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
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Osteocalcin (stimulated by 1,25dihydroxyvitamin D [1,25(OH)2D3])
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Type I collagen
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Bone sialoprotein
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Receptor activator of nuclear factor (NF)-κβ ligand (RANKL)
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Osteoprotegerin—binds RANKL to limit its activity
how does wnt affect bone?
Wnts are proteins that promote osteoblast survival and proliferation.
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Deficient Wnt causes osteopenia; excessive Wnt expression causes high bone mass.
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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
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Derived from hematopoietic cells in macrophage lineage
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Monocyte progenitors form giant cells by fusion
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Function
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Bone resorption
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Bone formation and resorption are linked
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Stimulated primarily by RANKL binding to RANK receptor on cell surface
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Osteoblasts (and tumor cells) express RANKL (Fig. 1.4):
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Binds to receptors on osteoclasts
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Stimulates differentiation into mature osteoclasts
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Inhibited by osteoprotegerin (OPG) binding to RANKL
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Occurs both normally and in certain conditions, including multiple myeloma and metastatic bone disease
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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
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Border consists of plasma membrane enfoldings that increase surface area
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Bind to bone surfaces through cell attachment (anchoring) proteins
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Integrin (αvβ3 or vitronectin receptor)
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Bone resorption occurs in depressions: Howship lacunae.
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Effectively seal the space below the osteoclast
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Synthesize tartrate-resistant acid phosphate
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Produce hydrogen ions through carbonic anhydrase
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Lower pH
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Increase solubility of hydroxyapatite crystals
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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
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Collagen (90% of organic components)
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Primarily type I (mnemonic: bone contains the word one)
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Type I collagen provides tensile strength of bone
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Hole zones (gaps) exist within the collagen fibril between the ends of molecules.
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Pores exist between the sides of parallel molecules.
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Mineral deposition (calcification) occurs within the hole zones and pores.
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Cross-linking decreases collagen solubility and increases its tensile strength.
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Proteoglycans
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Matrix proteins (noncollagenous)
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Osteocalcin: most abundant noncollagenous protein in bone
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Inhibited by PTH and stimulated by 1,25(OH)2D3
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Can be measured in serum or urine as a marker of bone turnover
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Inorganic (mineral) components: 60% of dry weight of bone
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Calcium hydroxyapatite [Ca10(PO4)6(OH)2]: provides compressive strength
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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?
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Examples:
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Embryonic formation of long bones
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Longitudinal growth (physis)
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Fracture callus
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Bone formed with demineralized bone matrix
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Undifferentiated cells secrete cartilaginous matrix and differentiate into chondrocytes.
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Matrix mineralizes and is invaded by vascular buds that bring osteoprogenitor cells.
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Osteoclasts resorb calcified cartilage; osteoblasts form bone.
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Bone replaces the cartilage model; cartilage is not converted to bone.
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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.
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Lysosomal storage diseases (e.g., Gaucher disease) can affect this zone.
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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.
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Achondroplasia causes defects in this zone (see Fig. 1.11).
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Growth hormone exerts its effect in the proliferative zone.
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Hypertrophic zone:
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Divided into three zones: maturation, degeneration, and provisional calcification
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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.
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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)
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Perichondrial ring of La Croix: dense fibrous tissue, primary membrane anchoring the periphery of the physis
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what is intramembraneous ossification?
Occurs without a cartilage model
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Undifferentiated mesenchymal cells aggregate into layers (or membranes), differentiate into osteoblasts, and deposit an organic matrix that mineralizes.
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Examples:
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Embryonic flat bone formation
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Bone formation during distraction osteogenesis
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Blastema bone (in young children with amputations)
What is oppositional ossification?
Osteoblasts align on the existing bone surface and lay down new bone.
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Examples:
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Periosteal bone enlargement (width)
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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).
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Such remodeling is modulated by hormones and cytokines.
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Compressive forces inhibit growth; tension stimulates it.
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Suggests that mechanical factors influence longitudinal growth, bone remodeling, and fracture repair
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May play a role in scoliosis and Blount disease
Describe Wolff’s law:
remodeling occurs in response to mechanical stress.
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Increasing mechanical stress increases bone gain.
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Removing external mechanical stress increases bone loss, which is reversible (to varying degrees) on remobilization.
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Piezoelectric remodeling occurs in response to electric charge.
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The compression side of bone is electronegative, stimulating osteoblasts (formation).
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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
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Fracture hematoma provides hematopoietic cells capable of secreting growth factors.
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Subsequently, fibroblasts, mesenchymal cells, and osteoprogenitor cells form granulation tissue around the fracture ends.
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Osteoblasts (from surrounding osteogenic precursor cells) and fibroblasts proliferate.
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Repair
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Primary callus response within 2 weeks
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For bone ends not in continuity, bridging (soft) callus occurs.
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Soft callus is later replaced through enchondral ossification by woven bone (hard callus).
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Medullary callus supplements the bridging callus, forming more slowly and later.
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Fracture healing varies with treatment method (Table 1.6).
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In an unstable fracture, type II collagen is expressed early, followed by type I collagen.
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Amount of callus is inversely proportional to extent of immobilization.
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Progenitor cell differentiation
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High strain promotes development of fibrous tissue.
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Low strain and high oxygen tension promote development of woven bone.
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Intermediate strain and low oxygen tension promote development of cartilage.
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Remodeling
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Remodeling begins in middle of repair phase and continues long after clinical healing (up to 7 years).
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Allows bone to assume its normal configuration and shape according to stress exposure (Wolff’s law)
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Throughout, woven bone is replaced with lamellar bone.
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Fracture healing is complete when the marrow space is repopulated.
Describe the types of fracture healing based on methods of stabilization:
Characterize growth fractors of bone:
Review the endocrine effects on bone healing:
Review the types of bone graft and their properties
Review the stages of graft healing
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
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)
Clinical manifestations of limb buds genetic abnormalities
Mutations
removal of AER
truncated limb
duplication of ZPA
mirror-image duplication of the limb
what role do WNT and SHH role have in spinal cord differentiation
review growth plate
growth plate periphery
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
