Lect. 7 specialized ct - bone

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Lect. 7 specialized ct - bone

  1. 1. SPECIALIZEDSPECIALIZED CONNECTIVE TISSUECONNECTIVE TISSUE BONEBONE
  2. 2. BONEBONE ► forms theforms the strong and rigidstrong and rigid endoskeletonendoskeleton to which skeletal muscles are attached toto which skeletal muscles are attached to permit movement.permit movement. ► acts as aacts as a calcium reservoircalcium reservoir and isand is important in calcium homeostasis.important in calcium homeostasis. ► heavyheavy and its architecture is optimallyand its architecture is optimally arranged to provide maximumarranged to provide maximum strengthstrength forfor the least weight.the least weight.
  3. 3. Bone as a tissueBone as a tissue ►composed of cellscomposed of cells ► predominantly collagenous ECMpredominantly collagenous ECM  type I collagentype I collagen  calledcalled osteoidosteoid  becomes mineralised by the deposition ofbecomes mineralised by the deposition of calcium hydroxyapatitecalcium hydroxyapatite  giving the bone considerable rigidity andgiving the bone considerable rigidity and strength.strength.
  4. 4. CELLS of the bone:CELLS of the bone: ► OsteoblastsOsteoblasts ► OsteocytesOsteocytes ► OsteoclastsOsteoclasts ► derived from aderived from a primitive mesenchymalprimitive mesenchymal (stem) cell called the(stem) cell called the osteoprogenitorosteoprogenitor cellcell.. ► multinucleatemultinucleate phagocytic cellsphagocytic cells derived from thederived from the macrophage-monocytemacrophage-monocyte cell linecell line
  5. 5. OsteoblastOsteoblast ► synthesise osteoid and mediate itssynthesise osteoid and mediate its mineralisationmineralisation ► they are found lined up along bonethey are found lined up along bone surfaces.surfaces.
  6. 6. ►OsteoblastOsteoblast
  7. 7. OsteocytesOsteocytes ► housed in lacuna (cavity) within matrixhoused in lacuna (cavity) within matrix ► represent inactive osteoblasts trappedrepresent inactive osteoblasts trapped within formed bonewithin formed bone ► they may assist in nutrition of bone.they may assist in nutrition of bone.
  8. 8. OsteoclastsOsteoclasts ► phagocyticphagocytic cellscells which are capable ofwhich are capable of eroding bone anderoding bone and which are important,which are important, along with osteoblasts,along with osteoblasts, in the constantin the constant turnover andturnover and refashioningrefashioning ofof bone.bone. ►
  9. 9. ► Intramembranous Ossification: Mesenchymal cells develop intoIntramembranous Ossification: Mesenchymal cells develop into osteoblasts and synthesize bony matrix, becomes osteocytes trappedosteoblasts and synthesize bony matrix, becomes osteocytes trapped in the bone matrix. Osteoclasts are multinucleated, phagocytotic cellsin the bone matrix. Osteoclasts are multinucleated, phagocytotic cells releases hydrolases into the matrix to decalcify the bony matrix.releases hydrolases into the matrix to decalcify the bony matrix.
  10. 10. Other important structures:Other important structures: ► Canaliculi – provide passages forCanaliculi – provide passages for circulation of ECF & diffusion ofcirculation of ECF & diffusion of metabolites between lacunae andmetabolites between lacunae and vessels of Haversian canals.vessels of Haversian canals. ►CT layersCT layers  Periosteum – covers external surfacePeriosteum – covers external surface  Endosteum – covers internal surfaceEndosteum – covers internal surface
  11. 11. Areas within bones:Areas within bones: ► Most bones have aMost bones have a dense rigid outer shelldense rigid outer shell ofof compact bonecompact bone,, thethe cortexcortex ► and a centraland a central medullarymedullary oror cancellouscancellous zone ofzone of thin interconnectingthin interconnecting narrow bonenarrow bone trabeculae.trabeculae.
  12. 12. Bone cortex – COMPACT boneBone cortex – COMPACT bone
  13. 13. ►The Osteon (Haversian system)The Osteon (Haversian system)
  14. 14. OsteonOsteon ►Calcified unit (hard bone); periosteumCalcified unit (hard bone); periosteum ►Haversian canal – contains a blood vesselHaversian canal – contains a blood vessel ►Volkmann’s canal – connects 2 haversianVolkmann’s canal – connects 2 haversian canalscanals ►Lamellae – contains canaliculiLamellae – contains canaliculi ►Lamellar ridge – contains lacunaeLamellar ridge – contains lacunae
  15. 15. SPONGY / CANCELLOUS boneSPONGY / CANCELLOUS bone ► a network of interconnecting strutsa network of interconnecting struts orientated in a position to provide theorientated in a position to provide the maximum strength for the minimum mass.maximum strength for the minimum mass. ►They are composed of lamellar bone withThey are composed of lamellar bone with scanty lacunae containing osteocytes.scanty lacunae containing osteocytes.
  16. 16. The trabeculaeThe trabeculae TT have a thin external coating ofhave a thin external coating of endosteum containing flat inactive osteoblasts.endosteum containing flat inactive osteoblasts.
  17. 17. Bone marrowBone marrow
  18. 18. Long bone section:Long bone section: ► CancellousCancellous (Spongy) bone –(Spongy) bone – porous withporous with trabeculaetrabeculae (collagenous tissue)(collagenous tissue) ► Compact Bone –Compact Bone – calcified or hardcalcified or hard bone (periosteum)bone (periosteum) ► Medullary cavity –Medullary cavity – contains the bonecontains the bone marrow (endosteum)marrow (endosteum)
  19. 19. Seatwork: by partnersSeatwork: by partners (use 1 yellow paper only)(use 1 yellow paper only) ► Outline the sequence of events during boneOutline the sequence of events during bone formation or OSTEOGENESIS.formation or OSTEOGENESIS.  Intramembranous ossificationIntramembranous ossification  Endochondral ossificationEndochondral ossification
  20. 20. Bone Formation orBone Formation or OSTEOGENESISOSTEOGENESIS ► Fetal development of bone occurs in twoFetal development of bone occurs in two ways:ways:  Endochondral ossification -Endochondral ossification - cartilage bonescartilage bones  Intramembranous ossification -Intramembranous ossification - membranemembrane bonesbones ► Both of which involve replacement ofBoth of which involve replacement of primitive collagenous supporting tissue byprimitive collagenous supporting tissue by bone.bone.
  21. 21. Endochondral OssificationEndochondral Ossification  requires a pre-existing cartilage model ► embryonal type ► hyaline ► continuously growingcontinuously growing  followed by deposition of bone matrix.  cartilage model is progressively replacedcartilage model is progressively replaced by bone.by bone. ►Occur in formation of long bones, vertebrae, pelvisOccur in formation of long bones, vertebrae, pelvis and bones of the base of the skull.and bones of the base of the skull.
  22. 22. EndochondralEndochondral ossificationossification 1.1. A hyaline cartilage model starts with a boneA hyaline cartilage model starts with a bone collar.collar.  Cartilage model - a small model of the longCartilage model - a small model of the long bone is first formed in solid hyaline cartilage.bone is first formed in solid hyaline cartilage.  Undergoes appositional growth to form anUndergoes appositional growth to form an elongated, dumb-bell shaped mass of cartilageelongated, dumb-bell shaped mass of cartilage 1.1. At diaphysis: Bone collar undergoAt diaphysis: Bone collar undergo intramembranous ossification and forms anintramembranous ossification and forms an calcified cartilage plate.calcified cartilage plate.
  23. 23. Intramembranous ossification in diaphysis of theIntramembranous ossification in diaphysis of the cartilage model of long bone.cartilage model of long bone. ► Chondrocytes enlarge greatly, resorbing theChondrocytes enlarge greatly, resorbing the surrounding cartilage so as to leave only slendersurrounding cartilage so as to leave only slender perforated trabeculae of cartilage matrix.perforated trabeculae of cartilage matrix. ► This cartilage matrix then becomes calcified andThis cartilage matrix then becomes calcified and the chondrocytes degenerate leaving large,the chondrocytes degenerate leaving large, interconnecting spaces.interconnecting spaces. ► Perichondrium develops osteogenic potential andPerichondrium develops osteogenic potential and assumes the role of periosteum.assumes the role of periosteum.
  24. 24. ► The periosteum then lays down a thin layer ofThe periosteum then lays down a thin layer of bone around the surface of the shaft.bone around the surface of the shaft. ► Primitive mesenchymal cells and blood vesselsPrimitive mesenchymal cells and blood vessels invade the spaces left within.invade the spaces left within. ► The primitive mesenchymal cells differentiate intoThe primitive mesenchymal cells differentiate into osteoblasts and blood-forming cells of the boneosteoblasts and blood-forming cells of the bone marrow.marrow. ► The osteoblasts form a layer of cells on theThe osteoblasts form a layer of cells on the surface of the calcified remnants of the cartilagesurface of the calcified remnants of the cartilage matrix and commence the formation of irregular,matrix and commence the formation of irregular, woven bone.woven bone.
  25. 25. 3.3. Nutrient vessels penetrate the diaphysisNutrient vessels penetrate the diaphysis after osteoclasts invaded the area, theafter osteoclasts invaded the area, the osteoprogenitor cells follow the movementosteoprogenitor cells follow the movement of blood vessels.of blood vessels. 4.4. At diaphyseal area: invading osteoblastsAt diaphyseal area: invading osteoblasts undergo matrix synthesis (undergo matrix synthesis (primary ossification center).
  26. 26. 5.5. EpiphysesEpiphyses:: Secondary ossificationSecondary ossification centers appear at thecenters appear at the swellings of theswellings of the cartilage modelcartilage model (epiphyses). During(epiphyses). During their expansion andtheir expansion and remodeling, theremodeling, the primary and secondaryprimary and secondary ossification centersossification centers produce cavities thatproduce cavities that are gradually filled withare gradually filled with bone marrow.bone marrow.
  27. 27. ► In the secondary ossification centers, cartilageIn the secondary ossification centers, cartilage remains in two regions:remains in two regions: 1. articular cartilage,1. articular cartilage, which persists throughoutwhich persists throughout adult life and does not contribute to bone growthadult life and does not contribute to bone growth elongation.elongation. 2. epiphyseal cartilage,2. epiphyseal cartilage, also called thealso called the epiphyseal plate,epiphyseal plate, which connects the epiphyseswhich connects the epiphyses to the diaphysis, is responsible for the boneto the diaphysis, is responsible for the bone elongation, this plate disappears in adults, which iselongation, this plate disappears in adults, which is why bone growth ceases in adulthood.why bone growth ceases in adulthood.
  28. 28. Zones of Bone GrowthZones of Bone Growth 1)1) Resting zoneResting zone - hyaline cartilage without morphological changes- hyaline cartilage without morphological changes in the cells.in the cells. 2)2) Proliferative zoneProliferative zone -- chondroblasts mitotically divides and formchondroblasts mitotically divides and form columns of stacked cells parallel to the long axis of the bone.columns of stacked cells parallel to the long axis of the bone. 3)3) Hypertrophic cartilage zoneHypertrophic cartilage zone - large chondrocytes with- large chondrocytes with glycogen-filled cytoplasm. The resorbed matrix is reduced to thinglycogen-filled cytoplasm. The resorbed matrix is reduced to thin septa between the chondrocytes.septa between the chondrocytes. 4)4) SSimultaneous with the death of chondrocytes in theimultaneous with the death of chondrocytes in the calcifiedcalcified cartilage zonecartilage zone ,, the thin septa of cartilage matrix becomethe thin septa of cartilage matrix become calcified by deposition of hydroxyapatite (bone mineralization).calcified by deposition of hydroxyapatite (bone mineralization). 5)5) Ossification zoneOssification zone -- endochondral bone tissue appears. Bloodendochondral bone tissue appears. Blood capillaries and osteoprogenitor cells formed by mitosis of cellscapillaries and osteoprogenitor cells formed by mitosis of cells originating from the periosteum invade the cavities left by theoriginating from the periosteum invade the cavities left by the chondrocytes. The osteoprogenitor cells form osteoblasts, whichchondrocytes. The osteoprogenitor cells form osteoblasts, which are distributed in a discontinuous layer over the septa of calcifiedare distributed in a discontinuous layer over the septa of calcified cartilage matrix. Ultimately, the osteoblasts deposit bone matrixcartilage matrix. Ultimately, the osteoblasts deposit bone matrix over the calcified cartilage matrix .over the calcified cartilage matrix .
  29. 29. Intramembranous OssificationIntramembranous Ossification ►no hyaline cartilage model ►occurs within 'membranes' of condensed,occurs within 'membranes' of condensed, primitive mesenchymal tissue.primitive mesenchymal tissue. ►mineralization of matrix by osteoblasts ►direct replacement of mesenchyme by bonedirect replacement of mesenchyme by bone ► occurs in the bones of the vault of the skull,occurs in the bones of the vault of the skull, the maxilla and most of the mandiblethe maxilla and most of the mandible
  30. 30. 1.1. Mesenchymal cellsMesenchymal cells  osteoblastsosteoblasts  begin synthesis and secretion of osteoid atbegin synthesis and secretion of osteoid at multiplemultiple centres of ossification. 1.1. Osteoblasts (trapped in lacunae)Osteoblasts (trapped in lacunae)  osteocytesosteocytes  fine cytoplasmic extensions shrink to form thefine cytoplasmic extensions shrink to form the fine processes contained within the canaliculi.fine processes contained within the canaliculi. 1.1. Osteoprogenitor cells at the surface of theOsteoprogenitor cells at the surface of the centres of ossification undergo mitoticcentres of ossification undergo mitotic division to produce further osteoblastsdivision to produce further osteoblasts which lay down more bone.which lay down more bone.
  31. 31. 4.4. Fusion of adjacent ossification centres toFusion of adjacent ossification centres to form bone (spongy in gross appearance).form bone (spongy in gross appearance).  ““woven bone” – collagen fibres of developingwoven bone” – collagen fibres of developing bone are randomly arranged in interlacingbone are randomly arranged in interlacing bundles.bundles. 4.4. Progressive remodelling of woven boneProgressive remodelling of woven bone into lamellar bone by osteoclasticinto lamellar bone by osteoclastic resorptionresorption 5.5. Osteoblastic deposition to form matureOsteoblastic deposition to form mature compact or trabecular bone.compact or trabecular bone. 6.6. The primitive mesenchyme remaining in theThe primitive mesenchyme remaining in the network of developing bone differentiates intonetwork of developing bone differentiates into bone marrow.bone marrow.
  32. 32. Intramembranous ossificationIntramembranous ossification: Mesenchymal cells condensation: Mesenchymal cells condensation (primary ossification center)(primary ossification center)  Osteoprogenitor cells (boneOsteoprogenitor cells (bone blastema)blastema)  Osteoblasts (synthesizes bony matrix)Osteoblasts (synthesizes bony matrix)  OsteocyteOsteocyte

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