Bone

Question 1

How do long bones develop?

Answer 1

Endochondral ossification

Question 2

How do flat bones develop and what do they develop from?

Answer 2

Intramembranous ossification, directly from mesenchymal tissue

Question 3

Where does intramembranous ossification take place?

Answer 3

Within condensations of mesenchymal tissue - NOT by replacement of a pre-existing hyaline cartilage template

Question 4

What are some examples of flat bones?

Answer 4

Skull, clavicle, scapula, pelvic bones

Question 5

Does intramembranous ossification contribute to lengthening of long bones?

Answer 5

No, it contributes to thickening of long bones at periosteal surfaces (appositional growth)

Question 6

Outline the stages of intramembranous ossification

Answer 6

A small cluster of mesenchymal stem cells (MSCs) form a tight cluster of cells (a nidus).

The MSCs become osteoprogenitor cells (each developing more Golgi apparatus and rough endoplasmic reticulum).

The osteoprogenitor cells become osteoblasts and lay down an extracellular matrix containing Type I collagen (osteoid).

The osteoid mineralises to form rudimentary bone tissue spicules, which are surrounded by osteoblasts, and contain osteocytes.

The spicules join to form trabeculae, which merge to form woven bone, which is finally replaced by the lamellae of mature compact bone.

Question 7

What do spicules connect to form?

Answer 7

Trabeculae

Question 8

Name 3 types of cell which remodel bone

Answer 8

Osteoblasts, osteocytes, osteoclasts

Question 9

What is the main unction of osteoblasts

Answer 9

To deposit new bone

Question 10

What is the main function of osteoclasts?

Answer 10

To resorb bone (and they hence lie in a depression)

Question 11

What are the 2 types of bone?

Answer 11

Compact (cortical) and Cancellous (trabecular)

Question 12

Where is compact (cortical) bone found?

Answer 12

Compact bone forms the external surfaces of bones and comprises ca. 80% of the body’s skeletal mass.

Question 13

Describe cancellous (trabecular) bone

Answer 13

Cancellous bone forms a network of fine bony columns or plates to combine strength with lightness. The spaces are filled by bone marrow.

Question 14

What do Haversian and Volkmann’s canals contain?

Answer 14

Blood vessels, lymph vessels, and nerves

Question 15

What is an osteon

Answer 15

The fundamental functional unit of much of compact bone. Roughly cylindrical structures.

Question 16

How do the canals lie in relation to the osteon?

Answer 16

Haversian - through the osteon (parallel)

Volkmann - across osteon (perpendicularly)

Question 17

What are osteocytes?

Answer 17

Osteoblasts which have become embedded in the material they have secreted

Question 18

How are osteocytes arranged?

Answer 18

Immature bone - randomly arranged

Mature bone - concentric lamellae of osteon s

Question 19

Where are resorption canals found?

Answer 19

Parallel with the osteons long axis

Question 20

How are nutrients passed between osteocytes?

Answer 20

The osteocytes have very slender cytoplasmic processes, which reach out to those of adjacent osteocytes, via canaliculi. These processes connect via gap juctions such that nutrients can be passed between osteocytes.

Question 21

Describe the structure of cancellous (trabecular) bone

Answer 21

The internal histological structure of the trabeculae is similar to that of compact bone, with the presence of osteocytes lying between lamellae. There are however no Haversian or Volkmann’s canals.

Question 22

Which cells are found in cancellous (trabecular) bone and where are they found?

Answer 22

Each trabeculum consists of numerous osteocytes embedded within irregular lamellae of bone. Osteoblast and osteoclasts on their surfaces act to remodel them.

Question 23

What lies in the lacunae between trabeculae

Answer 23

Adipose and haemopoietic cells

Question 24

Briefly describe how bone is resorbed

Answer 24

A cutting cone is boring a tunnel through the bone by the action of osteoclasts, which release H+ ions and lysosomal enzymes.

Question 25

How does bone resist fracture?

Answer 25

Bone resists fracture because it has great tensile and compressive strength, but also because it has a degree of flexibility.
The lamellae are thought to be able to slip, relative to each other, before excessive load causes fracture.

Question 26

What is a haematology and how can it form?

Answer 26

A solid swelling of clotted blood within tissues
When bone breaks there is bleeding from multiple broken blood vessels, resulting in a haematoma between the broken bone ends.

Question 27

Give a brief overview of fracture repair

Answer 27

1- A blood clot (haematoma) is formed in which granulation tissue arises.

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

3- Endochondral and intramembranous ossification give rise to a bony callus of spongy/cancellous bone

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

Question 28

Describe haematology formation including the types of cells involved

Answer 28

Blood vessels in bone and periosteum break.
A mass of clotted blood (haematoma) forms.
Bone cells at the fracture edge die (no blood supply).
Swelling and inflammation occur.
Phagocytic cells and osteoclasts begin to remove dead and damaged tissue.
Macrophages will eventually remove the blood clot.

Question 29

Describe fibrocartilaginous callus formation

Answer 29

New blood vessels infiltrate the fracture haematoma. A procallus (soft callus) of granulation tissue (i.e. tissue rich in capillaries and fibroblasts) develops.
Fibroblasts produce collagen fibres that span the break. Others differentiate into chondroblasts that give rise to a sleeve of hyaline cartilage. An externally bulging, fibrocartilaginous matrix thus splints the broken bone.
Concurrently, and more centrally, osteoblasts from the nearby periosteum and endosteum, (and multipotent cells from the bone marrow) invade the fracture site and begin bone reconstruction by forming spongy/ trabecular bone.

Question 30

Describe bony callus formation

Answer 30

As a result, within a week, new bone trabeculae begin to appear in the fibrocartilaginous callus.
The trabeculae develop as the former fibrocartilaginous callus is converted to a hard (bony) callus of cancellous bone.
Ultimately endochondral ossification replaces all cartilage with cancellous bone, but intramembranous ossification also produces new cancellous bone in the area.
These processes begin as soon as two days after fracture in young people.
Bony callus formation continues for about two months until a very firm union is formed.

Question 31

Describe bone remodelling

Answer 31

As soon as it is formed the callus of spongy, cancellous bone begins to be remodelled into compact bone, especially in the cortical region (i.e. in the region of the former bone shaft walls).
This process continues for several months.
The material bulging from the outside of the bone, and inwards, into the medullary cavity, is removed by osteoclasts.
The final shape of the remodelled area is the same as that of the original unbroken bone because it responds to the same set of mechanical stressors.

Question 32

Why are bone banks necessary?

Answer 32

If a fracture involves loss of bone fragments, then bony union and callus formation is not possible.
Since the 1970s, bone banks have become available to supply viable bone for grafting purposes. Bone fragments are frozen and used by orthopaedic surgeons.

Question 33

Define autograft, homograft and heterograft

Answer 33

If a fracture involves loss of bone fragments, then bony union and callus formation is not possible.
Since the 1970s, bone banks have become available to supply viable bone for grafting purposes. Bone fragments are frozen and used by orthopaedic surgeons.

Question 34

What is osteoporosis?

Answer 34

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

Question 35

What is the main characteristic of osteoporosis?

Answer 35

Depletion of bone mass always characterises the disease. Loss of mass within the trabecular bone is particularly relevant to increased susceptibility to fracture.

Question 36

What does osteoporosis reflect?

Answer 36

Osteoporosis always reflects enhanced bone resorption relative to formation

Question 37

The outer surfaces of the bony trabeculae of cancellous bone are regularly remodelled by osteoclast resorption and osteoblast deposition.
How is this different in osteoporosis?

Answer 37

Osteoporosis associated with aging results from incomplete filling of osteoclast resorption bays.

Question 38

At what stage of life does bone mass peak?

Answer 38

Bone mass peaks between 25 – 35 years.

It begins to decline in the 5th or 6th decade.

Question 39

What is the cause of type 1 primary osteoporosis and who is affected?

Answer 39

Type 1 occurs in postmenopausal women.

Type 1 is due to an increase in osteoclast number, a result of oestrogen withdrawal.

Question 40

What is the cause of type 2 primary osteoporosis and who is affected?

Answer 40

Type 2 occurs in elderly persons of both sexes (senile osteoporosis).
Type 2 generally occurs after age 70 and reflects attenuated osteoblast function.

Question 41

Name some risk factors of osteoporosis

Answer 41

Genetic: Peak bone mass is higher in blacks than in whites or asians

Insufficient calcium intake: recommended value for postmenopausal women
is 800 mg/day.

Exercise: immobilization of bone (prolonged bed rest or application of a cast) leads to accelerated bone loss. Physical activity is needed to maintain bone mass. The weightlessness experienced by astronauts can result in osteoporosis.

Cigarette smoking in women has been correlated with increased incidence of osteoporosis.