HSF Block 1 > Histo - Bone > Flashcards

Histo - Bone Flashcards

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1
Q

* Basic bone structure and function

A

Basics:

  • Bone = connective tissue
    • contains cells & ECM
  • Specialized as it mineralizes its matrix forming hard tissue

2 Main Functions: Mechanical & Metabolic

  1. Support
  2. Protection
    • keep organs away from damage (ribcage/skull)
  3. Site of blood cell formation
  4. Movement
  5. Storage
    • bone store fat, growth factors & some minerals
      • ie: Ca + P
  6. Homeostatic regulation of blood Ca
  7. Sound Transduction

Notes:

  • Skeleton = highly metabolic & dynamic
  • Undergoes continuous turnover
    • entire skeleton replaced every 7 - 10 years
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2
Q

* What is the difference between intramembranous and endochondral ossification?

A

Intramembranous:

  • “simpler” type of bone formation
  • development of flat/irregular bones
    • skull, sternum, mandible
    • remnants of what used to be dermal shield
  • Multi-potnent mesenchymal stem cell
    • differentiate directly into osteoBLASTS —> form bone

Endochondral:

  • “complex” type of bone formation
  • development of extremities & axial skeleton
    • bears weight; most bones develop this way
  • hyaline cartilage template –> replaced by bone
    • responsible for growth in length of bone

Similarities:

  • Both processes are essentially the same
    • same cells participate & do the same things
  • At biochem/cell level same events occur

Differences:

  • SITE of activity
  • ORGANIZATION of activity
  • NUMBERS of centers of ossificiation
  • WHAT is replaced
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3
Q

Explain bone composition

A

Basics:

  • Organic (35%)
    • 90% collagen (type I)
    • 5% proteoglycans
  • Inorganic (65%)
    • Hydroxyapatite solid-phase crystals [Ca10(PO4)6(OH)2]

Composition:

  • Varies based on function
    • ie: Ossicle (ear bones) = 80% mineral vs 20% organic
      • allows for transmission of vibration so rigidity = key
  • Bone structure = more significant for the purpose of fxn
    • ie: femur vs. vertebrae
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4
Q

* What is Macroscopic Bone?

A

Basics:

  • General stucture of bone = thicker bone shell surrounding spongy center
    • can be long, short, flat, irregular, sesamoidal in shape

Cortical bone (compact, lamellar):

  • Structure:
    • appears as a mass
    • lacks spaces
    • provides strength & resists bending
  • Function:
    • rigidity
    • supporting weight

Cancellous (trabeccular, spongy):

  • Structure:
    • appears spongy
    • numerous spaces
    • large SA
  • Function:
    • marrow space
    • ion homeostasis

Notes:

  • Femur = thick cortical component
    • acts as a lever; resists bending; suppports body weight
  • Vertebrae = more cancellous bone
    • greater flexibility due to compression force loads
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5
Q

What are the 2 Types of Bone Linings?

A

Periosteum:

  • 2 Layers:
    • ​Outer layer = fibrous
      • Dense fibeous covering of the bone
      • Sharpey’s fibers
        • coarse collagenous fibers
        • anchor tendons & ligaments
    • Inner layer = cellular (cambium)
      • deep to outer fibrous layer
      • osteoprogenitor cells can become osteoblasts
        • produce new bone
  • Functions:
    1. Connection of tendons & ligaments
    2. Maintain progenitor population of cells

Endosteum:

  • Fine reticular connective tissue
    • not easily seen
  • Contains osteoprogenitor cells
    • ​young bones, these cells —> osteoblasts
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6
Q

Where do you find periosteum?

A
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7
Q

Where do you find endosteum?

A
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8
Q

What do we see in this image?

A
  • Here we can see the collagen fibrils of the periosteum and that they are incorporated into the mineralized matrix
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9
Q

What are the components of mature bone?

A
  1. Osteons or Haversian Systems
  2. Concentric Lamellae
  3. Osteonal or Haversian Canal
  4. Lacunae
  5. Canaliculi
  6. Lateral or Volkman’s Canal
  7. Osteocyte
  8. Circumferential Lamellae
  9. Interstitial Lamellae
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10
Q

What is the difference between Mature Bone and Immature Bone?

A
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11
Q

What is this?

A

Mature Cortical Bone

  1. Haversian canal
  2. Volkmann’s canal
  3. Osteocyte lacuna
  4. Interstial lamellae
  5. Osteon
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12
Q

What is Intramembranous Ossification?

A

Basics:

  • Part I: Osteoblast Differentiation
    • Wnt Signaling
      • increase: beta-catenin/Runx2/Osterix
    • mesenchymal stem cell –> osteoproginator –> osteoblast
  • Part II: Osteogenesis
    1. Proliferation
    2. Osteoid Deposition
    3. Mineralization

Location:

  • Occurs w/in vascularized embryonic mesenchyme

Process:

  1. Mesenchymal stem cells = close proximity to newly arrived blood vessel
    • blastema = differentiate into osteoBLASTs
  2. Ossification center = established
  3. OsteoBLAST secrete and organic matrix (Type I Collagen)
    • adds inorganic components –> forms bony spicules
  4. As matrix develops, osteoBLASTs are pushed farther apart
    • cytoplasmic processes connect them together
    • functional syncytium = intracellular communication
  5. OsteoBLAST will eventually be surrounded/trapped inside new bone
    • become osteoCYTES
  6. Further development & remodeling = forms compact bone
    • osteons/Haversian systems
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13
Q

What is appositional growth?

A

Basics:

  • Layer or course of new bricks is added to the layer of bricks that is already there
    • growth by addition of new layers to those already formed
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14
Q

What is Endochondral Ossification?

A

Basics:

  • How most bones are formed
  • Cartilage precursor

Steps: Formation of 1° Ossification Center (~3mo in-utero)

  1. Chondrocytes in a matrix (Type II collagen - Hyaline Cartilage)
  2. Chrondrocytes undergo maturation –> hypertrophy
    • express Type X Collagen
    • Secrete angiogenic factor (VEGF)
    • Direct mineralization of proximal matrix
  3. Hypertrophic chondrocytes
    • signal to perichondrial cells (osteoprogenitor) –> osteoBLASTS
    • VEGF = causes vascular invasion
      • brings invasion of osteoprogenitor & hematopoietic cells
  4. Formation of Primary Ossification Center
    • Calcification of the matrix causes chondrocytes to apoptos
      • mid shaft
    • periosteal bone collar is formed

Steps: Formation of Secondary Ossification Center (postnatal)

  1. In epiphysis, hypertrophic chondrocytes apoptos
    • blood vessels invade space
  2. Most hyalinie cartilage = replaced by bone
    • EXCEPT: articular cartilage & growth plate
    • IHH: stimulates chondrocyte proliferation
  3. In mature bone (~14 yrs), chondrocytes in growth plate = replaced by bone
    • epiphyseal line
    • no more growth
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15
Q

How is Endochondral Ossification Regulated?

A

Basics:

  • Indian Hedgehog (IHH) = master regulator of endochrondral ossification

Process:

  1. pre-hypertrophic chondrocytes produce IHH
  2. IHH causes perichondrium –> osteoBLAST differentiation
    • bone collar formation
    • PTH-RP (parathyroid-related protein) secretion
  3. PTH-RP = induces proliferation of reserve zone chondrocytes
  4. PTH-RP = inhibits precocious chondrocyte hypertrophy
    • proximity to PTH-RP = controls cell fxn

Note:

  • IHH -/- mice = exhibit skeletal development defects
    • lack of endochondral bone formation
    • under dev of cartilage due to decrease chondrocyte proliferation
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16
Q

How does Endochondral Ossification lead to Growth?

17
Q

What are the layers of Endochondral Ossification?

18
Q

What is this?

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A
  • Inactive osteoblasts
    • can be activated to produce bone matrix
  • Bone surfaces which are NOT under remodeling or modeling
    • covered by elongated, thin cells
19
Q

What is an Osteoclast?

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A

Basics:

  • OsteoCLAST = bone resorptive cell = bone break down
  • Hematopoietic in origin
    • formed from aggregation of monocytes (marrow macrophages)
  • Bone destruction achieve cia secretion of H+ ions
    • lowers pH
    • dissolves mineral/cathepsin K

Characteristics:

  • polar cell, multiple INDEPENDENT round nuclei
  • large compared to osteoBLAST
    • amorphous shape
  • Ruffled border near bone (working surface)
    • Howship’s lacunae (pit) (carbonic anhydrase II)

Function:

  • Creation of a microenvironment
    • osteoCLAST & underlying bone matrix
  • Intergrins - on osteoclast
  • Osteopontin - in bone matrix
  • Sealing Zone = actin ring
  • Podosomes - actin & cytoskeletal proteins = anchors
20
Q

* How are Osteoclast Formed?

How is differentiation regulated?

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A

Basics:

  • Formed by cytoplasmic fusion of their mononuclear precursors
    • Myeloid lineage of hematopoietic cells
  • Osteoclast differentiation req. expression of OCP (osteoclast precursors) of c-Fos
    • RANKL activated transcription factor
  • RANK = receptor for RANKL
    • found on OCP
    • essential for osteoCLAST formation

Process:

  1. Monocyte (bone marrow) = reaches an area of bone formation/remodeling
    • M-CSF (Macrophage colony-stimulating factor) receptor on surface
  2. Monocyte –> Macrophage when M-CSF ligand from osteoblast binds
    • expression of RANK now on surface of osteoclast
  3. Osteoblast has RANKL receptor that binds to RANK
    • commits to osteoclastogenesis
    • now OCP (but cannot reabsorb bone yet)
  4. Resting osteoclast uncouples from osteoblast
  5. Sealing zone & ruffle border - maturation of osteoclast
    • req. alpha-beta integrin
    • now a Functional Osteoclast

Regulation:

  • Parathyroid hormone (PTH)
    • simulated M-CSF & RANKL expression in osteoblast
    • essential recruiters for osteoclastogenesis
  • Denosumab
    • monoclonal Ab to RANKL
    • binds instead of RANK –> decrease osteoclastogenesis
  • Osteoprotegerin
    • inhibits osteoblast-derived RANKL
    • regulated population of functional osteoclasts

NOTE:

  • OsteoBLAST control osteoCLAST differentiation (NOT function)
21
Q

What are the 4 layers?

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A

1- Proliferation Zone
2- Hypertrophic Zone
3- Ossification Front
4- Calcified Cartilage/Bone

22
Q

What is Bone Remodeling?

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A

Compact Bone Remodeling:

  1. Activation
  2. Resorption
  3. Reversal
  4. Formation

Trabecular Bone Remodeling:

  1. Resorption Space
  2. Osteoclast
  3. Osteoblast
  4. New Bone
23
Q

What is fracture repair?

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A

Basics:

  • Fracture repair begins immediately after fracture
  • Involves both intramembranous & endochondral bone formation

Process:

  1. Bleeding & hematoma formation at site of injury
  2. Growth factor & cytokine release induce recruitment of mesenchymal stem cells (periosteal)
  3. Osteogenic & chondrogenic differentation & proliferation occur
    • produce soft fracture callus
  4. Woven bone is produced
    • hard callus
  5. Bone remodeling occurs to produce organized lamellar bone

Note:

  • Intramembranous ossification = occurs away from fracture
    • vascularized tissue = favorable
    • less cartilage = faster healing
  • Endochondral ossifciation = occurs at site
    • hypoxic conditions = favorable
24
Q

What are conditions that impair fracture repair?

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A
  1. Aging
    • decrease rate of fracture healing
      • non-union = significant clinical problem
    • Possible change in progenitor cell population
    • Impaired osteoclastic response & blood vessel formation
  2. Diabetes
    • impaired bone healing
      • possibly due to reduced progenitor proliferation & reduced matrix deposition
    • Insulin treatment = reverses this
    • direct medullary delivery of insulin to fracture site = enhances repair
  3. Smoking
    • impairs bone healing
      • non mechanism established
    • smoking reduces blood vessel formation & possibly mesenchymal stem cell condensation & chondrogenesis
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