Musculoskeletal System

Question 1

osteogenic cell

Answer 1

precursor -> unspecialised stem cells in mesenchyme (i.e. embryonic connective tissue)
location -> surface of bone in periosteum and endosteum, inside central canals of compact bone
function -> resting layer, normally dormant, but can divide and supply developing bone with bone forming cells (i.e. osteoblasts)

Question 2

osteoblast

Answer 2

precursor -> osteogenic cell
location -> usually in a layer under the periosteum or endosteum, wherever new bone is being formed
function -> active layer, synthesis, deposition, calcification of osteoid, can inhibit activity of osteoclasts

Question 3

osteoid

Answer 3

• organic extracellular matrix of bone
• synthesised by osteoblasts prior to mineral deposition
• mainly collagen (i.e. 70% of osteoid) and proteoglycans, other proteins, water
• eventually infused with hydroxyapatite (i.e. calcification)
• at first occurs rapidly, slows down as water (i.e. mobile medium) is displaced
• makes bone strong but dense, so nutritive fluids cannot freely diffuse through it

Question 4

osteocyte

Answer 4

precursor -> osteoblast
location -> surrounded by bone, trapped within lacunae, long cellular processes inside canaliculi
function -> bone tissue maintenance, canaliculi form live lattice inside bone, can communicate with neighbouring cells, localised minor repair, rapid Ca2+ exchange (i.e. between blood and bone)

Question 5

osteoclasts

Answer 5

precursor -> fusion of monocyte progenitor cells, forms syncytium, can be signalled by osteocytes
location -> at sites where bone resorption is occurring
function ->
- secretes acid and enzymes (i.e. acidic environment activates secreted enzymes)
- dissolves the mineral and organic components of bone (i.e. decalcify → remove organic matter)
- ruffled border increases surface area for secretion and absorption
- forms a pit (i.e. Howship’s lacunae), keeps microenvironment in place
- clear zone adheres osteoclast to lacunae, ensures acid doesn’t escape
- absorbs released components by endocytosis
- releases calcium and phosphate into blood by exocytosis

Question 6

extracellular matrix of bone

Answer 6

resists torsion (i.e. compression + tension)
water -> not well hydrated
fibres -> organic, mainly type I collagen fibres, resists tension (i.e. stretching/pulling)
ground substance ->
- inorganic
- hydroxyapatite (i.e. calcium and phosphate reserve)
- resists compression (i.e. squeezing/crushing)

Question 7

appositional growth

Answer 7

• adding tissue to existing surface, usually to periosteum
• chemical signal → osteogenic cells on periosteum divide → form osteoblasts
• osteoblasts deposit osteoid → calcifies → osteoblasts become trapped in lacunae → form osteocytes
• when growth stops, osteoblasts can convert back into osteogenic cells or die by apoptosis

Question 8

bone resorption

Answer 8

• removing bone, usually from endosteum
• chemical signal → monocyte precursor cells leave blood vessels → start to fuse on bone surface → form osteoclasts → dissolve bone
• osteoclasts eventually die by apoptosis → resorption stops → blood vessels grow into new space

Question 9

interstitial growth

Answer 9

• occurs in softer tissues that can deform (i.e. not in bone)
• cells divide mitotically
• grows tissue from within → cells divide inside the tissue, secrete more extracellular matrix
• bone is designed to resist deformation, can only grow by appositional growth

Question 10

endochondral ossification

Answer 10

• how long bones grow in length, despite articular cartilage on ends
• epiphysis of long bones can come away from metaphysis, natural break
• ephiphyseal/growth plate in space, contains hyaline cartilage
• chondrocytes inside divide (i.e. interstitial growth), increase thickness of plate
• cartilage and chondrocytes in contact with metaphysis die
• providing surface for osteoblasts to lay down new bone, and for osteoclasts to remove cartilage
• eventually rate of cartilage resorption exceeds appositional growth of bone, epiphysis fuses → final bone

Question 11

lamellae

Answer 11

• layers/sheets of new bone deposited onto a surface
• typically put down in the same direction within a layer
• can alternate up to 90 degrees out of phase between layers
• enables bone to withstand forces from different directions → significantly stronger

Question 12

unit formation of spongy vs compact bone

Answer 12

spongy -> grows outwards into medullary cavity
compact -> grows inwards forming tunnels (i.e. haversian canals)

Question 13

function of spongy bone

Answer 13

• supports the outer cortex of compact bone in areas where forces occur from multiple directions
• helps reduce the weight of bone
• high surface area
• rapid turnover rate of Ca and P

Question 14

function of compact bone

Answer 14

• provide strong dense shell of bone on outside
• thickens areas that are exposed to large forces

Question 15

primary osteon formation

Answer 15

• formed around existing blood vessel
• occurs when bone is growing and new bone tissue is being deposited on existing surface (i.e. appositional growth)
• osteoblasts in active periosteum either side of a blood vessel put down new bone
• forms periosteal ridges around blood vessel
• ridges come together and fuse
• forms tunnel around blood vessel
• active periosteum becomes active endosteum that lines the tunnel
• osteoblasts in active endosteum build concentric lamellae onto walls of tunnel
• tunnel is filled inward toward centre
• forms new osteon
• active endosteum becomes resting endosteum

Question 16

secondary osteon formation

Answer 16

• osteocytes detect damage → releases signal, initiates process
• monocyte progenitor cells leave blood vessels → attach to surface of old bone → forms group of osteoclasts
• start boring their way through old bone → forms cutting cone, creating tunnel
• osteoblasts move in behind the cutting cone
• line the tunnel wall → form new active endosteum
• start depositing osteoid onto walls → calcifies → forms new lamella
• blood vessel grows into newly formed tunnel to supply the cells
• osteoblast continue to deposit new concentric lamellae onto wall of tunnel → slowly fills it in
• active area behind cutting cone is called closing cone
• some osteoblasts are strapped in newly deposited lamellar bone → form osteocytes
• tunnel is reduced to size of a typical Haversian canal
• remaining osteoblasts lining Haversial canal either die or become osteogenic cells (i.e. resting endosteum)
• forms new osteon

Question 17

function of joints

Answer 17

• movement
• force transmission
• growth

Question 18

functional classification of joints

Answer 18

synarthrosis
- immovable joints, high stability
- for growth and force transmission
- commonly found in axial skeleton
amphiarthrosis
- slightly movable, medium stability
- for force transmission
- commonly found in axial skeleton
diarthrosis
- majority of joints
- freely movable, low stability
- commonly found in appendicular skeleton

Question 19

synovial joint

Answer 19

• a structural classification of diarthrosis
• not restricted by properties of specific tissues which hold the ends of bones together (i.e. unlike synarthrosis/amphiarthrosis)
• ends of articulating bones are mostly free (i.e. apart from articular capsule)
• permits a wide range of motion
• all have articular cartilage, articular capsule, joint cavity, synovial fluid

Question 20

articular cartilage

Answer 20

• specialised type of hyaline cartilage (i.e. type of connective tissue)
• protects the end of bone that come together to form a joint
• absorbs shock and supports heavy loads for long periods
• provides a smooth, near frictionless surface when combined with synovial fluid
• contains no blood vessels, nerves or lymphatics

Question 21

cells of articular cartilage

Answer 21

• ~5%
• chondrocytes
• build, repair and maintain cartilage
• live in lacunae
• occur by themselves or in groups (i.e. nests) depending on the zone
• nourished by diffusion only

Question 22

ground substance of articular cartilage

Answer 22

water
- ~75% of wet weight
- fluid component
- ions and gases dissolve here, provide nutrients to chondrocytes
- can move in and out of tissue
GAGs and PGs
- ~25% of dry weight
- glycosaminoglycans → e.g. hyaluronic acid, chondroitin
- proteoglycans → e.g. aggrecan
- hydrophilic, provides swelling and hydrating mechanism for proper function of cartilage
- part of solid component, fixed inside tissue

Question 23

fibres of articular cartilage

Answer 23

• ~75% dry weight
• collagen, mostly type II (i.e. flexible)
• provides structural integrity to tissue
• has specific zonation patterns
• tethers surface zone to subchondral bone
• part of solid component, fixed inside tissue

Question 24

zonation of articular cartilage

Answer 24

surface zone
- ~5-10% of thickness
- low proteoglycan content
- fibres arranged in parallel to surface to resist shearing forces
- chondrocytes stuck between fibres → flat and oblong
middle zone
- ~40-45% of thickness
- higher proteoglycan content than surface zone, lower proteoglycan content than deep zone
- fibres in thick bundles, not as densely packed
- round chondrocytes, occur by themselves
- secrete extracellular matrix
deep zone
- ~40-45% of thickness
- highest proteoglycan content
- fibres arranged perpendicular to surface
- round chondrocytes, occur in nests
- secrete extracellular matrix, migrates to upper tissues
tide mark
- between functional zone and transitory zone
- low proteoglycan content
- calcified
calcified cartilage
- low proteoglycan content
- high in hydroxyapatite
- not as calcified as bone
- chondrocytes sit inside lacunae, attached onto calcifying bone
osteochondral junction
- between cartilage and bone
- rich in adhesive proteoglycans
- cement line here

Question 25

proteoglycan complex

Answer 25

glycosaminoglycan - repeating disaccharide unit - have negative charges - can be as long as 50 repeating units (i.e. CS) - e.g. chondroitin sulphate (CS), keratin sulphate (KS) proteoglycan - has core protein - many glycosaminoglycans attached - negative charges of GAGs repel → stand out like bristles - e.g. aggrecan (i.e. ~125 CS, ~50 KS) hyaluronic acid - very long GAG (i.e. ~25,000 repeating units) - proteoglycans attach - form extremely large proteoglycan complex - attach to collagen fibres

Question 26

loading cycle of articular cartilage

Answer 26

unloaded - negative charges on proteoglycan complexes attract positive ions (e.g. Ca2+, K+, Na+) from synovial fluid → increases ion concentration in matrix of cartilage - creates an osmotic gradient → draws water and dissolved O2 / nutrients into matrix → cartilage begins to swell - places increasing tension on collagen fibres → eventually swelling force = tension force (i.e. unloaded equilibrium) → cartilage stops swelling loaded - initially densely packed collagen in surface zone impedes fluid movement out - eventually with sustained pressure, fluid component (i.e. water, dissolved CO2, metabolites and positive ions) is squeezed out into synovial fluid/joint space - loss of fluid reduces volume of cartilage (i.e. creep) → pushes negative charges closer together - eventually compressive force = resistive force of solid compartment (i.e. loaded equilibrium) → cartilage stops shrinking

Question 27

fibrous layer of articular capsule

Answer 27

- outer layer of dense connective tissue, both irregular and regular - variable in thickness, thicker sections called capsular ligaments - made up of parallel but interlacing bundles of white collagen fibres, maintained by fibroblasts - continuous with periosteum, can perforate into bone (i.e. Sharpey’s fibres) - poorly vascularised (i.e. because of high tension forces, most blood vessels are transitory) - richly innervated (i.e. pain and proprioceptors) - resist tensional forces, check excessive/abnormal joint movement - supports and protects both synovial membrane and joint

Question 28

synovial membrane of articular capsule

Answer 28

- inner layer of loose connective tissue - variable thickness - lines all non-articular surfaces inside joint cavity (i.e. bc its a fragile layer), up to edge of articular cartilage - has villi that increase surface area intima - thin, ~1-3 cells thick - has cells called synoviocytes - secretes some components of synovial fluid (i.e. hyaluronic acid, lubricating proteins) subintima - helps maintain and protect articular capsule during normal movement - highly vascular, leakage of filtrate from blood plasma becomes synovial fluid - has fibroblasts, don’t put down as much collagen, more reticular and elastic fibres - has macrophages, cleans up debris in joint space - has adipocytes, packaging, acts like cushions

Question 29

synovial fluid

Answer 29

- clear/slightly yellow fluid - amount rarely exceeds 2mL, bc more fluid → larger distance of chondrocytes in cartilage from blood vessels in synovial membrane - ultrafiltrate of blood plasma that leaks out of blood vessels in subintima - other components secreted by synoviocytes (i.e. hyaluronic acid, lubricating proteins) - low concentrations of free cells (i.e. monocytes, lymphocytes, macrophages, synoviocytes) - lubricates joints, absorbs shock, metabolisation of chondrocytes, maintains joints

Question 30

function of muscle

Answer 30

- convert chemical energy (i.e. ATP) into mechanical energy - movement - stability - communication - control of body openings and passages - heat production

Question 31

tendon

Answer 31

- attachment of muscle to bone - dense regular connective tissue - contains a lot of collagen

Question 32

myofibril

Answer 32

- ~1 micron thick - sarcomeres → contractile units - Z-disc/line → boundary that separates sarcomeres, pulled closer together during contraction - A-band → dark band - I-band → light band, Z-disk through centre, shared by neighbouring sarcomeres, shortens during contraction

Question 33

myocyte

Answer 33

- muscle fibre/cell - ~10-100 microns thick (i.e. variable) - jam packed with myofibrils - has multiple nuclei (i.e. syncytium), pushed to the edges of cell - sarcoplasm → cell cytoplasm, fills area between myofibrils, contains mitochondria and myoglobin (i.e. O2 storage protein) - sarcolemma → fast conductor of action potentials down entire length of myocyte, allows for coordinated contraction

Question 34

endomysium

Answer 34

- loose irregular connective tissue - surrounds myocytes - contains nerves and capillaries that supply myocytes

Question 35

fascicle

Answer 35

- a bundle of myocytes - number of myocytes is variable

Question 36

perimysium

Answer 36

- dense irregular connective tissue - surrounds fascicles - blend into epi/endomysium

Question 37

epimysium

Answer 37

- dense irregular connective tissue - surrounds perimysium and entire muscle

Question 38

deep fascia

Answer 38

- dense regular/irregular connective tissue - underlies skin and superficial fascia - makes up outer wall of muscle compartments - fuses with periosteum when in contact with bone - in most areas epimysium glides under deep fascia, in some areas can act as attachment point for muscle - keeps blood vessels in place for muscles to act as venous pump investing fascia - deeper walls/septa that separate individual muscles or muscle groups - intermuscular septa → investing fascia between different groups of muscle - interosseous membrane → investing fascia between bones

Question 39

causes of hypertrophy

Answer 39

heavy resistance training (i.e. repetitive contraction of muscles to near maximal tension) anabolic steroids - variants of testosterone synthesised by pharmaceutical companies - increased protein synthesis in target tissues (incl. skeletal muscle, bone) - affects other tissues, causing side effects

Question 40

causes of atrophy

Answer 40

- muscles are not used or stimulated by motor neurons - complex pathology of diseases such as heart failure, diabetes, cancer, AIDs

Question 41

satellite cells

Answer 41

- some myoblasts fuse during embyonic stage to form myocytes - remaining myoblasts remain as individual cells and become satellite cells - satellite cells lie beside muscle fibres, outside sarcolemma, within the same basement membrane - the only cells in muscle that can divide and fuse with each other and with myocytes - repair any damage that may have occurred, replace muscle fibres that die from old age/injury

Question 42

functions of skeletal muscle connective tissue

Answer 42

- provide the organisation and scaffolding upon which the muscle is constructed, mostly in utero - provide a medium for blood vessels and nerves to gain access to myocytes - resist tension, prevent excessive stretching and therefore damage to myocytes - distribute the forces generated by muscle fibre contraction

Question 43

desmin

Answer 43

- protein that holds Z-lines of adjacent sarcomeres together - help align sarcomeres between myofibrils - allows sarcomeres to shorten together and pull in unison - why myocytes have a uniform striated appearance

Question 44

protein complex

Answer 44

- group of proteins containing dystrophin - form bridge between Z-lines of outermost myofibrils to sarcolemma, surrounding basement membrane, and endomysium - transmits contractile forces generated by sarcomeres to surrounding endomysium → coordinated contraction of neighbouring myocytes - contributes to strengthening of sarcolemma