BL - Bone And Intermembranous Ossification

Question 1

What is intramembranous ossification?

Answer 1

Development of bones from condensations of mesenchymal tissue (not involving cartilage). Most flat bones develop this way.

Question 2

Give some examples of flat bones.

Answer 2

Skull, clavicle, scapula, pelvic bones.

Question 3

What is a nidus?

Answer 3

A small cluster of mesenchymal stem cells formed during the first stage of intramembranous ossification.

Question 4

Describe the stages of intramembranous ossification. (Hint - nidus -> osteoprogenitor -> osteoblasts -> spicules -> trabeculae -> lamellae)

Answer 4

Mesenchymal cells form a nidus, MSCs become osteoprogenitor cells (more Golgi and RER), these become osteoblasts which lay down extracellular matrix with type I collagen (osteoid), osteoid mineralises to bone spicules, these join to form trabeculae, these merge to form woven bone, finally replaced by lamellae.

Question 5

What is an osteoblast called when it is surrounded by osteoid?

Answer 5

An osteocyte

Question 6

What makes osteoclasts easy to identify on images?

Answer 6

They have multiple nuclei.

Question 7

What is cancellous/spongy bone?

Answer 7

A network of fine bony columns, with the spaces filled by bone marrow. It combines strength with lightness.

Question 8

What is compact/cortical bone?

Answer 8

Dense bone that forms external surfaces and comprises most of the skeletal mass.

Question 9

What is the difference between osteocyte arrangement in mature and immature bone?

Answer 9

Immature bone - osteocytes randomly arranged. Mature bone - osteocytes arranged in concentric lamellae of osteons.

Question 10

What do the Haversian and Volkmann’s canals carry?

Answer 10

Blood vessels, lymph vessels and nerves.

Question 11

How can Haversian and Volkmann’s canals be differentiated on images?

Answer 11

Volkmann’s canals have no concentric lamellae surrounding them, while Haversian canals do.

Question 12

How are nutrients passed between osteocytes?

Answer 12

They have very slender cytoplasmic processes which reach out to adjacent osteocytes via canaliculi.

Question 13

What is the trabeculae of cancellous bone?

Answer 13

The network of spiny bone processes that make up the framework of spongy bone. No Haversian or Volkmann’s canals here.

Question 14

What do osteoclasts release when part of a cutting cone?

Answer 14

H+ ions and lysosomal enzymes, which bore through the bone.

Question 15

What is the composition of bone?

Answer 15

65% mineral, 23% collagen, 10% water, 2% non-collagen proteins

Question 16

What are the lamellae though to be able to do to help prevent fracture?

Answer 16

Slip over each other.

Question 17

How does bone resist fracture?

Answer 17

It has great tensile and compressive strength and a degree of flexibility.

Question 18

What are the four stages of fracture repair?

Answer 18

Haematoma formation, fibrocartilaginous callus formation, bony callus formation, bone remodelling.

Question 19

What happens during haematoma formation?

Answer 19

A mass of clotted blood forms due to broken blood vessels, bone cells at fracture edge die. Swelling/inflammation, phagocytic cells and osteoclasts start to remove dead tissue. Macrophages remove blood clot.

Question 20

Which stage of fracture repair is this? New blood vessels infiltrate haematoma, procallus forms, collagen fibres and hyaline cartilage splint the bone. Osteoblasts begin to form spongy bone at the fracture site.

Answer 20

Fibrocartilaginous callus formation.

Question 21

Describe the stages of bony callus formation.

Answer 21

Trabeculae develop as fibrocartilaginous callus is converted to a bony callus of cancellous bone. Endochondral ossification replaces all cartilage with cancellous bone.

Question 22

How long does bone remodelling (the final stage of fracture repair) take place for?

Answer 22

Several months

Question 23

Why is the final, remodelled bone the same shape as the bone before the fracture?

Answer 23

It responds to the same set of mechanical stressors.

Question 24

Tim breaks his leg (fool) and loses a chunk of bone, meaning bony union and callus formation is not possible. His brother donates a chunk of bone to replace it. What is this called?

Answer 24

Homograft.

Question 25

What is osteoporosis?

Answer 25

A metabolic bone disease when mineralised bone is decreased in mass to the point that it no longer provides sufficient mechanical support.

Question 26

Osteoporosis associated with aging results from incomplete filling of what?

Answer 26

Osteoclast resorption bays.

Question 27

At what age does bone mass peak?

Answer 27

25-35 years.

Question 28

Describe some risk factors for osteoporosis.

Answer 28

Genetic factors, insufficient calcium intake, insufficient calcium and vitamin D absorption, less exercise, cigarette smoking.

Question 29

Which type of osteoporosis is due to an increase in osteoclast number as a result of oestrogen withdrawal?

Answer 29

Type 1

Question 30

Which type of osteoporosis is due to attenuated osteoblast function?

Answer 30

Type 2