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Flashcards in Introduction to Bone Deck (50)
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_____ Is the Master Regulator of Bone Formation

RUNX2 (Cbfa1)

  • Also mediated by osterix (SP7)


What is the job of osteoblasts?

synthesize the organic components of the bone matrix


Osteoblasts differentiate from __________ ______ ______.

Mesenchymal Stem Cells


What are the two final destinations for osteoblasts in vivo?

  1. undergo apoptosis


  1. undergo terminal differentiation to osteocytes


What would you use Aliziran red for?

What would you use Alcian blue for?

  • Alizarin red stains calcified tissue
  • Alcian blue stains cartilage


How do osteocyte processes in the canalicular network communicate with one another?

via gap junctions

  • encoded by connexin 43


Where do osteoclasts arise from?

myeloid progenitor

  • monocyte/macrophage lineage


What do osteoclasts require in order to differentiate?

Communication with osteoblasts:

  • Require:
    1. M-CSF
      • recpetor: CSF1R 
    2. RANK-L


RANKL is produced by _________.



Describe the role of osteoprotegrin (OPG):

  • also produced by osteoblasts
  • protein that is a soluble decoy receptor for RANKL 
  • RANKL that binds to OPG is not able to bind RANK 
  • high OPG levels inhibit terminal osteoclast differentiation


What stimulates osteoclast activity?

What inhibits osteoclast activity?

  • Stimulation:
    • PTH
    • 1, 25 di-OH-D
  • Inhibition:
    • calcitonin


  • How do osteoclasts degrade bone?
  • What is secreted?
  • What is the morphology?

  • mature osteoclasts bind tightly to the bone surface via the sealing zone to enclose a compartment in which bone matrix can be degraded
  • osteoclasts secrete: 
    • HCl to dissolve the mineral
    • cathepsin K to degrade the bone matrix proteins
  • cell membrane of the osteoclast within the sealing zone assumes a characteristic ruffled border appearance


Osteocytes signal to the ____ _______.

bone surface


Bone Components:

  1. Cortical
  2. Trabecular

Cortical and trabecular bone are constituted of the same cells and the same matrix elements, but there are structural and functional differences

  1. Cortical
    • ​​80-90% of volume is calcified
    • Fulfills mainly a mechanical and protective function
    • Always found on outside of bones and surrounds trabecular bone
    • ~80% of bone
  2. Trabecular
    • ​​15-25% of volume is calcified
    • Fulfills mainly a metabolic function
    • ~20% of bone


Bone Components:

  1. Intramembranous 
  2. ​Endochondral

  1. Intramembranous
    • ​​Intramembranous bone is formed by formation of osteoblasts
    • bone is formed de novo
  2. Endochondral
    • ​​replace previously formed cartilage models
    • grow in length by proliferation of chondrocytes within the growth plate 
    • Linear growth ceases when the growth plates fuse


Give examples of intramembranous bone:


  1. many bones of the skull 
  2. ribs
  3. Also formed in the bone collar region of a healing fracture and at the periosteal surface of long bones as they model to achieve greater diameters


What is the growth plate?

a specialized structure present within growing endochondral bones


Give examples of endochondral bone:

long bones of the limbs


Describe growth of endochondral bone:

(i.e. what occurs in the growth plate)

  • Growth plate is a highly organized tissue in which chondrocytes are arrayed in columns
    • with different positions within the column occupied by cells at a distinct point of maturation
  • Chondrocytes in the proliferative zone divide
    • replenishing the growth plate
  • Chondrocytes then hypertrophy, undergo apoptosis, and are mineralized
  • Blood vessels invade the zone of calcified cartilage,
    • which is resorbed by chondroclasts
    • space is filled by osteoblasts and bone matrix


What does linear growth of long bones depend on?

Depends on the relative speed with which:

  • cells in the hypertrophic zone undergo apoptosis 
  • cells in the proliferative zone divide


In humans, the growth plate closes in ____ _________ in response to ________ _________?

In humans, the growth plates close in late adolescence in response to estrogen signaling


At any given time, about ___ of the skeleton is being remodeled


  • albeit not all at the same rate


Why is trabecular bone much more actively remodeled than cortical bone?

Trabecular bone has a more prominent role in maintaining mineral homeostasis


2 essential principles to understanding the bone remodeling cycle:

  1. bone resorption and bone formation are coupled processes
  2. bone resorption is relatively rapid, requiring ~2 weeks, while bone formation is slow, requiring 4-6 months for full mineralization to take place


Describe the remodeling process of trabecular bone:

  • Remodeling of trabecular bone provides a mechanism by which extracellular fluid can buffer its calcium and phosphate content 
  • PTH and 1, 25 di-OH vitamin D are potent activators of osteoclast activity
  • bone resorption involves dissolution of the bone mineral 
    • providing free Ca and PO4 that can enter the extracellular fluids and blood


What is essential for moment-to-moment mineral homeostasis?

  • excess Ca and PO4 can be deposited into the bone.
  • Amount of Ca exchanged by trabecular bone each day is approximately twice the amount that is absorbed from food or lost in urine each day
  • Active exchange of minerals between the bone and the circulation is essential for moment-to-moment mineral homeostasis


Define the concept of bone modeling:

What is it mediated by?

What is an example of bone modeling?

  • Bones do not only grow in length, but grow in radial size and change shape as well
  • mediated at least in part by physiological responses to mechanical loading
  • Example:
    • seen in elite racquet sport athletes, comparing dominant and non-dominant arms


At the level of the whole bone, _____ is the primary stimulus to modeling


  • fractional change in length


What appears to be the critical stimulus for bone modeling at the cellular level?

shear stress

  • requiring both:
    1. anchoring of osteocytes by focal adhesion complexes
    2. fluid flow sensed by each cell’s primary cilium


Describe the process of fracture healing:

  1. Hematoma formation at the fracture site
    • consequence of the inflammatory process.
  2. Soft callus forms via mesenchymal stem cells
  3. Blood vessels invade the soft callus
  4. Hard callus forms 
    • Bone replaces cartilage
    • very similar to that seen in the growth plate  
  5. Simultaneously, osteoblasts on the subperiosteal surface form an intramembranous bone collar
  6. Hard callus undergoes remodeling to form lamellar bone and modeling ⇒ reduce the bulge at the fracture site