TOB Session 7

Question 1

What is Endochondral Ossification?

Answer 1

When bone forms from cartilage. Cartilage is reabsorbed and replaced by bone.

Question 2

Where does Endochondral Ossification begin?

Answer 2

Endochondral ossification begins at the diaphysis in the primary centre of the bone, and then the secondary centres form at each end (epiphysis).

Question 3

How does diameter of the bone increase?

Answer 3

Bone is deposited at the periphery of the shaft.

Question 4

How does length of the bone increase?

Answer 4

It grows from the growth (epiphyseal) plates at the end of the bone.

Question 5

What is intramembranous ossification?

Answer 5

ossification from a sheet of mesenchyme or loose connective tissue. Bone development starts in a highly vascularised connective tissue.

Question 6

Into what do mesenchymal cells differentiate?

Answer 6

Osteoblasts. They are surrounded by collagen fibres and ground substance.

Question 7

What do osteoblasts secrete?

Answer 7

Uncalcified matrix (osteoid) which later becomes calcified)

Question 8

Where do osteocytes come from?

Answer 8

They are formed from osteoblasts.

Question 9

Where does compact bone come from?

Answer 9

It is formed from spongy bone.

Question 10

What is the name of the process by which bone structure is constantly changing?

Answer 10

Bone remodelling

Question 11

Describe the symptoms and causes of Osteogenesis Imperfecta.

Answer 11

The bones are very fragile and brittle and so fractures are very common. Deformity of joints and bones is also prevalent. Multiple fractures are usually present at or before birth. It is caused by a mutation in the genes coding for type I collagen (no glycine).

Question 12

Describe Osteoporosis.

Answer 12

The bones become thinner and more porous as the levels of sex hormones begin to fall with age. Bone density is reduced and so fractures become more common. Bones reabsorption exceeds bone formation.

Question 13

What are the risk factors of Osteoporosis?

Answer 13

Genetic predisposition, Iinsufficient calcium intake, insufficient calcium absorption and vitamin D, immobilization of bone (lack of exercise or prolonged bed rest) and cigarette smoking.

Question 14

What is Achondroplasia and what is it caused by?

Answer 14

Short limb Dwarfism. It is caused by an autosomal dominant point mutation in the fibroblast growth factor receptor-3 gene (FGFR3). This causes decreased endochondral ossification amongst other things.

Question 15

What are the characteristics of achondroplasia?

Answer 15

The limbs are very short, the torso is of normal length and the vault of the skull is enlarged with a small face. Intelligence is entirely normal.

Question 16

Why is Vitamin D important in normal bone development?

Answer 16

Vitamin D undergoes hydroxylation in the liver to form 1,25 dihydroxyvitamin D3. This increases calcium absorption by the small bowel and promotes mineralisation of bone.

Question 17

What is Rickets?

Answer 17

It is a childhood disease in which the bones do not harden due to a Vitamin D deficiency. Bones become solft and malformed as there is insufficient calcium deposition for adequate bone rigidity. The bone matrix does not calcify properly and the epiphyseal plate becomes distorted.

Question 18

What is osteomalacia?

Answer 18

It is the adult version of rickets, caused by a significant calcium deficiency. Symptoms include bone pain, back ache and muscle weakness. The deficiency in calcification of recently formed bone causes fragility and increases risk of fractures.

Question 19

What can Osteogenesis Imperfecta often be confused with?

Answer 19

Child abuse

Question 20

How do dietary and behavioural factors influence rickets and osteomalacia?

Answer 20

Osteomalacia and Rickets can both be a consequence of inadequate calcium or vitamin D intake in the diet. Low sun exposure can also cause vitamin D deficiency.

Question 21

Describe how osteoblast and osteoclast activity differs in a patient with osteoporosis compared to a healthy person.

Answer 21

In someone with osteoporosis, osteoclast activity exceeds osteoblast activity and so bone becomes fragile.

Question 22

How do the medullary canals change in someone with osteoporosis?

Answer 22

They become enlarged and gaps develop in the lamellae.

Question 23

Explain the importance of osteoporosis as a risk factor for fractures in the elderly

Answer 23

Elderly people with osteoporosis are more likely to suffer fractures from falling down.

Question 24

How do abnormal levels of Growth Hormone (GH) affect bone development?

Answer 24

Lack of growht hormone affects the epiphyseal cartilage causing pituitary dwarfism. Excessive GH can cause gigantism through promotion of epiphyseal growth plate activity in children.

Question 25

Why can excessive amounts of growth hormone not cause gigantism in adults? What happens instead?

Answer 25

Because there are no longer epiphyseal growth plates. Excessive GH in adults can cause acromegaly due to periosteal growth.

Question 26

How do sex hormones affect bone development?

Answer 26

Sex hormones influence the time of appearance and development of ossification centres. Precocious sexual maturity stunts growth due to premature fusion of epiphyses. Deficiencies of sex hormones can lead to delayed fusion of epiphyses causing excessive growth.

Question 27

What does Thyroid hormone deficiency cause (relating to bone development)?

Answer 27

Cretinism (stunted physical and mental development).

Question 28

How can decreased thyroid hormone production in a newborn be reversed (to prevent cretinism)?

Answer 28

By administration of thyroxine