Bone Development

Question 1

What is meant by ossification?

Answer 1

The process by which bone is formed

Question 2

What are the 2 types of ossification, and what do they depend on?

Answer 2

Intramembranous ossification - flat bones

Endochondrial ossification - long bones

Question 3

What is the first step in intramembranous ossification?

Answer 3

Mesenchymal cells in the centre of the bone cluster and differentiate into osteoblasts, forming ossification centres.

Question 4

What type of bone is formed by intramembranous ossification?

Answer 4

Flat bones - ribs, scapula, skull and pelvis

Question 5

Following the formation of ossification centres, how are trabeculae formed during intramembranous ossification?

Answer 5

Osteoid is secreted by osteoblasts. Trapped osteoblasts become osteocytes. The osteoid becomes calcified and forms needle-like structures of spongy bone called spicules. Spicules aggregate to form beam-like structures called trabeculae.

Question 6

What is ‘woven-bone’?

Answer 6

Woven bone is a mechanically weak, spongy bone which is formed from a random interconnected network of thickend trebaculae

Question 7

How and when is a periosteum formed in intramembranous ossification?

Answer 7

After trebaculae and woven bone formation, blood vessels on the outside of the spongy bone condense to form the periosteum, a fibrous sheath that covers bone.

Question 8

How is lamellar bone different from woven-bone?

Answer 8

Lamellar bone forms after woven-bone,surrounding it. It is compact and contains condensed osteoid and regular, parallel sheets of collagen making it much stronger than spongy bone.

Question 9

What are the 2 types of bone and how do they differ?

Answer 9

Spongy/cancellous bone forms an irregular network of fine bony columns, spaces are filled with bone marrow.

Cortical bone- forms external services, regular parallel sheets of collagen = strong.

Question 10

What are haversian and volkmann’s canals?

Answer 10

Haversians run veritally in the centre of the osteoid, volkmann’s canals run horizontally between haversian canals.
They carry blood vessels, lymph vessels and nerves.

Question 11

How does the arrangement of osteocytes differ in immature and mature bone of compact bone?

Answer 11

Mature bone has osteocytes arranged in concentric lamellae of osteons.
Immature bone has osteocytes which are fairly randomly arranged.

Question 12

Which plane do resorption canals run?

Answer 12

In parallel with the osteons long axis

Question 13

What is the function of cytoplasmic processes on the osteocytes?

Answer 13

Reach out to adjacent osteocytes, connecting via canaliculi to allow passage of nutrients between cells.

Question 14

How does the structure of spongy bone differ to compact bone?

Answer 14

The internal structure of trabeculae is similar to compact bone, as osteocytes lie between lamellae. No osteons, No haversian or volkmann’s canals.

Question 15

Outline the process of bone remodelling.

Answer 15

Cutting cut resorbs bone through the action of osteoclasts (release H+ ions and lysosomal enzymes) and synthesis new bone by osteoblasts.

Question 16

What is the composition of bone?

Answer 16

65% Mineral
23% Collagen
10% Water
2% Non- collagen proteins

Question 17

What do chondrocytes secrete?

Answer 17

Collagen and elastin to give strength and flexibility

Question 18

What is bone marrow the site for?

Answer 18

Haematopoiesis

Question 19

Outline the process that leads to osteoclast bone resorption.

Answer 19

Osteoclasts anchor to the bone surface and create a micro-environment underneath cell (‘sealed zone’) where it creates an acidic environment that dissolves bone mineral content. Enzymes released remove the remaining collagen.

Question 20

What factors influence the process of bone remodelling ?

Answer 20

Nutrition and physical activity - strengthen bone

Glucocorticoids - promote osteoclast activity

Question 21

Outline the processes of fracture repair.

Answer 21

1. Hematoma formation
2. Fibrocartilaginous callus formation (soft)
3. Bony callus formation (hard)
4. Bone remodelling

Question 22

Define osteoporosis.

Answer 22

Decrease in bone mass, making bones more fragile and susceptible to fracture.

Question 23

Define osteomalacia.

Answer 23

Inadequate mineralisation of bone due to vitamin D deficiency. Mineral bone loss.

Question 24

What is osteoporosis with aging characterised by?

Answer 24

Outer surfaces of trabeculae are regularly remodelled by osteoclasts and osteoblasts. Osteoporosis associated with aging results from incomplete filling of osteoclast resorption bays.

Question 25

What are the 2 types of osteoporosis?

Answer 25

Type 1- Post menopausal women. Increased osteoclast activity as a result of oestrogen withdrawal.
Type 2- In elderly of both sexes due to attenuated osteoblast function.

Question 26

What are risk factors for osteoporosis?

Answer 26

genetic
Insufficient calcium intake
Lack of exercise
Smoking has been linked

Question 27

Describe the process of endochondral ossification.

Answer 27

1. Periosteum differentiates into osteoblasts which form the BONY COLLAR on the diaphysis.
2. The central cavity calcifies
3. Periosteal bud invades the cavity, supplying osteogenic cells which begin to form spongy bone. = primary ossification centre
4. Secondary ossification centres form at epiphysis and begin to make spongy bone after birth.

Question 28

Outline what happens in step 1 of bone fracture repair.

Answer 28

1. Hematoma

Blood clot forms and bone cells on fracture edge die

Question 29

What is the second step of fracture repair?

Answer 29

Fibrocartilaginous callus is formed.
Fibroblasts secrete collagen, which span the break.
Angiogenesis

Question 30

What is the third step of fracture repair?

Answer 30

Bony callus formation.

Trabeculae develop as the fibrocartilaginous callus is converted to a hard bony callus of cancellous bone.

Question 31

What time of bone formation is involved in the formation of a bony callus?

Answer 31

Both endochondral and intramembranous

Question 32

What is the final step of fracture repair?

Answer 32

Bone remodelling

The callus of spongy bone is remodelled into compact bone.

Question 33

Name the 5 zones in epiphyseal plate growth.

Answer 33

1. Zone of reserve
2. Zone of proliferation
3. Zone of hypertrophy
4. Zone of calcification
5. Zone of resorption

Question 34

What happens in the zone of reserve?

Answer 34

Chondrocytes resting, no proliferation or matrix production

Question 35

What happens in the zone of proliferation?

Answer 35

Chondrocytes divide, forming columns and secreting matrix

Question 36

What happens in the zone of hypertrophy?

Answer 36

Chondrocytes grow in size, compressing the matrix into linear bands between the columns

Question 37

What happens in the zone of calcification?

Answer 37

Matrix becomes calcified and chondrocytes degenerate

Question 38

What happens in the zone of resorption?

Answer 38

The calcified matrix is in direct contact with the marrow cavity.
Small blood vessels and connective tissue invade the region occupied by dying chondrocytes, leaving calcified cartilage spicules. Bone is laid down on these spicules.