Introduction to Bone

Question 1

_____ Is the Master Regulator of Bone Formation

Answer 1

RUNX2 (Cbfa1)

• Also mediated by osterix (SP7)

Question 2

What is the job of osteoblasts?

Answer 2

synthesize the organic components of the bone matrix

Question 3

Osteoblasts differentiate from __________ ______ ______.

Answer 3

Mesenchymal Stem Cells

Question 4

What are the two final destinations for osteoblasts in vivo?

Answer 4

1. undergo apoptosis

or

1. undergo terminal differentiation to osteocytes

Question 5

What would you use Aliziran red for?

What would you use Alcian blue for?

Answer 5

• Alizarin red stains calcified tissue
• Alcian blue stains cartilage

Question 6

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

Answer 6

via gap junctions

• encoded by connexin 43

Question 7

Where do osteoclasts arise from?

Answer 7

myeloid progenitor

• monocyte/macrophage lineage

Question 8

What do osteoclasts require in order to differentiate?

Answer 8

Communication with osteoblasts:

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

Question 9

RANKL is produced by _________.

Answer 9

osteoblasts

Question 10

Describe the role of osteoprotegrin (OPG):

Answer 10

• 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

Question 11

What stimulates osteoclast activity?

What inhibits osteoclast activity?

Answer 11

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

Question 12

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

Answer 12

• 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

Question 13

Osteocytes signal to the ____ _______.

Answer 13

bone surface

Question 14

Bone Components:

1. Cortical
2. Trabecular

Answer 14

Cortical and trabecular bone are constituted of the s_ame 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

Question 15

Bone Components:

1. **Intramembranous **
2. ​Endochondral

Answer 15

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

Question 16

Give examples of intramembranous bone:

Answer 16

**Examples: **

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

Question 17

What is the growth plate?

Answer 17

a specialized structure present within growing endochondral bones

Question 18

Give examples of endochondral bone:

Answer 18

long bones of the limbs

Question 19

Describe growth of endochondral bone:

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

Answer 19

• 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

Question 20

What does linear growth of long bones depend on?

Answer 20

Depends on the relative speed with which:

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

Question 21

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

Answer 21

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

Question 22

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

Answer 22

10%

• albeit not all at the same rate

Question 23

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

Answer 23

Trabecular bone has a more prominent role in maintaining mineral homeostasis

Question 24

2 essential principles to understanding the bone remodeling cycle:

Answer 24

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

Question 25

Describe the remodeling process of trabecular bone:

Answer 25

* 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

Question 26

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

Answer 26

* _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**

Question 27

# Define the concept of bone modeling: What is it mediated by? What is an example of bone modeling?

Answer 27

* 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

Question 28

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

Answer 28

**strain** * fractional change in length

Question 29

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

Answer 29

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

Question 30

**Describe the process of fracture healing:**

Answer 30

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

Question 31

How long does it take for a fracture to heal?

Answer 31

Time required is _~8-12 weeks_, though the final remodeling and mineralization may take considerably longer

Question 32

# Define strain: What causes **negative strain**? What causes **positive strain**?

Answer 32

**Strain:** * _fractional change in the length_ of a structure as a _result of a force being applied to it_ 1. _Pure compression_ causes **negative strain** 2. _Pure stretching_ causes **positive strain**

Question 33

What does mechanical loading result in?

Answer 33

mechanical loading results in a mixture of compression and tension, as occurs in the case of either bending or shear

Question 34

Where does the compressive and tensile strength come from in bone?

Answer 34

1. _Compressive strength_ comes from the **mineral phase** * 2/3 of the bone ECM by mass 2. _Tensile strength_ comes from the **protein and water** * 1/3 of the bone ECM by mass

Question 35

What is the primary protein component of bone’s extracellular matrix?

Answer 35

**type 1 collagen**

Question 36

After collagen assembles there are spaces left in between the fibrils. What is a possible explanation of why the spaces are left?

Answer 36

The pattern of assembly leaves empty spaces that are thought to function as **nucleation sites for the mineral phase**

Question 37

Describe the process of cross-linking between collagen fibrils:

Answer 37

* _Adjacent molecules of collagen are covalently linked by cross-links_, initially formed enzymatically in a _vitamin C-dependent process to yield aliphatic bonds_ * Cross-links subsequently _mature non-enzymatically to aromatic pyridinolines and deoxypyridinolines_

Question 38

Why are _mature aromatic cross-links harder to break_ compared to the aliphatic cross-links?

Answer 38

_mature aromatic cross-links_ have **higher bond energy** than the _aliphatic cross-links_, and are therefore harder to break

Question 39

What allows individual collagen molecules to be stretched?

Answer 39

* The helical structure of the individual collagen molecules allows them to be stretched along the axis like springs * Provides yet another mechanism by which collagen imparts _tensile strength_ to bone matrix

Question 40

What are the **most abundant ions** present in bone mineral? What other ions are also present?

Answer 40

* **Ca and PO₄ are the most abundant ions** in bone mineral * Significant amounts of Mg, Na, OH, CO₃, SO₄, and Cl are also present

Question 41

Bone is a mineral that is described as **"apatite"**. What is **"apatite"**? How is it formed?

Answer 41

* Apatite is a mineral that **results from substitution of hydroxyapatite with other ions** * This substitution of the mineral _decreases the regular structure of the mineral lattice_ * reduces the energy needed to solubilize ion * This occurs at some cost to the _stiffness_ (resistance to bending) _of the mineral_ * _Important in promoting bone’s function in mineral homeostasis_

Question 42

Is the mineral phase of bone a passive or active process? What are the inhibitors and modulators of bone mineralization?

Answer 42

* Mineral phase of bone is intercalated into the collagen fibril structure, and appears to be deposited passively * _Both sulfate and pyrophosphate are potent inhibitors of mineralization_ * _Proteins in the SIBLING_ (small integrin-binding ligand, N-linked glycoprotein) _family are modulators of mineralization_ * account for most of the non-collagenous protein secreted by osteoblasts

Question 43

What provides toughness to bone?

Answer 43

**lamellar structure of bone**

Question 44

**Osteogenesis imperfecta:** * **Cause:** * **Cells affected:**

Answer 44

* Cause: * _mutations in the genes encoding type 1 collagen_ or critical enzymes in the _assembly and processing of type 1 collagen_ * results in deficient production or improper assembly of the extracellular matrix * _susceptibility to fracture_ * **Cells affected:** * **purely osteoblast disease**

Question 45

**Osteomalacia:** * **Cause:**

Answer 45

**"Rickets"** * inadequate mineralization of the bone matrix * can be a consequence of several different conditions: * including various malabsorptive disorders * vitamin D deficiency or resistance * phosphate wasting disorders * low Ca diet

Question 46

What can **osteomalacia** result in?

Answer 46

* Osteomalacia results in **weak, undermineralized bones** * When osteomalacia occurs in a growing individual: * **bone modeling is abnormal** * long bones assume a bowed shape * This is called rickets. * A disorder in which the **bone cells function normally**, but **lack of the appropriate mineral substrates** leads to abnormal bone function

Question 47

**Sclerosteosis and Van Buchem’s disease**

Answer 47

* disorders in which the skeletal mass is abnormally high, due to **mutations of the protein sclerostin** * disturbs the mechanosensory system, resulting in _bones perceiving that they are being loaded even when they are not_ * These can be thought of as **osteocyte diseases** _Clincial Manifestations:_ * Nerve entrapment leads to facial palsy, deafness * Some mutations also cause syndactyly

Question 48

**Osteopetrosis**

Answer 48

* group of diseases in which **osteoclast function or maturation is impaired** * As a result, **remodeling is deficient** * Severe cases are lethal, as the marrow space is not formed and hematopoiesis is severely impaired as a result * **Inability to remodel bone results in:** * _dense and massive bones that are weak_ * _unable to assume the lamellar structure_ that remodeling produces * **Pure osteoclast disease**

Question 49

**Paget’s disease of bone**

Answer 49

* Disorder characterized by **focally excessive and disorganized bone remodeling** * Can result in _pain, nerve entrapment, weakening of the affected areas_ * _presence of woven (as opposed to lamellar) bone_ * bone deformity arising as the result of _aberrant modeling_ * **Remodeling regulation disease**

Question 50

1. pure osteoblast disease ⇒ 2. pure osteoclast disease ⇒ 3. remodeling regulation disease ⇒ 4. osteocyte disease ⇒ 5. lack of the appropriate mineral substrates leading to abnormal bone function ⇒

Answer 50

1. pure osteoblast disease ⇒ **osteogenesis imperfecta** 2. pure osteoclast disease ⇒ **o****steopetrosis** 3. remodeling regulation disease ⇒ **Paget’s disease of bone** 4. osteocyte disease ⇒ **Sclerosteosis or Van Buchem’s disease** 5. lack of the appropriate mineral substrates leading to abnormal bone function ⇒ **osteomalacia**