Bone Pathology-Steve Weisbrode

Bone Pathology-Steve Weisbrode
4/18/10- Madison Wisconsin

Bone Development and Structure

Names of Bone:
Cortical bone – synonyms (osteonal in adults that remodel cortical bone; lamellar)
Trabecular bone – Spongy, cancellous, medullary bone (differs from avian egg laying “medullary

Bone surfaces: Vascular channels or haversian systems, endosteal surface, periosteal surface, trabecular surface

Woven bone- Rapidly deposited, fetal bone and fracture repair bone, arranged as haphazard collagen fibers

Lamellar bone- slowly deposited inorganized sheets, parallel arrangement of collagen fibers- normal adult bone. Adult trabecular bone and adult cortical bone is all lamellar, and each layer is called a lamella.

Synovial fossa- Loss of articular cartilage over time from the non-weight-bearing surface of the large animals (not dog and acat). They are not present at birth but begin to be visible at 3-6 months of age

Bone growth:
Osteoblasts move backward in space when forming tissue (hard)
In utero development: Cartilage precursor of mesodermal origin (endochondral), or directly from a primitive neurectodermal tissue
with osseous potential (intramembranous).

Apoptosis occurs in regions that become joints so that individual bones become apparent.
Histologic organization:
Osteoblasts →  lamina limitans (biochemically altered un-mineralized osteoid; appears green in undemineralized plastic embedded Massons trichrome stain; very thin, mostly not seen with light microscopcy and H&E sections) →  normal unmineralized osteoid (will become mineralized; appears red with Massons trichrome) → Mineralized osteoid (bone; appears green with masons trichrome *note that the color is the same as the lamina limitans but in this case its mineralized*

Some or all osteoblasts will become osteocytes, but some may undergo Apoptosis.
Osteoblasts control initiation of bone resorption and interact with osteoclasts to pump calcium out of the bone.

Bird growth plates- vascular invasion of unmineralized cartilage
Mammals have only trans-plate vessels in cartilage canals (vascular channels)

Woven bone = Rapid bone → haphazard collagen deposition

Lamellar bone = Slowly deposited in coordinated sequence of alternating fiber orientation

Electron Microscopy:
Osteoblasts have cytoplasmic processes within osteoid that connect to osteocytes embedded in Osteoid→  can appear as microvescicular blebs →  these are the same microvesicles that initiate mineralization

Active osteoblasts have abundant RER and mitochondria

Inactive bone- increased N:C ratio, no cytoplasmic blebs

Type 1 collagen- microfibrils have periodicity when viewd by EM (67 nm) → Tropocollagen 3000 Angstroms

Collagen is deposited in different directions by coordinated osteoblast production→ leads to the polarization effect (alternating dark and light bands in lamellar bone) This is due to unknown factors

Non-Collagenous Matrix Proteins in Osteoid:

Deposited by osteoblasts:
Growth Factors: TGF-beta, PDGF, BMP
Glues: Osteonectin, sialoprotein
Involved in mineralization: Sialoprotein
Indicator of bone formation: Osteocalcin (vit K dependent biomarker, can be found in
serum, and better than Alk Phos as indicator of bone formation)
Central core of Hyaluronan
Radially arranged sulfated glycosaminoglycans
Chondroitin and keratin sulfate

Inorganic Bone Matrix:
Mineralization of Osteoid or chondroid
Osteoblast cytoplasmic processes →  matrix vesicles
Matrix Vesicles:
Phospholipid bilayer
Contain Calcium and Phosphate
Phase 1: Pyrophosphatase located on membranes cleave diphosphate groups (precursor of ADP)
into phosphate groups → go into matrix vesicles
diphosphate bond is inhibitory to mineralization (P-O-P)
Calcium is attracted to general area by physiologic diffusion
Calcium is pumped into matrix vesicles
Calcium binds to phosphate and initially forms an amorphous matrix within the vesicles
→ slowly becomes crystallized onto acicular crystals within the matrix vesicle
Phase 2: Vesicle membrane ruptures due to crystal formation
Exposed crystal attracts calcium and phosphate and grows larger (propogation)
Apatite crystals are attracted to gap regions in tropocollagen molecules. Gaps→ progress to diffuse mineralization

Lamina limitans remains unmineralized (no one knows why)

Osteocytes and Chondrocytes in matrix:

  • Osteoblasts that become trapped in osteoid become osteocytes.
  • Osteocytes retain cytoplasmic processes connected to osteoblasts through canaliculi.
  • Osteocytes are not in direct contact with osteoid, but have a fluid filled space that can condunct electric charges through canaliuculi
  • Chondrocytes have receptors for and are in contact with the surrounding collagen (the spaces seen on H&E are articfactual).
  • There is communication between the osteoblasts and osteocyte via the cytoplasmic processes in the canaliculi.
  • Inhibitors of mineralization (P-0-P bond) in canaliculi keep fluid from being plugged with mineral.

Damage, mechanical forces, use all affect communication between osteocytes and osteoblasts.

Streaming electric potentials- electric signals through fluid in canaliculi

Piezoelectric potential- generated by movement/twisting of crystals and collagen
Hematopoietic cell origin (monocyte/macrophage lineage)
Bone resorption via MMP’s collagenase, acid secretions- enzymes either secreted by osteoclasts via vesicles, or released from the ECM resorption

Osteoclast Activity:

  • Bind to fully mineralized osteoid or chondroid matrix
  • Requires that osteoblasts remove the lamina limitans (un-mineralized osteoid) before
  • Osteoclasts bind to RGD ligand (sequence of amino acids) in mineralized osteoid or chondroid (both type 1 collagen (osteoid), and type II collagen (chondroid) have RGD sequence) and form seals at the periphery (sealing zone)
  • Ruffled border is where the secretion of acid and enzymes happen


Carbonic anhydrase convert water and Co2 into Hydrogen ions which are excreted into the mineralized ECM.  H+ combines with Calcium Phosphate and forms HPO4 + Ca++ thereby demineralizing bone

Enzymes from the osteoclast are secreted from vesicles into the ECM which breakdown the collagen in the osteoid.

TGF beta is released from ECM by OB and activates OC to secrete acid

Negative feedback to OC via TGF-beta released from the osteoid by enzymes and Osteoprotegrin from OB inhibit OC activity

Endochondral Bone Growth:
Embryo: Cartilage model created → Vascularization of diaphysis → Primary ossification center →  secondary epiphyseal vsacularization →  secondary ossification center

  • Cartilage zones:
    • Resting zone
    • Proliferative zone
    • Hypertrophic zone
    • Mineralized zone
    • Chondrocyte apoptosis/necrosis
    • Mineralized longitudinal septa
    • Vascular invasion → chondrocytes and horizontal septa removed by pericytes,
    • and osteoblasts produce osteoid to line the Mineralized Longitudinal Septa →  Primary spongiosa

Modeling vs Remodeling:

  • Modeling: Change in size or shape without prior osteoclast activity (growth, fracture, acetabulum in DJD
    • Bone deposits at sites of compression, and resorbs at sites of tension
    • Mechanical use is detected by streaming potentials, and piezoelectric potentials (electric charge from twisting of crystals and collagen), stretch receptors and pressure receptors.
  • Remodeling:
    • Resoprtion of bone deposited during growth
    • Replacing it with adult bone
    • Important in cortical bone (originally a mixture of rapidly deposited woven bone, and slowly deposited non-osteonal compact lamellar bone)

Bone remodeling units:


  • cortical bone-
    • cutting cone:
      • Leading cluster of OC followed by osteoblasts and blood vessels
      • Parallel to the long axis of the bone. They are flexible.
      • Structure is composed of a central vascular channel (haversian canal) surrounded by layers of concentric lamellar bone.
      • Cementing lines separate osteons from surrounding bone, but not between the lamellae.
      • Haversian systems are characterized as primary, secondary, or tertiary based on whether there has been osteonalization prior (ie overlapping osteons).
      • Lamina limitans present only on the inside surface of the osteon, so there is no need for osteoblast removing it unless the osteon crosses another osteon
  • Trabecular bone
    • Bone Remodeling Unit (BRU)
      • Simply ½ of the osteonal remodeling unit. – All trabecular bone are covered by a lamina limitans.

Bone Remodeling Formula: Q A R R F Q

Quiescence → Activation → Resorption → Reversal → Formation  → Quiescence
Activation: Growth signals (PTH, Vit D, PGE2, PGI2, IL-1, TNF) → OB (cell retraction, collagenase production); Mononuclear cells become OC → remove lamina limitans

Osteoid resorption: PTH → OB → ODF –RANK → OC activation: RANK is bound or diffusible


  • PTH activates osteoblasts
  • Osteoblasts secrete collagenases and remove the lamina limitans
  • OB secrete Osteoclast Differentiation Factor (ODF/ RANK Ligand) which binds to the Receptor RANK on osteoclasts.
  • Deactivation: Osteoprotegrin from osteoblasts inhibit effect of ODF
  • Mineralized Longitudinal Septa → Primary trabeculae (Thicker and fewer) →  Secondary Trabeculae (Thicker and fewer) → Tertiary trabceulae (Thicker and fewer).  Generally there is less chondroid in each one

Species differences:

No remodeling: Rat. Mouse, birds

Minimal:  Rabbit
Cementing lines:

  • Resting/Reversal line
    • Divides a region of resorption from a region of addition.
    • The two areas are not united, but remain separate.
    • Cementing lines are high in proteoglycans and mineral but low in osteoid.
    • It acts as a slippery surface between the osteon and the surrounding bone.
    • This helps dissipate fractures and allows flexibility of the bone.
    • Bones soaking in any liquid will leach Proteoglycans from cartilage and cementing lines, which will make them look less basophilic

Pathologic bone resorption is marked by scalloped cementing lines, physiologic resorption or reversal is marked by smooth cementing lines.

Intramembranous bone formation:

  • Growth in bone width (cortical bone of most species)
  • Addition by the periosteal osteoblasts
  • Resorption at the endosteum
  • Cut back zone at the Metaphysis →  more osteoclast activity at the periosteum and compaction at the endosteum

Types of Cortical Bone:

  • Circumferential Lamellar →  found in non weight bearing neonates: Bone is layed down compacted in a lamellar form (slowly)
  • Plexiform osteonal  → Formed as anastomosing trabeculae of lamellar bone with central vascular channels (species that need to bear weight early= large animals), and then is compacted (filled in ) by quick production  of woven bone, then remodeled by osteonalization.
  • Rats may have cartilage in their cortical bone. This is from their development from physeal cartilage, and lack of remodeling. The cortical bone is simply compacted.

Articular Cartilage:

  • Avascular, relies on fluid dynamics from the synovium and subchondral bone
  • Articular/epiphyseal complex-
    • present during growth → Epiphyseal portion just below the Tidemark- basophilic line of proteoglycans between the mineralized vascular cartilage of the AE complex and the non-mineralized avascular articular cartilage.
    • Tidemark is a physiochemical interaction that happens during mineralization that alters the proteoglycan content in cartilage thereby making it more basophilic.
  • Collagen fibers-
    • Tangential at surface, transitional below, and large radial fiber region (nearly perpendicular to the articular surface. Fibers anchor into mineralized cartilage layer.
    • Cartilage is Type II collagen, and there is an abrubt transition between the AE cartilage and subchondral bone.
  • Articular cartilage-
    • Chondrification of the cartilage canals → Physiologic thrombosis of vessels in cartilage canal with replacement by chondroid → prelude to mineralization

Other Structures:

Vertebrae- Horizontal physis should be gone by 5 months
Mandible- the horizontal ramus is membranous bone → multilobular tumor of bone: Multilobular tumor of bone recapitulates normal zone of intramembranous ossification → Fibrous and cambium periosteal layer, then mineralized cartilage and bone (not necessary to have cartilage)

Cartilage forms from same cells, under low oxygen saturation
Mandibular suture- mineralized sharpeys fibers, no cambium layer

Bone responses to Injury:

  • Failure of Endochondral Ossification (retained physeal cartilage)
    • Primary Osteochondrosis
      • Absence of :
        • Mineralization
        • Chondrocyte death
        • Vascularization.
      • Looks similar to Ca/Phos imbalance or Vit D deficiency
      • AKA physeal dysplasia
      • Chondrocytes clusters/clones (proliferating repairing chondrocytes)
      • Impaired vascularization
  • Inflammation
  • Trauma/ infraction-
    • The presence of the fracture above the MLS indicate that mineralization, chondrocytes death, and vascular invasion had taken place prior to the fracture. Not primary osteochondrosis. This type of injury may happen many times, and be non-significant, and will heal quickly.
  • Growth Retardation Lattice
    • Failure of OC remodeling in a site of endochondral ossification
    • Retained primary trabeculae
    • Flared metaphysic, failed cutback, metaphyseal sclerosis
    • Acquired, temporary
      • CDV, BVDV, Lead toxicity
    • Osteopetrosis:
      • Congenital Angus disease
  • Growth Arrest Line
    • Transverse trabeculation indicates premature temporary physeal closure due to slowed longitudinal growth with normal remodeling
  • Ectopic Growth plate/ Osteochondromatosis/ Multiple cartilaginous exostosis
    • Misplaced growth plate
    • Typically single, quick
    • Different than osteochondromatosis in cats- admixed cartilage and bone, fibrous CT, multifocal, multi joints.  Vit A and FELV
  • Angular Limb Deformity:
    • Bowed leg
    • Concave aspect = compression→ net formation on the leading edge, and Net Formation (increased F > R) in the trailing edge
    • Convex aspect= Tension →  Net resorption (increased R > F on leading edge)
    • Net effect is bowed leg with a thicker cortical surface where the compression (concave surface) is.
  • Osteoporosis:
    • Decreased bone mass with clinical disease (fractures/pain etc)
    • Osteopenia =  decreased bone mass with no clinical disease
  • Nutritional causes
    • Normal AF/R with derceased F
    • Starvation/ negative energy balance
    • Loss of trabeculae, metaphyseal buttressing
    • Cortex may be normal thickness
    • Post-menopausal OP
    • Increased AF/R with Decreased F
    • Only develops in animals with regular estrous cycles (not dogs)
  • Osteopenia of Disuse:
    • Normal mechanical use impedes bone resorption →  maintenance of bone mass
    • Increased AF/R with decreased F
    • Periosteal contour may change due to modeling
    • Stress shielding may lead to segmental osteopenia
    • Cancellozation of cortical bone (cortex becomes porous)
  • Osteosclerosis
    • Decreased AF/R with Normal F (?)
    • Periosteal modeling
    • Accumulation of microfractures → Increased AF but the Net effect is decreased AF/R due to severe lack of R
  • IVDD → Compression of vertebral endplates dude to loss of IVD, end plate fusion and spondylosis
  • Legg-Calves-Perthes (Idiopathic Necrosis of the Femoral Head):
    • No clinical disease during the infarction →  dead bone is structurally sound
    • Signs occur during revascularization and remodeling to remove dead bone
    • Inadequate replacement of bone → Compression and fractures
    • May revascularize with creeping substitution →  clinically silent: We don’t know how often this occurs and heals without pathology
    • Histologically- can see new woven bone on top of dead lamellar bone without resorption (no cementing line)
  • Reactive Periosteal Bone:
    • Gradual transition between fibrous periosteal layer, periosteal osteoblastic cambium layer, and the woven trabecular zone
    • Causes:
      • Trauma
      • inflammation
      • neoplasia
      • vascular
      • Viral (avian retrovirus)
      • Inherited (congenital hyperostosis, craniomandibular osteopathy)
      • Sterile inflammation- HOD (Metaphyseal osteopathy)
      • Nutritional/metabolic- Hypervitaminosis A (sweet potatoes), Cervical hyperostosis in cats
  • Congenital Hyperostisis → Not normal periosteal reaction because the trabeculae are oriented perpendicular to the cortical surface
  • Hypertrophic Osteodystrophy (HOD, metaphyseal Osteopathy)
    • Reactive bone in the metaphysic secondary to metaphyseal sterile inflammation, necrotizing and suppurative
    • Periosteum holds epiphysis to metaphysic → Mechanical instability
    • Cortical bone is porous at the cutback zone → suppurative inflammation leaks to Periosteum
  • Nutritional Secondary Hyperparathyroidism (fibrous osteodystrophy)
    • Calcium/ Phosphorous imbalance: diet high in Phos, Low in Ca, Vit D deficiency
    • Prolonged hyperparathyroidism due to continued low serum calcium

Histopathology Cases

Proliferative Bone lesions:

  • Features to Evaluate
    • Transition between surrounding CT and Productive OB and osteoid
    • Organization of the periosteum, surrounding CT,
    • Cortical bone, remodeling of inner layers
    • Atypical cells- Osteoblasts, Cartilage, and surrounding Cells lining the osteoid trabeculae
    • Underlying cause (neoplasia, inflammation, fracture, hemorrhage)
    • Abrubt transitions = malignant
    • Gradual transition = Benign neoplasm or hyperplasia
    • Normal osteoblasts ligning osteoid = benign/hyperplastic
    • Atypical mesenchymal cells between trabeculae = malignant
  • Ossifying fibroma- Lytic, expansile, intraosseus with osteoblasts lining trabeculae in a matrix of fibrous spindle cells
  • Fibrous dysplasia- Same as ossifying fibroma but without the osteoblasts lining the trabeculae
    • Alternate diagnosis- fibrosarcoma, low grade with osseous metaplasia
  • Fracture callus:
    • Common to have cartilage
    • Low oxygen levels form cartilage, not osteoid →  then ossify (endochondral ossification)
    • Normal oxygen levels→ membranous bone formation
    • Lots of movement→ fibrous tissue
  • Scurvey: inadequate osteoid formation (altered OB and collagen formation-scorbutic osteopathy) and altered osteoclast function (impaired remodeling)
  • Dentinal dysplasia and atrophy

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