some can have more
Skeletal muscle - fundamental unit
Myocyte
Skeletal muscle: Myocyte - size/length
Can be from mm to cm long
Size: 10-fold difference from min to max
Skeletal muscle: Myocyte - nuclei
Many nuclei - up to 100
Are a syncytium
Skeletal muscle: Myocyte - cell membrane
Unique cell membrane called sarcolemma
Conducts APs very quickly
Skeletal muscle: Myocyte - sarcoplasm
Inside cell
Has a lipid reserve and myoglobin
Skeletal muscle: Myocyte - myoglobin
A protein that can store O2
Not as good as haemoglobin (~4x more) but still gives cells an O2 store
Can function anaerobically but less efficient
Skeletal muscle: Myocyte - vascular?
Very vascular tissue
Skeletal muscle: Myocyte - strength
Quite delicate
Skeletal muscle: Myofibrils
Contractile organelles that run the length of the cell
Skeletal muscle - contractile unit
Sarcomere
Skeletal muscle: Myofibrils - structure
Have sarcomeres next to each other, each of which is defined in its boundary by a Z-disc/band/line
Skeletal muscle: Myofibrils - sarcomere arrangement
All arranged in series
When contract –> pulls Z-discs closer tgt
Skeletal muscle: Myofibrils - sarcomere bands
A band = dark band in middle of sarcomere
I band = has Z disc running through it, and is shared by neighbouring sarcomeres
Myo-
Muscle
Sarco-
Flesh
Myofibril, myocyte, fascile and muscle
Myofibril = many sarcomeres Myocyte/myofibre = bundle of myofibrils Fascicle = bundle of myocytes Muscle = bundle of fascicles
Skeletal muscle: Fascicle - no of myocytes
Variable - can be a few or hundreds depending on muscle
Skeletal muscle: Fascicle - endomysium
Loose irregular CT
Runs around myocytes and packages them within the fascicle
Supporting tissue
Allows capillaries and motor neurons to run down cell
Skeletal muscle: Fascicle - what puts down the endomysium
Fibroblasts
Skeletal muscle: Fascicle - BM
Immediately outside the sarcolemma is a BM which is partly secreted by myocyte and fibroblasts
A thin, specialised CT that blends with endomysium
Skeletal muscle: Muscle - no of fascicles
Highly variable
Skeletal muscle: Muscle - perimysium
Dense irregular CT bundling fascicles tgt
Thicker bundles of collagen and more dense
Bigger vessels
Skeletal muscle: Muscle - epimysium
Dense irregular CT
Surrounds perimysium and entire muscle
Skeletal muscle: Muscle - continuum?
Structures blend with each other
Skeletal muscle: Order of layers (superficial to deep)
(skin) (superficial fascia / subcutaneous layer) (deep fascia) Muscle Epimysium Perimysium Fascicle Endomysium Myocyte Sarcolemma Sarcoplasm Myofibril
Skeletal muscle: What does the deep fascia cover
Most of your muscles
Fascia
A collagenous sheet-like material found all over the body
Often has regional names
Skeletal muscle: Where is the deep fascia particularly important
Appendicular skeleton
Skeletal muscle: Superficial fascia / subcutaneous tissue
Fatty layer under skin
Acts like a cushion and thermal blanket
Skeletal muscle: Deep fascia - intermuscular septa
Where deep fascia comes away from outer layer and goes deep
Septa
Wall / partition
Skeletal muscle: Deep fascia - interosseous membrane
A piece of fascia that links 2 bones
Skeletal muscle: Deep fascia - investing fascia
Intermuscular septa and interosseous membranes
A continuation of the deep fascia that leaves the outer wall and goes deep
Usually anchors onto deeper structures (often bone)
Skeletal muscle: Deep fascia - compartments
Muscle in a limb often divided into compartments
Groups muscles with some commonality
Skeletal muscle: Deep fascia - compartment - specificity
Epimysium is specific to that muscle, whereas deep fascia isn’t specific to a tissue
Skeletal muscle: Deep fascia and epimysium
In most areas the epimysium can move and glide under deep fascia
Sometimes deep fascia will blend with epimysium depending on muscle
Skeletal muscle: Deep fascia - compartment - common function
If you have a compartment with a common function, the other side of the limb will have a compartment that are antagonists
Skeletal muscle: Deep fascia - compartment - supply
Muscles in a compartment often have the same blood and nerve supply
Skeletal muscle: What is fascia made of
Collagen, which doesn’t stretch easily so if muscles in compartment contract, the belly expands
Veins have valves so if mucles contract –> compress veins against compartment –> aids venous return back to heart
Skeletal muscle: Deep fascia - compartment - swelling
If excessive swelling, veins may be compressed so hard against the wall that they get occluded –> still have arteriole supply but drainage is affected –> edema
Usually happens in trauma
Skeletal muscle: Deep fascia - type of CT
Dense CT (regular and irregular)
Skeletal muscle: Deep fascia - When investing fascia comes in contact with bone…
It fuses with the periosteum
Skeletal muscle: In some areas, the deep fascia is part of the ______ and can act as a an attachment point
Muscle tendon
Hyperplasia
When a tissue or organ increases in size due to an increase in cell no
Does skeletal muscle undergo hyperplasia
Not typically; they usually undergo hypertrophy
Skeletal muscle: Hypertrophy
Increase in muscle size is due to increases in size of individual myocytes as more myofibrils are packed into each muscle cell
Skeletal muscle: What factors can stimulate skeletal muscle hypertrophy
Repetitive contraction of muscles to near maximal tension (heavy resistance training)
Anabolic steroids
Skeletal muscle: Anabolic steroids
Variants of testosterone
Skeletal muscle: Anabolic steroids - function
Increase protein synthesis through interactions with specific target tissues, e.g. skeletal muscle and bone
Skeletal muscle: Anabolic steroids - side effects
Removes regulatory process of testosterone levels --> Acne Hair loss Excess hair gain in wrong places Liver failure Shrivelled testes Infertility Increased susceptibility to coronary disease Mood swings
Skeletal muscle: What were anabolic steroids originally designed for
To help people who had diseases that caused their muscles to waste away - retards this process by overstimulating cells that manufacture protein
Skeletal muscle: Atrophy
When the muscle decreases in size due to reduction of myofibrils in myocytes
Skeletal muscle: Atrophy - when does it occur
When muscles aren’t used or stimulated by motor neurons –> can result in paralysis
Also as part of diseases, e.g. heart failure, diabetes, cancer, AIDS
Skeletal muscle: Atrophy - when does normal loss of muscle mass start
Age of 20 years
Rate is accelerated after age of 50
By 80, ~40% of our muscle mass will be lost
Skeletal muscle: Atrophy - is it reversible
If atrophy is not permitted to proceed too far, it can often be reversed
But, hypoplasia is not reversible
Skeletal muscle: Atrophy - what is muscle replaced by
Fat and CT
Skeletal muscle: Atrophy - hypoplasia
Where muscle loss occurs due to the loss of myocyte
Difficult to reverse
Skeletal muscle: How are myocytes created
By fusion of many myoblasts during embryonic stage of life = syncitium
Skeletal muscle: Myocytes - division
Since they contain many nuclei and are v large cells, can’t divide by mitosis
Skeletal muscle: Formation of satellite cells / myoblasts
During formation of myocytes, not all myoblasts fuse
Some remain as individual cells –> satellite cells
Skeletal muscle: Satellite cells / myoblasts - where is it found
They lie beside the muscle fibres, outside the sarcolemma but within the same BM
Skeletal muscle: Satellite cells / myoblasts - division
They are the only cells in muscle that can divide (mitosis) and fuse with each other and myocytes to repair damage
Skeletal muscle: Satellite cells / myoblasts - limited ability?
Have a limited ability to replace muscle fibres that die from old age or injury
Skeletal muscle: At what age do you start growing your muscles
~8 weeks
Skeletal muscle: How many myocytes do you have when you’re born
About the number of myocytes for your life
Skeletal muscle: What are myoblasts
Cells that put down / build your muscle
Denervation of skeletal muscle
Since many muscles have a dual nerve supply, if you lose one supply, the other one can try pick it up –> some myocytes become overstimulated –> lose fine control of muscle
Myostatin
Turns off satellite cells
Skeletal muscle: Recruiting myocytes as we need them allows for…
Smoother action
Collagen fibres at MTJ blend with…
Collagen in the endomysium
Tendon is an extension of…
Fused endo, peri and epimysium of muscle
Deep fascia groups…
Muscles with similar function tgt
What is the most abundant cartilage
Hyaline cartilage
Alignment of osteons is along…
Lines of physical stress on a long bone
Intrinsic ligaments
Thicker part of fibrous layer
Bone with large amount of osteoid is likely to be…
More flexible
What type of joint is most likely used for growth
Synarthrosis
