363 Final

Marrow

• Found in nonmineralized tissue (all bones have except for ear ossicles)

• Composed of blood vessels, nerves, and various types of cells

• Generates the principal cells found in blood

• More red in young people, and turns yellow with age

Heterotopic Ossification

Building bone tissue outside of the bone organ, occurs following neurological trauma and can cause spinal cord injuries

Trabecular bone

• High porosity bone found in cuboidal bones, flat bones, and ends of long bones

• Bone matrix forms plates and rods called trabeculae

• Sometimes organized orthogonally, but often trabeculae are randomly arranged

• More so in epiphyseal sections of long bones

• Helps distribute load to periosteal surface

Cortical bone

• Dense bone found in shafts Of long bones and forming shell around vertebral bodies and other Spongy bone

• Contains Haversian and Vokmann’s canals as well as resorption cavities

Haversian canals

Structure in cortical bone aligned longitudinally, contain capillaries and nerves, about the size of a human hair (50 um)

Vokmann's canals

Short transverse aligned canals in cortical bone connecting Haversian canals, contain blood vessels/nerves

Resorption cavities

Temporary spaces created by osteoclasts during initial stage of bone remodeling

Lamellar bone

• Slowly formed, highly organized

• Parallel layers of anisotropic matrix of mineral crystals and collagen fibers

Woven bone

• Quickly formed, poorly organized

• Randomly arranged mineral and collagen fibers

• Sites of fracture healing, tendon/ligament attachments

  • Bone has to 'fix' as quickly as possible and throws collagen and mineral all over site to strengthen it

Primary bone

• Tissue laid down de novo on existing surface (e.g., periosteal surface during growth)

• Circumferential lamellar bone and plexiform bone

Secondary bone

• Bone resulting from remodeling, has been 'turned over'

• In compact bone consists of secondary osteons

• Most adult bone is secondary bone

T/F bone is both a tissue and an organ

True

Bone volume fraction (Bv)

Bv = Vm/Vt

• Vm = volume mineral matrix

• Vt = total volume

Porosity (Pv)

Pv = Vv/Vt

• Vv = void space

• Vt = total volume

Bone apparent density (p)

mass of a bone divided by its volume

p = bone mass/vt = (PmVm + PvVv)/ Vt

Bone ash fraction

Degree of mineralization of bone tissue, independent of porosity

pm = (PoVo + PaVa + PwVw)/Vm

where subscripts o, a, and m = organic, mineral, and water

Osteoclasts

• CLEAVE

• Multinucleated cells formed by the fusion of monocytes originating in bone marrow

  1. Demineralizes tissue with acids

  2. Resorbs collagen (organic component) by releasing enzymes

Osteoblasts

• BUILD

• Mononuclear cells that produce osteoid, the organic portion of the bone matrix

  • Osteoid = organic bone tissue (collagen)

• Differentiated from mesenchymal cells

Osteocytes

• Osteoblasts 'trap themselves' within bone and over time become this type of bone cell

• They sit in cavities called lacunae and communicate with each other via dendrinic processes called canaliculi

• Highly mechanically sensitive - respond to stress and strain

• Responsible for orchestrating bone remodeling and adaptation and adjustments needed within those processes

Bone lining cell

A buried osteoblast that does not become an osteocyte and instead “escapes'“

quiescent on bone surface

Bone modeling

• The independent action of osteoclasts and osteoblasts on different surfaces

• Produces changes in bone size and shape

• Highly active during growth and development and greatly decreases after skeletal maturity

  • Never goes away but it is easier to get larger structural adaptations during childhood/youth

• Is highly influenced by physical activity during childhood

Bone remodeling

• The sequential, coupled action of osteoclast and osteoblasts

  1. Osteoclasts cleave and absorb

  2. Osteoblasts build

• Does not usually influence bone size and shape

• It removes a portion of old bone and replaces it with newly formed bone

• Occurs throughout life but decreases after growth

Trabecular remodeling

Clasts resorb a little pocket of bone along surface of the bone

Cortical remodeling

• No surface so the cells create a surface by tunneling through the bone marrow

• Osteoclasts go ahead of osteoblasts

• Older cortical bone is more porous and vascular due to this

A-R-F sequence

1. Activation: differentiation of precursor cells to produce osteoclasts (3 days)

2. Resorption: osteoclasts resorb bone (30 days)

3. Formation: osteoblasts build bone (3 months)

Axial stress

• Perpendicular to surface

• Can be compressive or tensile

• F/A

Shear stress

Parallel to surface

F/A

Principal stresses are ____ degrees apart

90

Max shear stress is oriented ____ degrees between the principal stresses

45

When axial stress is at the maximum or minimum shear stress is ___

0

Strain

change in length / original length = F / AE

can be axial or shear and within axial category can be compressive or tensile

Poisson’s ratio

A material loaded in one direction will undergo strains both parallel and perpendicular to the direction of load

• V = transverse strain/axial strain

Elastic modulus

Stress / Strain

larger modulus = more stress needed to deform an object

Orthotropic

Material properties vary in all directions

Characteristics of loading that affect mechanical properties of bone (4)

1. Sample orientation

2. Sample hydration

3. Strain rate (viscoelasticity)

4. Loading mode

Viscoelasticity

 Time-dependent mechanical behaviour

• all biological materials are viscoelastic

• faster loading = stiffer and stronger(but more brittle)

What are the 3 toughening mechanisms of bone?

1. Collagen fibers create bridges between cracks

2. Uncracked ligaments create bridging between cracks

3. Microcracks (energy release via heat)

Stress by bending formula

Mx/I

• M = moment

• x = position

• I = areal moment of inertia

Normal stress due to axial force and bending

= (F/A) + (Mx/I)

What is beam theory?

• Used to calculate normal and shear stresses and strains acting on a cross section of bone

• 3 primary assumptions:

  1. The beam has a constant cross sectional geometry

  2. The beam has a longitudinal plane of symmetry

  3. The beam is made of homogeneous material

What are the 3 types of cartilage?

• Hyaline

• Elastic

• Fibrocartilage

Hyaline cartilage

• Most prevalent cartilage found in adults

• Includes articular cartilage that covers joint surfaces

Elastic cartilage

• Found in external ear, eustachian tubes, and epiglottis.

• More flexible and elastic than hyaline

Fibrocartilage

• Found in intervertebral disks, meniscus, tendon-bone attachments

• Can form when hyaline cartilage is damaged

T/F Cartilage has blood and nerve supply

False, it has neither

Extracellular matrix

• produced by chondrocytes

• mostly type II collagen and proteoglycans (aggrecan), rest is interstitial fluid

• responsible for mechanical properties of cartilage

Chondrocytes

• Live in lacunae like osteocytes, but do not have cell to cell connections

• Metabolically active (synthesis and degradation of ECM)

  • mechanical compression = signal for metabolic activity

Proteoglycans

• Negatively-charged, mutual repulsion

• Provide high compressive strength

  • (Cartilage would have no compressive strength without PGs!!!)

• Most common PG = aggrecan

Interaction between PGs and collagens

• Collagens form fibrillar network

  • Hold water in

• PGs bind to collagen fibrillar network

  • Push water away

• Water fills this molecular framework

What are the 3 things that vary with depth in cartilage?

• Amount of collagen/PGs

• Orientation of collagen fibers

• Shape and size of chondrocytes

What are the 4 zones of articular cartilage?

• Superficial Tangential Zone (STZ)

• Middle zone

• Deep zone

• Calcified zone

Superficial Tangential Zone (STZ)

• Tangential fibres

• 10-20% of tissue thickness

• Highest collagen and water content

• Lowest PG content

• Collagen fibers oriented parallel to surface

• Chondrocytes are elliptical with their axes aligned with the surface

Middle zone

• 40-60% of the thickness

• Randomly arranged collagen orientation

• Chondrocytes are round and randomly distributed

• Highest proteoglycan content

Deep zone

• 30% of the thickness

• Collagen oriented perpendicular to the surface

• Chondrocytes are arranged in a columnar fashion perpendicular to the calcified cartilage

• "anchored" into calcified cartilage

Calcified zone

A layer of calcified cartilage anchored to underlying subchondral bone

Functions of cartilage

• Transfers and distributes loads between bones, thereby lowering joint stress

• Allows load-bearing surfaces to articulate with very low friction

• Its is "not" a shock absorber as is frequently stated

Mechanical properties of cartilage (4)

• Inhomogeneous

• Biphasic

• Viscoelastic

• Anisotropic

Biphasic

• Articular cartilage is considered a fluid-filled biphasic porous permeable material

• Fluid-saturated porous material

• Two phases: fluid (interstitial fluid) and solid (collagens, PGs, and cells)

Viscoelasticity is dependent on:

• Time

• Internal friction (high internal friction=water doesn’t escape)

Creep

When a fixed amount of stress is applied and held, strain will rapidly increase and then slowly increase until a certain point

At equilibrium, load is balanced by the _____ phase alone

Solid

Stress relaxation

Under constant strain, stress slowly decreases over time due to fluid escaping ECM (like a wet sponge)

Creep mechanism in cartilage

• Fluid exudation is most rapid initially (initial rapid rate of deformation)

• Fluid exudation decreases gradually until it completely ceases

• Load is equilibrated by solid matrix and the fluid friction

Permeability ________ when strain increases

decreases

What are the 3 regions of tendon?

• External (free) tendon

• Aponeurosis (internal tendon)

  • Attachment area for muscle fibers (myotendinous junction)

• Bone-tendon junction (osteotendinous junction)

Rupture is most common in this area of tendon

Free tendon

Myotendinous Junction

• Muscle joins with tendon:

• Increased surface/contact area:

  • Decreased stress

  • Changes form of stress:

    • Tensile -> Shear

Osteotendinous junction

• Tendon joins with bone. Gradual transition:

  • From tendon -> fibrocartilage

  • Fibrocartilage -> mineralized fibrocartilage

  • Mineralized fibrocartilage -> bone

Viscoelasticity in tendon is a combination of:

• Viscous properties: Fluids resistance to fluid flow

  • 55-70% of a tendon is water

  • Internal frictional forces between adjacent collagen fibers/fibrils/fascicles etc.

• Elastic properties: Ability to return to original shape once unloaded

  • Collagen Triple Helix structure

  • Crimp

  • Elastin

Fatigue life

The number of cycles of submaximal stress/strain a material can withstand before failure

Material properties

Independent of size and shape

• ex. elastic and shear modulus

Structural properties

Dependent on size, shape and CSA

• ex. stress, force, displacement

Homogeneity

Material acts the same throughout regardless of location

Isotropy

Material acts the same throughout regardless of direction

(does not exist in real life)

Describe the organizational scales of muscle from smallest to largest

1. Sarcomere

2. Myofibril (contractile unit)

3. Fiber (cell)

4. Fascicle

5. Whole muscle

Epimysium

Surrounds whole muscle

Perimysium

Surrounds muscle fascicles

Endomysium

Surrounds muscle fibers

Unipennate muscle

A muscle with one pennation angle

• ex. medial gastrocnemius

Bipennate muscle

A muscle with 2 pennation angles

• ex. tibialis anterior

Aponeurosis

A continuation of distal and proximal tendons onto the muscle

• more randomly arranged collagen fibrils than in tendon

T/F aponeurosis is stiffest in the longitudinal direction and overall less stiff than tendon

True

Fascicle

A bundle of fibers surrounded by perimysium

Muscle Fiber

A single cell formed during development from the fusion of several undifferentiated immature cells known as myoblasts

Fiber bundles promote muscle _____

Stiffness

Myofibrils

• Contractile machinery of muscle

• Repeating units of sarcomeres with a characteristic striation pattern

Describe the anatomy of a sarcomere

• Z-line

  • outer bounds

• M-line

  • myosin tails

• A-band

  • length of myosin

• I-band

  • region of only actin

• H-zone

  • region of only myosin

Z-line

• Defines outer bounds of each sarcomere

• Attachment of actin/thin filaments

• Appears as dark lines on electron micrograph

M-line

• Attachment of myosin filaments

A-band

• Does NOT change in length

• Length of myosin/thick filament

• Referred to as "Anisotropic band" due to how it polarizes light under an electron micrograph

I-band

• Shortens with contraction

• Region of only actin filaments

• Referred to as "Isotropic band" due to how it does not polarize light under an electron micrograph

H-zone

• Shortens with contraction

• Region of only myosin filaments

Titin

• Largest protein found

• Links thick myosin filaments to Z-disc

Motor Unit

A single motor neuron and all the muscle fibers it innervates

Innervation

Acetylcholine (Ach) is released at the neuromuscular junction and travels across the synaptic cleft to the motor end plate

Excitation

This generates an action potential (AP) along the fibre in nerves, APS can only travel in one direction, but in muscle, they can travel in both directions

Activation

Action potential causes release of calcium into the cell which triggers cross-bridge cycling

What are the steps involved in cross-bridge cycling?

1. Active site on actin is exposed as Ca2+ binds troponin

2. Myosin head forms cross bridge with actin

3. During power stroke, myosin head bends, and ADP and Pi are released

4. A new ATP molecule binds to myosin head, causing cross-bridge to detach

5. ATP hydrolyzes to ADP and Pi which returns myosin to "cocked" position

T/F The biceps brachii is an example of a unipennate muscle

False, it is a fusiform muscle

T/F Human muscles are each composed of a single fiber type

False