Skeletal System 2

Question 1

When does Intramembranous Ossification begin?

Answer 1

• Begins during embryonic development
• Bone develops from fibrous tissue called mesenchyme

Question 2

What are the steps involved in Intramembranous Ossification?

Answer 2

1. Mesenchymal cells differentiate into osteogenic cells
2. Osteogenic cells become osteoblasts and gather to become the ossification center
3. Osteoblasts secrete unmineralized material called osteoid which allows for collagen deposition and mineral (calcium and phosphate) deposit, allowing the tissue to harden
4. Osteoblasts trapped by the hardened matrix become osteocytes

Question 3

What are the steps involved in Intramembranous Ossification after Osteoblasts become osteocytes?

Answer 3

5. As osteoblasts transform into osteocytes, osteogenic cells in the surrounding connective tissue differentiate into new osteoblasts at the edges of the growing bone
6. Areas where osteoid is secreted around capillaries
   becomes spongy bone
7. Compact bone develops superficial to spongy bone

Question 4

Intramembranous Ossification video

Answer 4

https://www.youtube.com/watch?v=MZGRiUdg0RA

Question 5

What are the initial steps in Endochondral Ossification?

Answer 5

a) Mesenchymal cells differentiate into chondrocytes that produce a cartilage template for bone

b) Blood vessels on the edge of the cartilage model bring osteoblasts that deposit a bony layer

c) Capillaries penetrate cartilage and deposit bone inside cartilage model, forming primary ossification cente

Question 6

What are the final steps in Endochondral Ossification?

Answer 6

d) Cartilage and chondrocytes continue to grow at ends of the bone while medullary cavity expands and remodels

e) Secondary ossification centers develop after birth

f) Hyaline cartilage remains at epiphyseal (growth) plate and
at joint surface as articular cartilage until the end of puberty

Question 7

Why do Bones grow in Length?

Answer 7

Question 8

Endochondral Ossification

Answer 8

https://www.youtube.com/watch?v=eBeyApWuGEI

Question 9

What does it mean when we say bones are dynamic?

Answer 9

Bones are a dynamic tissue that continue to remodel even after they are done growing

Question 10

When do Bones Remodel?

Answer 10

Bones remodel when:

• [Ca 2+ ] in the blood changes
• There is mechanical stress applied to the bones
  (i.e., pull from the muscles and gravity)

Question 11

Why do bones remodel after sensing mechanical stress?

Answer 11

1. Exercise increases mechanical stress on bones
2. Mechanical forces are converted to biological signals (primarily by osteocytes) that signal bone remodeling
3. Bending causes both compressiveand tensile stress cause fluid movement

Question 12

How do bones remodel after sensing mechanical stress?

Answer 12

Fluid flows through the lacunocanalicular system and around osteocytes

• Aids in the transport of nutrients and waste
• Exerts a shear stress that may stimulate an osteogenic response and new bone formatio

Question 13

What are Bone Fractures?

4

Answer 13

1. Blood vessels rupture when the bone breaks, forming a hematoma
2. Fibrocartilage callus (made of bone, cartilage, bone matrix and collagen fibres) fills the gap where the bone was broken
3. Fibrocartilage callus is replaced with spongy bone
4. Bony callus is remodeled in response to mechanical stresses, a patch is formed over the break

Question 14

If an individual is bedridden or physically inactive, what effect do you anticpate this to have on their bone health?

Answer 14

no mechanical stress on the bone will cause the bone to detoriate.

Answer 15

bone has more blood supply than ligaments and tendons so they heal better/quicker

Question 16

Would you recommend physical activity to someone who is healing from a fractured bone? Assume the risk of reinjury to the bone has passed?

Answer 16

Yes, to an extent because since there is no risk of injury, mechanical stress is required to continue bone remodelling.

Question 17

What is the Bone’s Role as a Calcium Reservoir?

Answer 17

• Bone stores over 99% of the body’s calcium, phosphate and carbonate
• Calcium plays a crucial role in normal neurological function and muscular activity (as we know!)

Question 18

What happens when Plasma [Ca2+] is too high?

Answer 18

• Hormones will be released to stimulate osteoblast activity
• Increase bone formation
• Ca2+ from the plasma stored in bone
• become stronger

Question 19

What happens when Plasma [Ca2+] is too low?

Answer 19

• Hormones will be released to stimulate osteoclast activity
• Increased bone resorption
• Ca2+ is released from the bone into the plasma

Question 20

DNTK

What are Hormones that favour Bone Formation and Increased Bone Mass?

Answer 20

Question 21

DNTK

What are Hormones that favour Increased Bone Resoption and Decreased Bone Mass?

Answer 21

Question 22

What is Paget’s Disease?

Answer 22

Homeostatic Imbalance - Paget’s Disease

• Osteoclast activity > osteoblast activity
• Causes weak, brittle bones that are misshapen, and prone to fracture
• Treatment: drugs that decrease the activity of osteoclasts and surgery in some cases

Question 23

What are Rickets?

Answer 23

Homeostatic Imbalance – Rickets

• Disease in children where bones fail to calcify
• Bones soften as a result and weight-bearing bones become bowed
• Usually due to lack of Ca2+ or lack vitamin D (required to absorb Ca2+ into the bloodstream)

no as common in canada where lack of vitamin D is not an issue

Question 24

What is Bone Mineral Density?

Answer 24

• The relative value of bone mineral (Ca2+ and phosphate) per measured bone area (g/cm2 or mg/cm2)
• Higher bone mineral density = stronger bones
• Measured with specialized equip

Question 25

How is Bone Mineral Density Measured?

Answer 25

Duel-Energy X-Ray Absorptiometry (DEXA)
* Measures bone mineral density and body composition
* DEXA of a healthy, physically active, 37- year old female
* Areas with “brighter” white, indicate areas of greater bond mineral density

Question 26

What are Osteopenia and Osteoporosis?

Answer 26

Both conditions of reduced bone mineral density

• Can be diagnosed with DEXA
• Osteopenia – T-score between -1 and -2.5
• Osteoporosis – T-score < -2.5

Question 27

An individual has severe malnutrition. What effect might this have on bone mineral denisty? Why?

Answer 27

result in lower bone density due to lack of calcium. Bone will have to breakdown to release the calcium necessary for other processes