Basic Science- Bone Flashcards
What are the histologic features of bone?
What are the different types of bone and how are they each characterized?
What are the cellular origins of bone and cartilage cells?
List the factors the affect osteoblasts and osteoclasts?
which transcription factors turn mscs into bone?
Transcription factor RUNX2 and bone morphogenetic protein (BMP) direct mesenchymal cells to the osteoblast lineage.
what do osteoblasts make?
Alkaline phosphatase
•
Osteocalcin (stimulated by 1,25dihydroxyvitamin D [1,25(OH)2D3])
•
Type I collagen
•
Bone sialoprotein
•
Receptor activator of nuclear factor (NF)-κβ ligand (RANKL)
•
Osteoprotegerin—binds RANKL to limit its activity
how does wnt affect bone?
Wnts are proteins that promote osteoblast survival and proliferation.
•
Deficient Wnt causes osteopenia; excessive Wnt expression causes high bone mass.
•
Wnts can be sequestered by other secreted molecules such as sclerostin (Scl) and Dickkopf-related protein 1 (Dkk-1).
how does sclerostin interact with bone formation?
Sclerostin secreted by osteocytes helps negative feedback on osteoblasts’ bone deposition
bone cells and their interaction
cartoon of how osteoclasts are regulated
key testable facts about osteoclasts
Multinucleated irregular giant cells
•
Derived from hematopoietic cells in macrophage lineage
•
Monocyte progenitors form giant cells by fusion
•
Function
•
Bone resorption
•
Bone formation and resorption are linked
•
Stimulated primarily by RANKL binding to RANK receptor on cell surface
•
Osteoblasts (and tumor cells) express RANKL (Fig. 1.4):
•
Binds to receptors on osteoclasts
•
Stimulates differentiation into mature osteoclasts
•
Inhibited by osteoprotegerin (OPG) binding to RANKL
•
Occurs both normally and in certain conditions, including multiple myeloma and metastatic bone disease
•
Denosumab is a monoclonal antibody that targets and inhibits RANKL binding to the RANK receptor
what are the resorptive methods of osteoclasts?
Osteoclasts possess a ruffled (brush) border and surrounding clear zone
•
Border consists of plasma membrane enfoldings that increase surface area
•
Bind to bone surfaces through cell attachment (anchoring) proteins
•
Integrin (αvβ3 or vitronectin receptor)
•
Bone resorption occurs in depressions: Howship lacunae.
•
Effectively seal the space below the osteoclast
•
Synthesize tartrate-resistant acid phosphate
•
Produce hydrogen ions through carbonic anhydrase
•
Lower pH
•
Increase solubility of hydroxyapatite crystals
•
Organic matrix then removed by proteolytic digestion through activity of the lysosomal enzyme cathepsin K
List the Organic and Inorganic components of bone:
Organic components: 40% of dry weight of bone
•
Collagen (90% of organic components)
•
Primarily type I (mnemonic: bone contains the word one)
•
Type I collagen provides tensile strength of bone
•
Hole zones (gaps) exist within the collagen fibril between the ends of molecules.
•
Pores exist between the sides of parallel molecules.
•
Mineral deposition (calcification) occurs within the hole zones and pores.
•
Cross-linking decreases collagen solubility and increases its tensile strength.
•
Proteoglycans
•
Matrix proteins (noncollagenous)
•
Osteocalcin: most abundant noncollagenous protein in bone
•
Inhibited by PTH and stimulated by 1,25(OH)2D3
•
Can be measured in serum or urine as a marker of bone turnover
•
Inorganic (mineral) components: 60% of dry weight of bone
•
Calcium hydroxyapatite [Ca10(PO4)6(OH)2]: provides compressive strength
•
Calcium phosphate (brushite)
what is the most abundant noncollagenous protein in bone?
Osteocalcin
what provides the compressive strength of boen
Calcium hydroxyappetite
different from subchondroplasty (calcium phosphate)
Describe the different types of bone formation:
what is enchondral bone formation?
•
Examples:
•
Embryonic formation of long bones
•
Longitudinal growth (physis)
•
Fracture callus
•
Bone formed with demineralized bone matrix
•
Undifferentiated cells secrete cartilaginous matrix and differentiate into chondrocytes.
•
Matrix mineralizes and is invaded by vascular buds that bring osteoprogenitor cells.
•
Osteoclasts resorb calcified cartilage; osteoblasts form bone.
•
Bone replaces the cartilage model; cartilage is not converted to bone.
•
Embryonic formation of long bones
How is endochondral ossification stimulated?
Differentiation stimulated in part by binding of Wnt protein to the lipoprotein receptor–related protein 5 (LRP5) or LRP6 receptor
Review of the growth plate
Review the Cartoon of MSCs in bone
Cartoon of the enchondral ossification of bone
Review the clinically relevant zones of the growth plate:
Reserve zone: cells store lipids, glycogen, and proteoglycan aggregates; decreased oxygen tension occurs in this zone.
•
Lysosomal storage diseases (e.g., Gaucher disease) can affect this zone.
•
Proliferative zone: growth is longitudinal, with stacking of chondrocytes (the top cell is the dividing “mother” cell), cellular proliferation, and matrix production; increases in oxygen tension and proteoglycans inhibit calcification.
•
Achondroplasia causes defects in this zone (see Fig. 1.11).
•
Growth hormone exerts its effect in the proliferative zone.
•
Hypertrophic zone:
•
Divided into three zones: maturation, degeneration, and provisional calcification
•
Normal matrix mineralization occurs in the lower hypertrophic zone: chondrocytes increase five times in size, accumulate calcium in their mitochondria, die, and release calcium from matrix vesicles.
•
Chondrocyte maturation is regulated by systemic hormones and local growth factors (PTH-related peptide inhibits chondrocyte maturation; Indian hedgehog protein is produced by chondrocytes and regulates the expression of PTH-related peptide).
Osteoblasts migrate from sinusoidal vessels and use cartilage as a scaffolding for bone formation.
Low oxygen tension and decreased proteoglycan aggregates aid in this process.
This zone widens in rickets (see Fig. 1.11), with little or no provisional calcification.
Mucopolysaccharide diseases (see Fig. 1.11) affect this zone, leading to chondrocyte degeneration.
Physeal fractures probably traverse several zones, depending on the type of loading (Fig. 1.12).
describe the grooves of ranvier and perichondral ring of lacroix
Groove of Ranvier: supplies chondrocytes to the periphery for lateral growth (width)
•
Perichondrial ring of La Croix: dense fibrous tissue, primary membrane anchoring the periphery of the physis
•
what is intramembraneous ossification?
Occurs without a cartilage model
•
Undifferentiated mesenchymal cells aggregate into layers (or membranes), differentiate into osteoblasts, and deposit an organic matrix that mineralizes.
•
Examples:
•
Embryonic flat bone formation
•
Bone formation during distraction osteogenesis
•
Blastema bone (in young children with amputations)
What is oppositional ossification?
Osteoblasts align on the existing bone surface and lay down new bone.
•
Examples:
•
Periosteal bone enlargement (width)
•
Bone formation phase of bone remodeling
Describe bone remodeling
what is the Heuter Volkmann Law?
remodeling occurs in small packets of cells known as basic multicellular units (BMUs).
•
Such remodeling is modulated by hormones and cytokines.
•
Compressive forces inhibit growth; tension stimulates it.
•
Suggests that mechanical factors influence longitudinal growth, bone remodeling, and fracture repair
•
May play a role in scoliosis and Blount disease
Describe Wolff’s law:
remodeling occurs in response to mechanical stress.
•
Increasing mechanical stress increases bone gain.
•
Removing external mechanical stress increases bone loss, which is reversible (to varying degrees) on remobilization.
•
Piezoelectric remodeling occurs in response to electric charge.
•
The compression side of bone is electronegative, stimulating osteoblasts (formation).
•
The tension side of bone is electropositive, stimulating osteoclasts (resorption).
what are the biomechanical and biological factors affecting bone repair?
Describe the stages of fracture repair:
Inflammation
•
Fracture hematoma provides hematopoietic cells capable of secreting growth factors.
•
Subsequently, fibroblasts, mesenchymal cells, and osteoprogenitor cells form granulation tissue around the fracture ends.
•
Osteoblasts (from surrounding osteogenic precursor cells) and fibroblasts proliferate.
•
Repair
•
Primary callus response within 2 weeks
•
For bone ends not in continuity, bridging (soft) callus occurs.
•
Soft callus is later replaced through enchondral ossification by woven bone (hard callus).
•
Medullary callus supplements the bridging callus, forming more slowly and later.
•
Fracture healing varies with treatment method (Table 1.6).
•
In an unstable fracture, type II collagen is expressed early, followed by type I collagen.
•
Amount of callus is inversely proportional to extent of immobilization.
•
Progenitor cell differentiation
•
High strain promotes development of fibrous tissue.
•
Low strain and high oxygen tension promote development of woven bone.
•
Intermediate strain and low oxygen tension promote development of cartilage.
•
Remodeling
•
Remodeling begins in middle of repair phase and continues long after clinical healing (up to 7 years).
•
Allows bone to assume its normal configuration and shape according to stress exposure (Wolff’s law)
•
Throughout, woven bone is replaced with lamellar bone.
•
Fracture healing is complete when the marrow space is repopulated.
Describe the types of fracture healing based on methods of stabilization:
key testing points about progenitor cell differentiation:
High strain promotes development of fibrous tissue.
•
Low strain and high oxygen tension promote development of woven bone.
•
Intermediate strain and low oxygen tension promote development of cartilage.
Characterize growth fractors of bone:
Key tested concepts about BMP:
•
BMP-2: acute open tibial fractures
•
BMP-3: no osteogenic activity
•
BMP-4: associated with fibrodysplasia ossificans progressiva
•
BMP-7: tibial nonunions
what do BMP do?
BMPs activate intracellular signal molecules called SMADs to cause osteoblastic differentiation
Review the endocrine effects on bone healing:
how is NSaids related to fracture healing?
Cyclooxygenase-2 (COX-2) activity is required for normal enchondral ossification during fracture healing.
Should you prescribe quinoles during a fracture?
Quinolone antibiotics are: (cipro, levoquin)
Toxic to chondrocytes and inhibit fracture healing
Does ultrasound help fracture healing?
•
Low-intensity pulsed ultrasound (30 mW/cm2) accelerates fracture healing and increases the mechanical strength of callus
•
A cellular response to the mechanical energy of ultrasound has been postulated.
are proteins diet helpful in healing?
In experimental models, oral supplementation with essential amino acids improves bone mineral density in fracture callus.
Characterize bone graft properties
Osteoconductive matrix: acts as a scaffold or framework for bone growth
•
Osteoinductive factors: growth factors (BMP) that stimulate bone formation
•
Osteogenic cells: primitive mesenchymal cells, osteoblasts, and osteocytes
•
Structural
Review the types of bone graft and their properties
Review the different bone graft types:
Cortical bone graft
•
Slower incorporation: remodels existing haversian systems through resorption (weakens the graft) and then deposits new bone (restores strength)
•
Resorption confined to osteon borders; interstitial lamellae are preserved.
•
Used for structural defects
•
Insufficiency fracture eventually occurs in 25% of massive grafts.
•
Cancellous graft
•
Useful for grafting nonunion and cavitary defects
•
Revascularizes and incorporates quickly
•
Osteoblasts lay down new bone on old trabeculae, which are later remodeled (“creeping substitution”).
•
Vascularized bone graft
•
Although technically difficult to implant, allows more rapid union and cell preservation; best for irradiated tissues or large tissue defects (morbidity may occur at donor site [e.g., fibula])
•
Nonvascular bone grafts are more common
•
Allograft bone
•
Types
•
Fresh: increased immunogenicity
•
Fresh frozen: less immunogenic than fresh; BMP preserved
•
Freeze dried (lyophilized “croutons”): loses structural integrity and depletes BMP, is least immunogenic, is purely osteoconductive, has lowest risk of viral transmission
•
Bone matrix gelatin (a digested source of BMP): demineralized bone matrix is osteoconductive and osteoinductive.
•
Osteoarticular (osteochondral) allograft
•
Immunogenic (cartilage is vulnerable to inflammatory mediators of immune response)
•
Articular cartilage preserved with glycerol or DMSO
•
Cryogenically preserved grafts (leave few viable chondrocytes)
•
Tissue-matched (syngeneic) osteochondral grafts (produce minimal immunogenic effects and incorporate well)
•
Antigenicity
•
Allograft bone possesses a spectrum of potential antigens, primarily from cell surface glycoproteins.
•
Classes I and II cellular antigens in allograft are recognized by T lymphocytes in the host.
•
Primary mechanism of rejection is cellular rather than humoral.
•
Incorporation related to cellularity and MHC incompatibility.
•
Cellular components that contribute to antigenicity are marrow origin, endothelium, and retinacular activating cells.
•
Marrow cells incite the greatest immunogenic response.
•
Extracellular matrix components that contribute to antigenicity are as follows:
•
Type I collagen (organic matrix): stimulates cell-mediated and humoral responses
•
Noncollagenous matrix (proteoglycans, osteopontin, osteocalcin, other glycoproteins)
•
Hydroxyapatite does not elicit immune response.
•
Demineralized bone matrix
•
Acidic extraction of bone matrix from allograft
•
Osteoconductive without structural support
•
Minimally osteoinductive despite preservation of osteoinductive molecules
•
Synthetic bone grafts: calcium, silicon, or aluminum
•
Calcium phosphate–based grafts: capable of osseoconduction and osseointegration
•
Biodegrade very slowly
•
Highest compressive strength of any graft material
•
Many prepared as ceramics (heated apatite crystals fused into crystals [sintered])
•
Tricalcium phosphate
•
Hydroxyapatite; purified bovine dermal fibrillar collagen plus ceramic hydroxyapatite granules and tricalcium phosphate granules
•
Calcium sulfate: osteoconductive
•
Rapidly resorbed
•
Calcium carbonate (chemically unaltered marine coral): resorbed and replaced by bone (osteoconductive)
•
Coralline hydroxyapatite: calcium carbonate skeleton is converted to calcium phosphate through a thermoexchange process.
•
Silicate-based: incorporate silicon as silicate (silicon dioxide); bioactive glasses and glass-ionomer cement
•
Aluminum oxide: alumina ceramic bonds to bone in response to stress and strain between implant and bone
Review the stages of graft healing
Review heterotopic ossification:
Heterotopic ossification
•
Ectopic bone forms in soft tissues.
•
Most commonly in response to injury or surgical dissection
•
Myositis ossificans: heterotopic ossification in muscle
•
Increased risk with traumatic brain injury
•
Recurrence after resection is likely if neurologic compromise is severe.
•
Timing of surgery for heterotopic ossification after traumatic brain injury is important:
•
Time since injury (3–6 months)
•
Evidence of bone maturation on radiographs (sharp demarcation, trabecular pattern)
•
Heterotopic ossification may be resected after total hip arthroplasty (THA).
•
Resection should be delayed for 6 months or longer after THA.
•
Adjuvant radiation therapy may prevent recurrence of heterotopic ossification.
•
Optimal therapy: single preoperative or postoperative dose of 600–800 rad/cGy (6-8 Gy)
•
Prevents proliferation and differentiation of primordial mesenchymal cells into osteoprogenitor cells
•
Preoperative radiation (600–800 rad/cGy) may be given in a single fraction up to 24 hours prior to surgery.
•
Helps prevent heterotopic ossification after THA in patients at high risk for this development
•
Incidence of heterotopic ossification after THA among patients with Paget disease is approximately 50%.
Review Distraction Osteogenesis
Definition: distraction-stimulated formation of bone
•
Clinical applications:
•
Limb lengthening
•
Deformity correction (via differential lengthening)
•
Segmental bone loss (via bone transport)
•
Biologic features:
•
Under optimal stability, intramembranous ossification occurs.
•
Under instability, bone forms through enchondral ossification.
•
Under extreme instability, pseudarthrosis may occur.
•
Three histologic phases:
•
Latency phase (5–7 days)
•
Distraction phase (1 mm/day [≈1 inch/mo])
•
Consolidation phase (typically twice as long as distraction phase)
•
Optimal conditions during distraction osteogenesis:
•
Low-energy corticotomy/osteotomy
•
Minimal soft tissue stripping at corticotomy site (preserves blood supply)
•
Stable external fixation and elimination of torsion, shear, and bending moments
•
Latency period (no lengthening) 5–7 days
•
Distraction: 0.25 mm three or four times per day (0.75–1.0 mm/day)
•
Neutral fixation interval (no distraction) during consolidation
•
Normal physiologic use of the extremity, including weight bearing
Bone Organic components-orthobullets
Bone is made up of organic component
40% of dry weight
inorganic component
60% of dry weight
Organic component
Components include collagen
90% of organic component
primarily type I collagen
provides tensile strength
it is a triple helix composed of one alpha-2 and two alpha-1 chains
proteoglycans
responsible for compressive strength
inhibit mineralization
composed of glycosaminoglycan-protein complexes
matrix proteins
includes noncollagenous proteins
function to promote mineralization and bone formationthree main types of proteins involved in bone matrixosteocalcinmost abundant non-collagenous protein in the matrix (10%-20% of total)
produced by mature osteoblasts
functionpromotes mineralization and formation of bone
directly involved in regulation of bone density
attracts osteoclasts
signaling
stimulated by 1,25 dihydroxyvitamin D3
inhibited by PTH
clinical application marker of bone turnover
can be measured in urine or serum
osteonectin
secreted by platelets and osteoblasts
function
believed to have a role in regulating calcium or organizing mineral in matrix
osteopontinfunction
cell-binding protein
cytokine and growth factors
small amounts present in matrix
aid in bone cell differentiation, activation, growth, and turnover
include
IL-1, IL-6, IGF, TGF-beta, BMPs
Inorganic component
Components include calcium hydroxyapatite (Ca10(PO4)6(OH)2
provides compressive strength
osteocalcium phosphate (brushite)
Osteopetrosis
condition caused by a genetic defect resulting in absence of osteoclastic bone resorption
a mouse RANKL knockout model creates a osteopetrosis-like condition
Paget disease
felt to be caused by alterations in cytoplastmic binding to RANK or mutations in the OPG gene
Osteolytic bone metastasis
found to be mediated by the RANK and RANKL pathway
RANKL is produced directly by the cancer cells
blocking of RANKL by OPG results in decreased skeletal metastasis in animal models
bisphosphonates decrease skeletal events in cancer metastasis
Osteolysis following joint arthroplasty
polyethylene wear debris is phagocytized by macrophage leads to activation of the macrophage
additional macrophages are recruited with release of additional cytokines including RANKL
RANKL activates osteoclasts which leads to bone resorption around implants
Clinical conditions related to bone biology
Conditions related to PTH
hypoparathyrodism
pseudohypoparathyroidism
renal osteodystrophy
Conditions related to Vitamin D
Rickets
Vitamin D pathway
skin
liver
kidney
Intramembranous ossification
One of the two essential processes during
fetal development bone formation
fracture healing
also commonly known as contact healing, and Haversian remodeling
Physiology
occurs without a cartilage model (unlike enchondral ossification)
Examples of intramembranous ossification
embryonic flat bone formation (skull, maxilla, mandible, pelvis, clavicle, subperiosteal surface of long bone)
distraction osteogenesis bone formation
blastem bone (occurs in children with amputations)
fracture healing with rigid fixation (compression plate)
one component of healing with intramedullary nailing
Associated conditions conditions with defects in intramembranous ossification cleidocranial dysplasia
caused by defect in intramembranous ossification
caused by mutation in CBFA1 (also know as Runx2) located on chromosome 6
Mechanism
Steps of intramembranous bone formation
aggregation of undifferentiated mesenchymal cells
osteoblast differentiation
organic matrix deposition
Regulation and signaling controlled by pathway called canonical Wnt and Hedgehog signaling
beta-catenin enters cells and induces cells to form osteoblasts which then proceed with intramembranous bone formation
important transcription factors include CBFA1 (also know as Runx2) and osterix (OSX)
sclerostin, created by the SOST gene, decreases bone mass by inhibiting the Wnt pathway
sclerostin vs WNT interaction
embryology-
limb bud
appears to be under the control of fibroblast growth factors (FGF)
enlargement of the limb bud is due to the interaction between the apical ectodermal ridge (AER) and the mesodermal cells in the progress zone.
first identifiable by transvaginal ultrasound at 8 weeks
Steps of limb development
notochord expresses Shh
Shh regulates limb bud formation
limb bud is combination of lateral plate mesoderm and somatic mesoderm
growing outwards into ectoderm (called apical ectodermal ridge)
limb bud formed at embryonic stage 12 (26 days after fertilization)
mesenchyme condenses into preskeletal blastemal at core of limb bud
chondrification occurs where mesenchyme differentiates into chondrocytes
All upper limb bones are endochondral except distal parts of distal phalanges (membranous)
From proximal (humerus, 36 days after fertilization) to distal (distal phalanges, 50 days)
Factors required for chondrification
transcription factors – Sox-5, Sox-6, Sox-9
transforming growth factor superfamily – TGF-b, BMP-2
FGF family
receptor mutation leads to acrocephalosyndactyly (Apert syndrome)
patients with severe craniofacial features have mild hand syndactyly (gain of function in FGFR2c affinity for FGF2 expressed in craniofacial area )
patients with mild craniofacial features have severe hand syndactyly (loss of function in FGFR2c specificity for FGF2, and is now able to bind FGF10, more expressed in hands)
retinoids
hedgehog gene products
PTHrP
cadherins
WNT5a and WNT7a
Formation of joints requires repression of chondrogenesis at sites of future joints
proteins involved – WNT4, WNT14, growth and differentiation factor 5 (also known as cartilage-derived morphogenetic protein 1)
shoulder interzone appears at 36 days, hand interzones appear at 47 days
Finger separation
digital rays are evident within hand paddle at stage 17 (41 days)
interdigital mesenchyme cells undergo programmed cell death (stage 19 to 22)( days 47-54)
transcription factor Msx2 is expressed in interdigital mesenchyme, regulates BMP4-mediated programmed cell death pathway
transcription factor Hox-7 also expressed in interdigital zones
sonic hedgehog
notochord expresses Shh
Shh regulates limb bud formation
limb bud is combination of lateral plate mesoderm and somatic mesoderm
growing outwards into ectoderm (called apical ectodermal ridge)
limb bud formed at embryonic stage 12 (26 days after fertilization)
embryology:
Chondrification
All upper limb bones are endochondral except distal parts of distal phalanges (membranous)
From proximal (humerus, 36 days after fertilization) to distal (distal phalanges, 50 days)
Factors required for chondrification
transcription factors – Sox-5, Sox-6, Sox-9
transforming growth factor superfamily – TGF-b, B
MP-2
FGF family
- receptor mutation leads to acrocephalosyndactyly (Apert syndrome)*
- patients with severe craniofacial features have mild hand syndactyly (gain of function in FGFR2c affinity for FGF2 expressed in craniofacial area )*
- patients with mild craniofacial features have severe hand syndactyly (loss of function in FGFR2c specificity for FGF2, and is now able to bind FGF10, more expressed in hands)*
retinoids
hedgehog gene products
PTHrP
cadherins
WNT5a and WNT7a
Radial/Ulnar Development
second signaling center to appear is ZPA (zone of polarizing activity), along posterior limb bud
grafting ZPA on anterior limb margin leads to mirror-image digit duplication (ulnar dimelia, or mirror hand)
signaling molecule is Shh compound (dose dependent) normal
high concentration of Shh on posterior (ulnar) side for small finger development
low concentration of Shh on anterior (radial) side for thumb development
posterior/ulnar side abnormalities abnormal upregulation of Shh in the ZPA results in polydactly on the ulnar (posterior) side
extent of duplication is dose dependent (higher dose = more replication)
downregulation of Shh (on the posterior/ulnar side) leads to loss of ulnar digits
anterior/radial side abnormalities
abnormal upregulation of Shh in the anterior aspect of the limb bud (where Shh concentration is supposed to be low) leads to loss of thumb
timing
posterior elements (little finger/ulna) are formed EARLY prior to anterior elements which are formed LATE (radius/thumb)
disruption of AP patterning will result in loss of later forming elements (radius/thumb)
WNT gene for limb bud
hint-fingernails
Wnt genes (Wnt7a)
expressed in dorsal (non-AER) ectoderm (Wnt signalling center)
dorsal-ventral growth
Mutations
Clinical manifestations of limb buds genetic abnormalities
Mutations
removal of AER
truncated limb
duplication of ZPA
mirror-image duplication of the limb
dorsal ventral development
hird signaling center is non-AER limb ectoderm /Wnt signalling center (progress zone, PZ)
dorsal limb ectoderm expresses WNT7a
activates Lmx1b (LIM-homeodomain factor) to regulate dorsal patterning
WNT7a is responsible for all dorsal features (including nails)
ventral ectoderm expresses en-1 (engrailed-1 protein, antagonistic to WNT7a)
inhibits WNT7a (and restricts it to dorsal ectoderm)
allows ventral limb development
key genes for limb bud embryogenesis
Sonic Hedgehog (Shh) genes
secreted by ZPA
involved with HOX gene expression
anterior-posterior (radioulnar) growth
anterior (radial) mesoderm expresses ALX4
posterior (ulnar) mesoderm expresses Hox8
concentration gradient dictates formation of digits
little finger develops where there is highest Shh concentration
thumb develops where there is lowest Shh concentration
activates Gremlin
Gremlin inhibits BMPs that would otherwise block FGF expression in the AER
Hox genes anterior-posterior (radioulnar) patterning
together with Shh
regulate somatization of the axial skeleton, essentially patterning digit formation
Wnt genes (Wnt7a)
expressed in dorsal (non-AER) ectoderm (Wnt signalling center)
dorsal-ventral growth
review spine embryologic development
Somites
the spinal column originates from pairs of mesodermal structures known as somites
somites develop in a cranial to caudal direction on either side of the notochord and neural tube
this process is dependent on the presence of the paraxis gene
somite layers sclerotome
layer will become the vertebral bodies and annulus fibrosus
myotome
will lead to myoblasts
dermatome
becomes skin
Dorso-vental patterningdorso-vental patterning of the neural tube determined by counteracting activities ofSonic Hedgehog (Shh)
in the floor plate and notochord (ventral)
canonical Wnt/β-catenin
in the roof plate (dorsal)
Metameric shift phenomenon
the phenomenon of how the spinal nerves, which originally ran in the center of the sclerotome, exit between the two vertebral bodies at each level.
Progressionneural crest
forms PNS, pia mater, spinal ganglia, sympathetic trunk
neural tube
forms spinal cord
notochord
forms anterior vertebral bodies and nucleus pulposus
Ossification centersvertebrae have 3 primary ossification centers
centrum (anterior vertebral body)
neural arch (posterior elements, pedicles, small portion of anterior vertebra)
costal element (anterior part of lateral mass, transverse process, or rib)
Intervertebral disc
nucleus pulposus forms from notochord
annulus fibrosus forms from sclerotome
what role do WNT and SHH role have in spinal cord differentiation
review growth plate
growth plate periphery
what is wolfe’s law?
bone remodels in response to mechanical stress
what is hueter volkman’s law
theory that bone remodels in small packets of cells known as Basic Multicellular Units (BMUs)
theory suggest that mechanical forces influence longitudinal growth
compressive forces inhibit growth
may play role in scoliosis
what does sclerostin do?
sclerostin inhibits osteoblastogenenesis to decrease bone formation
what are the clinical applications of BMP?
FDA-approved uses rhBMP-2
single-level ALIF from L2 to S1 levels in degenerative disc disease together with the lumbar tapered fusion device (LT Cage; Medtronic)
open tibial shaft fractures stabilized with an IM nail and treated within 14 days of the initial injury
rhBMP-7
as an alternative to autograft in recalcitrant long bone nonunions where use of autograft is unfeasible and alternative treatments have failed
as an alternative to autograft in compromised patients (with osteoporosis, smoking or diabetes) requiring revision posterolateral/intertransverse lumbar fusion for whom autologous bone and bone marrow harvest are not feasible or are not expected to promote fusion
BMP review
orthobullets
BMPs belong to the TGF-B superfamily
BMP 2,4,6, and 7 all exhibit osteoinductive activity
BMP 3 does not exhibit osteoinductive activity
Mutations in BMP-4 are associated with Fibrodysplasia ossificans progressiva
Mechanism osteoinductive
leads to bone formation
activates mesenchymal cells to transform into osteoblasts and produce bone
has been foung to increase chondrogenic phenotype and matrix synthesis in intervertebral discs
Signaling Pathways and Cellular Targets
BMP targets undifferentiated perivascular mesenchymal cells
activates a transmembrane serine/threonine kinase receptor that leads to the activation of intracellular signaling molecules called SMADs
SMADS are primary intracellular signaling mediators
currently eight known SMADs, and the activation of different SMADs within a cell leads to different cellular responses.
Peroxisome proliferator-activated receptor gamma (PPAR-gamma or PPARG)
adipocyte gene
what are the clinical manifestions of rickets?
Vitamin D-resistant (familial hypophosphatemic)
most common form of heritable rickets
presents at 1-2 years of age
caused by inability of renal tubules to absorb phosphate
GFR is normal
vitamin D3 response is impaired
genetics X-linked dominant
most common form
results from mutation in PHEX gene
leads to increased levels of FGF23, which decreases renal phosphate absorption and suppresses renal 25-(OH)-1α-hydroxylase activity
autosomal dominant
results from mutation in FGF23
leads to decreased FGF23 degradation
autosomal recessive
results from mutation in dentin matrix protein 1 (DMP1) gene
leads to impaired osteocyte maturation and bone mineralization, and increased levels of FGF23
Vitamin D-deficient (nutritional) results from decreased dietary intake of Vitamin D
rare now that Vitamin D is added to milk
presents at 6 months - 3 years of age
risk factors
premature infants
black children > 6 months who are still breastfed
patients with malabsorption syndromes (celiac sprue) or chronic parenteral nutrition
Asian immigrants
patients with unusual dietary choices (vegetarian diet)
pathophysiology
low Vitamin D levels lead to decreased intestinal absorption of calcium
low calcium levels leads to a compensatory increase in PTH and bone resorption
bone resorption leads to increased alkaline phosphatase levels
Vitamin D-dependent (type I & type II)
rare disorder
leads to clinical features similar to Vitamin D-deficient rickets but more severe
clinical characteristics type I
hypotonia, muscle weakness, growth failure, hypocalcemic seizures, joint pain/deformity, fractures in early infancy
type II
hypotonia, muscle weakness, growth failure, hypocalcemic seizures, growth retardation, bone pain, severe dental caries or dental hypoplasia
pathophysiology type I
results from autosomal recessive mutation in renal 25-(OH)-1α-hydroxylase
prevents conversion of inactive form of vitamin D to active form
responsible gene 12q14
type II
results from autosomal recessive mutation in intracellular receptor for 1,25-(OH)2-vitamin D
what is toughness?
Toughness refers to the amount of energy that a material can absorb before failing. On a load-displacement plot, the toughness is equal to the area under the stress-strain curve,
what is strength?
Strength refers to the magnitude of the load at the point where the plate breaks,
what is brittleness?
The opposite of toughness
