Lecture 18 - Bone Function & Repair

Question 1

What are the mechanical, synthetic and metabolic functions of bone?

Answer 1

Mechanical = protect important organs, provide overall framework for human body and form basis of levers involved in movement.

Synthetic = Haemopoiesis (holds and protects red bone marrow)

Metabolic = mineral storage (Ca2+ and PO4), fat storage, acid-base homeostasis

Question 2

What are the 2 types of ossification and what types of growth do they display?

Answer 2

1) Endochondral ossification - formation of long bones from a cartilage template, continued lengthening by ossification at epiphyseal growth plates (appositional growth) - growth at edges
2) Intra-membranous ossification - formation of bone from clusters of mesenchymal stem cells in the centre of bone (interstitial growth) - growth from the middle out

Question 3

What are the main differences between endochondral and intra-membranous ossification?

Answer 3

Intramembranous takes place within condensation of mesenchymal tissue, endochrondral by replacement of hyaline cartilage template.

Intramembranous ossification contributes to bone thickening of long bones, at their periosteal surfaces (appositional growth).

Question 4

What is the difference between immature and mature bone?

Answer 4

• Immature bone has osteocytes in random arrangements
• Mature bone has osteocytes arranged in lamellae of osteons.
• Resorption canals in mature bone run parallel with osteons’ long axis.

Question 5

Which one of compact or cancellous bone contains Haversian’s and Volkmann’s canals?
What do these structures do?

Answer 5

• compact bone HAVE them
• cancellous/spongy bone do NOT HAVE them
• They carry blood vessels, lymph vessels and nerves

Question 6

Why does bone have great tensile and compressive strength?

Answer 6

Lamallae within cortical bone are able to slip relative to each other to resist fracture - although excessive load can still cause fractures.

Question 7

What are the main factors affecting bone stability?

Answer 7

• Mostly activity of osteoblasts (bone deposition) and osteoclasts (bone resorption), however osteocytes have some ocsteoblast/clast activity too.
• Other important factors include nutrition - Vit D, Vit C and Vik K and B12.

Question 8

Describe the 4 stages of bone repair after a fracture occurs.

Answer 8

1) Blood clot (haematoma) is formed in which granulation tissue arises, eventually removed by macrophage
2) Fibrocartilaginous callus is formed, first a procallus of granulation tissue is replaced by a fibrocalliganous callus in which bony trabeculae are developing.
3) Bonus callus formation - endochondral and intramemranous ossification give rise to a bony callus of spongy/cancellous bone.
4) Cancellous bone replaced by compact cortical bone until remodelling is complete

Question 9

What is the genetic basis of Osteogenesis imperfecta?
What are the clinical features?
Who does it mainly affect?

Answer 9

• Mutation in COL1A (collagen type 1) gene, leading to incorrect production of collagen 1 fibres.
• Weak bones, increased risk of fracture, shortened stature, blue sclera
• Neonates + children

Question 10

What is rickets caused by?

What are the clinical symptoms?

Answer 10

• Vit D deficiency, leading to poor Ca2+ mobilisation and ineffective mineralisation
• Leads to soft bones, shortened heigh, bow legs, pain while walking

NB: Rickets in adults is called Osteomalacia.

Question 11

What are the 2 main divisions of osteoporosis?

What is each one due to?

Answer 11

1) Primary - can be type 1, which occurs in postmenopausal women, due to increased osteoclast number (as there is a loss of oestrogen which keeps osteoclast number under control) or type 2, which occurs in the elderly, due to loss of osteoblast function (due to loss of oestrogen/androgen)
2) Secondary - result of drug therapy (corticosteroids), is a major risk factor for bone fracture in elderly.

Question 12

What are 3 modifiable risk factors for osteoperosis?

Answer 12

1) Insufficient Ca2+ intake
2) Exercise - immobilisation of bone leads to accelerated bone loss, physical activity is required
3) Cigarette smoking

Question 13

What is Achondroplasia caused by?
What does it consequently affect?
What does it result in?

Answer 13

• Inherited mutation in the FGF3 receptor gene
• FGF promotes collagen formation from cartilage, therefore endochondral ossification affected (but not intramembranous).
• Results in short stature, but normal sized head and torso (long bones cannot lengthen properly, e.g.: femur!)