S12) Bone

Question 1

What is an osteoblast?

Answer 1

An osteoblast is a cell which secretes the substance of bone

Question 2

What is endochondral ossification?

Answer 2

Endochondral ossification is the process in which most of the bones of the body develop involving the replacement of a pre-existing hyaline cartilage template by bone

Question 3

Describe the 6 steps involved in long bone development by endochondral ossification

Answer 3

⇒ Initial cartilage model

⇒ Collar of periosteal bone appears in shaft

⇒ Central cartilage calcifies, nutrient artery penetrate, primary ossification centre forms

⇒ Medulla becomes cancellous bone, cartilage forms epiphyseal growth plates, epiphyses develop secondary ossification centres

⇒ Epiphyses ossify and growth plates move apart, lengthening bone

⇒ Epiphyseal growth plates replace by bone, hyaline cartilage persists

Question 4

What is intramembranous ossification?

Answer 4

Intramembranous ossification is the embryonic development of flat bones from an embryonic tissue called the mesenchyme

Question 5

Where does intramembranous ossification occur?

Answer 5

It takes place within condensations of mesenchymal tissue and not by replacement of a pre-existing hyaline cartilage template

Question 6

Flat bones develop by intramembranous ossification.

Provide some examples

Answer 6

• Skull
• Clavicle
• Scapula
• Pelvic bones

Question 7

Describe the 7 steps involved in flat bone development by intramembranous ossification

Answer 7

⇒ Small cluster of mesenchymal stem cells form a nidus

⇒ MSCs become osteoprogenitor cells

⇒ Osteoprogenitor cells become osteoblasts and lay down an extracellular matrix of Type I collagen (now, an osteoid)

⇒ Osteoid mineralises to form rudimentary bone tissue spicules

⇒ Spicules join to form trabeculae

⇒ Trabeculae merge to form woven bone

⇒ Woven bone is replaced by mature compact bone

Question 8

What is an osteocyte?

Answer 8

An osteocyte is a bone cell formed when an osteoblast becomes embedded in the material it has secreted

Question 9

Describe the structure of osteocytes

Answer 9

• The osteocytes have very slender cytoplasmic processes, which reach out to adjacent osteocytes, via canaliculi
• These processes connect via gap junctions so nutrients can be passed between osteocytes

Question 10

Which form of ossification increases the length of the bone?

Answer 10

Endochondral ossification (interstitial growth)

Question 11

Which form of ossification increases the girth of the bone?

Answer 11

Intramembranous ossification (appositional growth)

Question 12

What is a synovial joint?

Answer 12

A synovial joint is a moveable joint containing an articular capsule with synovial fluid & membrane wherein juxtaposed ends are covered by hyaline/fibrocartilage

Question 13

Identify the different zones seen in a LS through an epiphyseal growth plate?

Answer 13

• Zone of reserve cartilage
• Zone of proliferation
• Zone of hypertrophy
• Zone of calcified cartilage
• Zone of resorption

Question 14

What happens in the zone of reserve cartilage?

Answer 14

• No cellular proliferation
• No active matrix production

Question 15

What happens the zone of proliferation?

Answer 15

• Cells actively dividing to form columns
• Cells enlarge and secrete matrix

Question 16

What happens in the zone of hypertrophy?

Answer 16

• Cells enlarge greatly
• Matrix compresses into linear bands between cell columns

Question 17

What happens in the zone of calcified cartilage?

Answer 17

• Enlarged cells begin to degenerate
• Matrix calcifies

Question 18

What happens in the zone of resorption?

Answer 18

• Calcified matrix is in direct contact with the marrow cavity
• Small blood vessels and connective tissue invade
• Calcified cartilage is left as spicules between them

Question 19

Describe the structure of cancellous bone

Answer 19

• The spaces are filled with bone marrow
• Osteocytes lie between lamellae
• No Haversian or Volkmann’s canals

Question 20

Describe the functions of cancellous bone and compact bone

Answer 20

• Cancellous bone forms a network of fine bony columns/plates to combine strength with lightness
• Compact bone forms the external surfaces of bones and comprises approx. 80% of the body’s skeletal mass

Question 21

What do Haversian and Volkmann’s canals do?

Answer 21

Haversian and Volkmann’s canals carry blood vessels, lymph vessels and nerves

Question 22

Distinguish between immature and mature bone

Answer 22

• Immature bone has osteocytes which are randomly arranged
• Mature bone has osteocytes arranged in the concentric lamellae of osteons

Question 23

Describe the composition of bone

Answer 23

Question 24

What is an osteoclast and what does it do?

Answer 24

An osteoclast is large multinucleate bone cell which remodels bone by releasing H⁺ and lysosomal enzymes

Question 25

Describe the 4 steps involved in fracture repair

Answer 25

⇒ Haematoma formation ⇒ Fibrocartilaginous callus formation ⇒ Bony callus formation ⇒ Bone remodeling

Question 26

Describe haematoma formation in fracture repair

Answer 26

A blood clot (haematoma) is formed from which granulation tissue arises

Question 27

Describe fibrocartilaginous callus formation in fracture repair

Answer 27

The procallus of granulation tissue is replaced by a fibrocartilaginous callus in which bony trabeculae are developing

Question 28

Describe bony callus formation in fracture repair

Answer 28

Endochondral and intramembranous ossification give rise to a bony callus of spongy/canccellous bone

Question 29

Describe bone remodeling in fracture repair

Answer 29

Cancellous bone is replaced by compact cortical bone until remodelling is complete

Question 30

In terms of bone banks, define the following terms: - Autograft - Homograft - Heterograft

Answer 30

- **Autograft:** donor is the recipient (most successful) **- Homograft**: donor is a different human (may be rejected as foreign) **- Heterograft**: donor is of a different species (least successful)

Question 31

What is osteoporosis?

Answer 31

**Osteoporosis** is a metabolic bone disease in which mineralised bone is decreased in mass to the point that it no longer provides adequate mechanical support

Question 32

Differentiate between Type 1 and 2 osteoporosis

Answer 32

**- Type 1 Osteoporosis** (more common)**:** I. Occurs in postmenopausal women II. Due to increased in osteoclast number (oestrogen withdrawal) - **Type 2 Osteoporosis** (senile osteoporosis): I. Occurs in elderly persons of both sexes II. Due to decreased osteoblast function

Question 33

What are 5 risk factors of osteoporosis?

Answer 33

- Genetics - Insufficient calcium intake - Exercise - Smoking - Insufficient Vitamin D

Question 34

What is Rheumatoid Arthritis?

Answer 34

**- RA** is a long-lasting autoimmune disorder that primarily affects the joints - It results in warm, swollen, painful joints as well as pain and stiffness which often worsen following rest

Question 35

What is Perthe's disease?

Answer 35

**- Perthes'** **disease** is a rare childhood condition that affects the hip occurring when the blood supply to the head of the femur is temporarily disrupted - Without adequate blood supply, bone cells die (avascular necrosis)

Question 36

Should a fractured bone be totally immobilised?

Answer 36

- No, some movement stimulates the healing of the fracture - If immobilisation lasts too long, the joint may become stiff and muscles may shorten or shrink

Question 37

What is the difference between osteoporosis and osteomalacia?

Answer 37

- **Osteoporosis**: decreased bone mass with a normal ratio of mineral to matrix, bones are porous and brittle - **Osteomalacia**: decreased ratio of mineral to matrix, bones are soft

Question 38

What is achondroplasia?

Answer 38

- **Achondroplasia** is a bone growth disorder that causes disproportionate dwarfism - It occurs as a sporadic mutation most commonly, but can be inherited as an autosomal dominant disorder