factors in initiation of ossification Flashcards
The immediate conversion of mesenchymal cells to osteoblasts; occurs where? and what is the exception?
primarily in the bones of the skull, including cranial suture lines; An exception includes the periosteum surrounding the outer surface of long bones; also considered as intramembranous
what are the processes intramembranous ossification?
stage 1- Ossification Centre Formation
stage 2- Calcification
stage 3- Trabeculae Formation
stage 4- Periosteum Development
what happens in stage 1?
Mesenchyme condenses into membranous sheet
Osteoprogenitor cells found here differentiate into osteoblasts and collagen fibres surround the cells
what happens in stage 2?
Osteoblasts secrete osteoid and some are enveloped in intracellular matrix, forming lacunae and converting into osteocytes
Osteoid calcifies to form spicules of bone
what happens in stage 3?
The remainder of cells synthesize and secrete osteoid, then calcify to become woven bone trabeculae
Cancellous bone formed from interconnecting network of trabeculae, including spaces for separation (form red bone marrow)
what happens in stage 4?
The periosteum, which contains osteoblasts, constructs a bone layer beneath its surface and forms an outer compact bone layer
what is Endochondral Ossification?
This process involves the conversion of mesenchymal cells to chondroblasts, producing the cartilaginous template, and then the mineralised mode is replaced by bone.
main mode of ossification within the centra of vertebrae
1st process in Endochondral Ossification?
Condensation and differentiation of mesenchyme into chondrocytes, leads to production of main hyaline cartilage mode
2nd process in Endochondral Ossification?
Hypertrophy and division of chondrocytes occurs, hyaline model begins to calcify. As calcification progresses, diffusion of nutrients through calcified matrix dramatically reduces
3rd process in Endochondral Ossification?
Chondrocytes apoptose and hollow cartilage matrix now serves as framework for deposition of bone
4th process in Endochondral Ossification?
Due to osteogenic potential of inner perichondral cells, a thin collar of bone surrounds midpoint of diaphysis, referred to as periosteum. Above and below bony collar, periosteum continuous with perichondrium
5th process in Endochondral Ossification?
Mesenchyme near periosteum differentiate into osteoprogenitor cells and blood vessels invade calcified hyaline model
6th process in Endochondral Ossification?
Osteoprogenitor cells differentiate into osteoblasts, secreting osteoid which is quickly mineralised to bone. Osteoblasts surrounded in lacunae referred to as osteocytes (cell to cell connection through canaliculi)
7th process in Endochondral Ossification?
Primary ossification centre (POC) first appears in diaphysis of long bone, secondary ossification centres (SOCs) occur at epiphyses
8th process in Endochondral Ossification?
Epiphyseal growth plate develops soon after formation of POC at boundaries of diaphysis and epiphyses, these are important for longitudinal growth and diametric expansion until growth stops
9th process in Endochondral Ossification?
Expansion of POC and SOCs eventually replaces all cartilage with bone, including EGP, at the end of puberty. Hyaline cartilage surrounding outside of epiphyses remains; articular cartilage
what are some factors initiating ossification?
- transcription factors
what do SRY-box containing gene 9 (Sox9) do?
differentiation of mesenchymal stem cells to chondrocytes, absence can cause achondroplasia
what do Run-related transcription factor 2 (Runx2): do?
expressed in pre-hypertrophic and hypertrophic chondrocytes, and osteoblasts, consequences of removal are fatal due to lack of mineralized skeleton
what is Ihh/ PTHrP?
form an importantfeedback loopwhich regulates part of endochondral ossification
what does Ihh stand for?
indian hedgehog
what does PTHrP stand for?
parathyroid hormone-related peptide
Ihh secreted by what?
prehypertrophic chondrocytes, in turn allowing PTHrP to control the differentiation of chondrocytes near the articular surface of the growth plate
If removed, what happens to PTHrP expression
decreases, pathway ensures hypertrophic differentiation doesn’t occur too early
the Ihh/ PTHrP pathway ensures what?
This pathway normally ensures that hypertrophic differentiation doesn’t occur too early, so the number of columnar chondrocytes is maintained and hence the length of the columns. Bones are able to be formed in an organised fashion, as this loop ensures there is a balance between proliferation and maturation of chondrocytes.
when Ihh is absent what occurs?
Osteoblast differentiation is also halted when Ihh is absent.
Beta-catenin-dependent pathways are called what?
canonical pathways
Signalling without Beta-catenin are called what?
non-canonical pathways
Wnt/Beta-catenin signalling allows what?
osteoblasts to differentiate, removal from mouse models inhibited this process and increased osteoclastogenesis
Absence of Wnt3 causes what?
failure of both arms and legs to develop (tetra-amelia)
Wnt3 are what?
glycoproteins
what do Wnt3 signal?
They signal either via B-catenin-dependent pathways, which are calledcanonicalpathways, or without B-catenin, which arenoncanonicalpathways.
It has been suggested thatWnt/B-catenin signallingallows what?
signallingallows osteoblasts to differentiate.
Bone morphogenetic proteins (BMPs) are part of what?
the Transforming Growth Factor-Beta protein family
When BMPs are removed what happens?
When some are removed, can result in death or prominent defects
what do BMPs bind to?
Bind to specific membrane receptors
BMP 2: allows what?
BMP 2: allows proliferation and maturation of chondrocytes, without it chondrodysplasia develops, also significant in fracture healing and mutations can cause cleft palate
BMP 11 allows what?
BMP 11: responsible for axial skeleton patterning anterior to posterior, change in expression of Hox genes
BMPs induce what
induce Runx2 expression
FGFR3 does what?
thought to control chondrocyte proliferation, present in proliferating zone of growth plate
Mutations of FGFR3 can cause?
can cause achondroplasia
FGF18: expressed in what?
perichondrium, likely most common activator of FGFR3
FGF2: expressed in what?
limb buds, likely connections to Ihh and BMP2 expression, both pathways affected when removed
FGFR1/2 important in what?
important in endochondral ossification, particularly in limb development. FGFR1 expressed in hypertrophic chondrocytes and FGFR2 in osteoblasts
what is Mechanobiology?
A field, which focuses on how cell tissue mechanics and physical forces affect cell behaviour and tissue morphogenesis
what is Mechanosensing?
The cell sensing mechanical signals created by the environment
what is Mechanotransduction?
Translating the sensed mechanical signals into a response
in 1892 what was suggested by wolff?
Wolff suggested that bone undergoes trabecular remodelling to deal with the relevant pressures and stresses. In other words, “tissue (bone) will adapt to best fulfil its mechanical function”
So, if the load on a bone progressively increased over time, what would happen?
then so too would the density and bone mass of that bone, to deal with the stresses
Oppositely, a reduction of load on that bone, would lead to what?
would lead to a reduction of bone tissue
Osteocytes encased within the bone matrix, are believed to do what?
coordinate osteoblast and osteoclast production, responding to mechanical stimuli i.e strain. Mechanical signals are transformed into biochemical signals which ultimately regulate bone remodelling
in 1980 Harold Frost created what?
the concept of a control system in which a minimum effective strain (MES) is necessary for various stages of bone maintenance, using a thermostat as the basis of his idea
Parathyroid Hormone (PTH) makes osteoblasts more what?
sensitive to mechanical stimuli, so these set points (MES) are altered to make a response more likely
Continuous hypersecretion can lead to what?
bone resorption
if mechanical stimulus (strain) is low what occurs?
remodelling removes unneeded bone
if mechanical stimulus is high what occurs?
results in overall bone gain
if mechanical stimulus is too high what occurs?
internal damage to bone matrix. remodelling acts to repair damage by creating woven bone or on existing surfaces surfaces or new bone within the marrow cavity
Modelling and remodelling are what processes?
antagonistic processes
Remodelling is what?
removing bone by resorption or maintaining bone by resorption/formation
Modelling is what?
adding bone (no coupling of the two)
When the mechanical stimulus is high, modelling is what?
activated and remodelling is suppressed
When the mechanical stimulus is low, remodelling is what?
activated and modelling is suppressed
what is mechanobiology?
is a term used to describe how physical forces and cell changes influence development, differentiation and degeneration
nucleus pulposus is formed from what?
notochord
Annulus fibrosus develops from what?
sclerotome, first derived from somites
Annulus fibrosus made up of what?
circumferential layers of collagen fibres, with type II constituting inner layers and type I the outer layers
the structure is able to do what?
Able to resist axial forces and give tensile strength
Nucleus pulposus is known to be what?
exposed to more hydrostatic pressures
Nucleus pulposus sensitive to what?
mechanical forces throughout development
Immature nucleus pulposus cells from different levels of vertebral column presented with what?
with distinct characteristics when viewed microscopically
lumbar cells appear drastically different from the caudal cells why?
unique stresses and compression levels on lumbar and caudal regions
Lumbar vertebral bodies are generally exposed to what?
greater weight-bearing forces compared to those at the caudal level, so they present with smaller vacuoles and more cell processes.
Effects of axial stresses and compression have also been studied on the annulus fibrosus in animal models and a large degree of stretching has been found to what?
reduce proteoglycan synthesis by annulus fibrosus cells
intralamellar compartment is believed to what?
mechanobiologically active
Iatridis et al. (2013) looked further into the effects of stress on the metabolism of intervertebral discs and stated what?
that the response is dependent on loading magnitude, frequency and duration, They found that mechanical loading induced specific mRNA and enzyme changes
Ausk et al. (2017) discussed what?
the effects transient muscle paralysis by mode of Botulinum Toxin A (BTxA) injection
what did ausk discover?
Discovered this stimulates bone resorption within the medullary cavities of nearby bones, with associated inflammation
Greater nuclei numbers and greater size of osteoclasts were produced 72 hours after BTxA exposure. Remodelling process was not in equilibrium thus a decrease in bone density occurred
Aliprantis et al. (2012) also researched this concept and came to the conclusion what?
that transient muscle paralysis leads to decrease in bone density due to osteoclastogenesis
Differentiation and activation of osteoclasts occurred within where?
the tibial bone marrow cavity which was activated by the upregulated expression of the RANKL protein (also known as osteoclast differentiation factor)
Increase of RANKL of untreated (Contra) limb, at days 3 and 7 have a what?
increasing trend, however differences were not statistically significant
Foster et al. (2001) investigated what?
the effect of Botulinum as a treatment for chronic lower back pain
forester concluded what?
that paravertebral administration of BTxA in patients suffering from low back pain had positive effects and improved function
With new studies being presented, this may be deemed as what?
a controversial administration due to the possible negative effects of bone mineral density decrease, and its related symptoms
motor neurons do what?
control muscle movement
what is spinal muscular atrophy
Group of inherited neurodegenerative disorders comprising of muscle atrophy and weakness, due to loss of spinal motor neurones, typically of the anterior horn
SMN1 gene gives instructions for what?
for making functional survival motor neuron (SMN) protein. SMN2 is the mutated pseudogeneof SMN1; around 99% identical.
More copies of SMN2 gene =?
lower the severity of the condition of SMA
age of on set SMA1 (Infant)?
0-6 months
Most infants with SMA1 will not survive past the age?
2
what occurs in SMA1?
Intercostal muscle weakness, paradoxical breathing, muscle weakness of legs. Lack of hip and neck flexion; letter results in neck lag.
SMN2 copies= 1 to 3
age of on set of SMA2 (Intermediate)?
6-18 months
result of SMA2?
Can stay seated independently, but ability may be lost over time. Independent walking is not possible. Proximal leg weakness and progressive intercostal muscle weakness and scoliosis. Survival into adulthood. SMN2 copies = 2 to 4
onset age of SMA3 (Juvenile)?
over 18 months
result of SMA3?
Learn to walk independently but may lose ability over time. Proximal leg muscle weakness persists. Restrictive lung disease and scoliosis not as prominent. Average life expectancy into adulthood SMN2 copies = 3 to 4.
age of onset of SMA4 (Adult)
over 21 years
result of SMA4?
Present with proximal girdle weakness. Intercostal muscles not usually affected, and scoliosis is rare. Muscle cramps may occur. Normal life expectancy. May lose ambulation within 20 years of onset. SMN2 copies = 4 to 5
what is Amyotrophic Lateral Sclerosis (ALS) or Motor Neurone Disease?
Neuromuscular disease defined as death of motor neurones with subsequent progressive skeletal muscle atrophy
what is Duchenne Muscular Dystrophy (DMD)?
Recessive condition normally characterised by muscle weakness and wasting due to mutation of dystrophin gene, resulting in absence of dystrophin, can lead to premature death
In a review by Morgenroth et al. (2012) it is discussed what?
loss of muscle tension and wasting which occurs in DMD is a cause of poor bone quality, decreased bone mineral density and osteopenia/osteoporosis
what is the limitation of morgenroth?
However, it is not clear as to whether this is the only explanation, also believed that, due to its potent anti-inflammatory action, glucocorticoid (GC) therapy could compromise bone mineral quality and density.
what is Myasthenia Gravis (MG)?
Autoimmune disorder which leads to weakness of skeletal muscles including the diaphragm, seriously compromising effectiveness of breathing
Immune system disrupts what?
transfer of neurotransmitter acetylcholine (ACh) across synapse by blocking ACh receptors (AChR) with antibody anti-AChR, slowing transmission of impulses to effector muscle
MG presents similar problems as in what?
to DMD, with glucocorticoid being one of the treatment plans. However, patients with MG following this treatment plan are also at risk of osteopenia or osteoporosis, presenting with low bone mineral density
What is a Mechanical Stimulus?
Detectable change in internal or external environment
E.g. high pressure, which causes bones in our bodies to change
Role of Osteocytes
In an adult, 90-95% of bone cells are osteocytes
One osteocyte body can have up to 60 long processes radiating from it
These connect with one another to form the lacunar-canalicular system
This system allows communication of osteocytes and surface cells
How osteocytes are able to sense mechanical stimuli and act accordingly ?
not completely understood
what is mechanotransduction?
allows physical forces to be transformed into biochemical responses and then for cells to respond biologically
what is the Biochemical
responses?
Could be activity of signalling molecules, but mainly thought to be ion channels
what are the Biological
responses?
Osteoclast and osteoblast activity
what is Interstitial Fluid Flow ?
Force applied to pericellular matrix enveloping osteocyte cells
what is created in Interstitial Fluid Flow ?
pressure created
Flow of interstitial fluid initiated throughout what?
lacunar-canalicular network, to Haversian or Volkmann’ channels
what does flow of interstial fluid cause?
Results in shear stress, stretching osteocyte cell membranes and causing biochemical response
what does interstitial fluid flow allow?
Allows transport of essential nutrients and removal of waste products
