Cellular structure of bone

Question 1

What are the functions of bone?

Answer 1

• Support and movement:attachment site for muscles
• Protection for internal organs
• Provides home for bone marrow
• Act as a mineral reservoir
• Endocrine function as it is a source of some non-classical hormones

Question 2

What is the composition of bone?

Answer 2

• Protein:organic osteoid matrix(25%)
• Minerals(75%)
• Cells

Question 3

What type of collagen does the organic protein matrix have and what does that give?

Answer 3

Mainly type 1 collagen

-Gives both flexibility and tensile strength

Question 4

What is bone mineral mainly made up of?

Answer 4

Mainly hydroxyapatite

Question 5

What is bone minerals composed of?

Answer 5

Composed of both calcium and phosphate

Question 6

What qualities does bone mineral have and what does this give?

Answer 6

Its rigid and brittle:

-Gives high compressive strength

Question 7

What cells are present in the bone?

Answer 7

• Osteoblasts
• Osteoclasts
• Osteocytes

Question 8

What cells are present in the bone marrow?

Answer 8

• Mesenchymal stem cells

- Haematopoietic cells

Question 9

What are osteoblasts ?

Answer 9

They are bone forming cells

Question 10

What are osteoblasts derived from?

Answer 10

Derived from mesenchymal stem cells

Question 11

What do osteoblasts secrete?

Answer 11

Secrete osteoid collagen matrix of bone

Question 12

What do osteoblasts promote?

Answer 12

Promote mineralization of osteoid

Question 13

What are osteoclasts?

Answer 13

They are bone reabsorbing cells

Question 14

What are osteoclasts derived from?

Answer 14

Derived from haematopoietic stem cells

Question 15

Structure of osteoclasts?

Answer 15

Large and multinucleate

Question 16

What do osteoclasts secrete?

Answer 16

Secrete acid to dissolve bone mineral enzymes to digest organic matrix

Question 17

What is the life cycle of the osteoclasts controlled by?

Answer 17

Controlled by apoptosis

Question 18

Where do osteocytes come from?

Answer 18

Come from terminally differentiated osteoblasts

Question 19

What are osteocytes encased in?

Answer 19

Encased in bone mineral matrix

Question 20

What do osteocytes extend?

Answer 20

Extend multiple dendrites via minute canals in bone matrix

Question 21

What does the lacuno canalicular system maintain?

Answer 21

Maintains communication with bone surface and blood vessels

Question 22

How does bone remodelling occur in cortical bone?

Answer 22

• There is a longitudinal system of canals and spaces
• There will be a leading edge, where cells differentiate into osteoclasts and start to digest the bone
• Osteoblasts differentiate and lay down new bone here

Question 23

How does bone remodeling occur in trabecular bone?

Answer 23

• Osteoclasts will eat away at the bone and osteoblasts will form new bone
• The lining cells on the surface of the bone detach underneath it and form a basic multicellular unit

Question 24

What are the stages in bone remodelling?

Answer 24

1. Activation
   - Promotion of the differentiation of new osteoclasts
2. Resorption
   - Duration of the osteoclast activity
   - Removing old bone and creating the pits
3. Reversal
   - Osteoclast apoptosis, terminating osteoclast activity and overlapping with osteoblast differentiation
4. Formation
   - Osteoblasts line the surface, leading to . the formation of osteoid which eventually become mineralised

Question 25

What are the controls of remodelling?

Answer 25

-Load bearing exercise
-Cytokines and other local signals
-Endocrine
-Oestrogen inhibits osteocyte apoptosis and promotes
osteoclast apoptosis

Question 26

What is osteoporosis known as?

Answer 26

A loss of bone mass beyond a certain threshold

Question 27

What is RANK?

Answer 27

Surface receptors on pre-osteoclasts, stimulates osteoclast differentiation

Question 28

What is RANK LIGAND?

Answer 28

Produced by pre-osteoblasts and osteocytes

Question 29

What is OPG?

Answer 29

Is a decoy receptor produced by osteocytes and bind to RANK-L, preventing action of RANK

Question 30

What is the Wnt signalling pathway required for?

Answer 30

Required for osteoblast differentiation

Question 31

What is Wnt signalling pathway negatively regulated by?

Answer 31

Negatively regulated by DKK and sclerostin

Question 32

What are the steps in the Wnt signalling pathway?

Answer 32

1.Wnt protein has G coupled receptors called Frizzled
2.Frizzled requires a co-receptor LRP5/6 which both need to interact in order to activate Wnt signalling pathway
3.Beta catenin is inactivated until Wnt binds which causes beta-catenin to be free
-This then accumulates and then translocates to the
nucleus where it initiates transcription

Question 33

What is osteoporosis pseudoglioma caused by?

Answer 33

By the inactivation of LRP5 with no co-receptor

Question 34

What is sclerosteosis and van buchem disease caused by?

Answer 34

By the mutation of the SOST gene, inactivating sclerostin protein

Question 35

What is osteoporosis caused by?

Answer 35

By a mutation which inactivates RANK-L protein

Question 36

What does osteoporosis cause?

Answer 36

Causes:

• Loss of bone density
• Increased fracture risk
• Increase in bone resorption over formation