Muscoskeletal 2: Bone growth

Question 1

What is the precursor, location and function of Osteoclast

Answer 1

Osteoclasts are from a separate lineage: they come from the fusion of monocyte progenitor cells.

Location: At sites where bone reabsorption is occuring

Function: Secretes acid to dissolve the mineral and enzymes to dissolve the organic components of bone

Question 2

How does the osteoclast control the destructive enzymes

Answer 2

It has a clear zone close to its ruffled border which collects the enzymes that it secretes and forces it to be endocytosed so it can be neutralised. The enzymes are also only active in an acid environment.

Question 3

In what layers are the osteogenic cells in inactive bone and what does inactive bone not have ?

Answer 3

1. Layer under blood vessels in the periosteum, on top of the osteocytes
2. Under the mineralised bone, in the endosteum, on top of the medullary cavity where there is bone, marrow and bv.

Bones don’t have osteoblasts if inactive

Question 4

What does bone remodelling mean

Answer 4

Ongoing replacement of old bone tissue, involving bone reabsorption (removal of minerals and collagen fibres) by osteoclasts and appositional growth (addition of minerals and collagen fibres) by osteoblasts on existing bone.

Question 5

Describe the process of appositional growth 3 steps from active to inactive.

Answer 5

1. Osteogenic cells divide, forming osteoblasts which deposit osteoid
2. Some osteoblasts become trapped in the lacunae where they will eventually become osteocytes
3. When growth stops, osteoblasts can convert back to osteogenic cells or die. The osteoid is fully calcified

Question 6

Describe the process of bone reabsorption 3 steps from active to inactive.

Answer 6

1. Monocyte precursor cells leave the BV and start to fuse on the bone surface
2. Osteoclasts form and start dissolving bone, releasing the endocytosed minerals to interstitial fluid-> bv
3. Osteoclasts eventually die and reabsorption stops
4. Blood vessels grow into the new space. (resting)

Question 7

Why does the bone grown appositional instead of interstitial

Answer 7

The tissue bone is too rigid for interstitial growth which involves the cells dividing inside the tissue, excreting more ECM and growing the tissue from within. It can only grow by having more stuff added on top.

Question 8

Are the appositional growth and bone reabsorption always happening at the same time. Where can they occur

Answer 8

No, they occur throughout the skeleton independently of each other. They can occur in the endosteum or the periosteum

Question 9

How do long bones grow in length and why does it do that

Answer 9

Endochondral ossification= There is a plate of hyaline cartilage that can grow by interstitial growth underneath the epiphysis which is the epiphyseal plate. This cartilage grows, dies then is replaced by bone.
It does it this way because it can’t lay bone down on the articulate cartilage on the ends.

Question 10

What is the two types of mature lamellar bone and what characterises this bone subset

Answer 10

Mature lamellar bone is characterised by many layers of bone, with collagen fibres put down in the same direction within one layer but alternating out of phase between layers. This enables bone to withstand forces from different directions.
The two types are spongey and compact bone

Question 11

Describe the structure of spongey/cancellous/trabecular bone and where you find it usually

Answer 11

Made of lamellae arranged in irregular thin columns called trabeculae. The spaces in between are filled with bone marrow (called medullary cavity). Usually found in the interior of a bone, always covered by compact bone for protection.

Question 12

What are the main different characteristics of spongey bone that is different to compact bone

Answer 12

1. spongey bone is light which allows the skeleton to move more readily
2. spongey bone supports red bone marrow so is the site of haemotpoeisis
3. Spongey bone is the site where bone remodelling is greater because the greater SA means that its easier for osteoclasts to settle on trabeculae to + and - Ca and P

Question 13

What is the maximum width of a trabeculae in the narrowest axis and why

Answer 13

0.4 mm, as the osteocytes in the middle of the trabeculae receive their nutrients via diffusion from blood vessels in the medullary space so increasing distance too much means they die.

Question 14

Describe the structure and organisation of compact/cortical bone and where you find it usually

Answer 14

Made of repeating osteons/haversian systems. This consists of concentric lamellae arranged around a central canal containing blood vessels and nerves. There are alternating arrangements of collagen fibres between lamellae.

Question 15

Compare how spongey bone and compact bone resist stresses- and therefore perform function

Answer 15

Spongey bone isn’t for heavy stresses but its trabeculae are orientated to resist stress from multiple directions and help to support outer cortex of compact bone.

Whereas Compact bone is for heavier stresses, resisting bending because osteons are aligned in the same direction and parallel to the length of the diaphysis. It also thickens in area exposed to large forces.

Question 16

Whats the difference between circumferential lamellae and interstitial lamellae in compact bone

Answer 16

Interstitial lamellae is areas between neighbouring osteons that contain lamellae with osteocytes and canaliculi. They are fragements of older osteons destroyed during rebuilding.
Whereas Circumferential lamellae is developed during initial bone formation and line the inner and outer boundaries of the compact bone.

Question 17

What lines the central canal

Answer 17

endosteum

Question 18

Describe 4 steps how a primary osteon forms from appositional growth

Answer 18

1. Osteoblasts in the active periosteum either side of the blood vessel put own new bone forming ridges
2. Bone continues ridges come together and fuse forming a tunnel around the blood vessel. The tunnel is now lined with endosteum.
3. The osteoblasts in the endosteum build concentric lamellae onto the walls of the tunnel. The tunnel fills inward toward centre
4. The bone continues to grow outwards as the osteoblasts in the periosteum build new circumferential lamellae.
   Process repeats as new ridges fold over blood vessels

Question 19

Where and what purpose of primary osteon formation happen compared to secondary osteon formation

Answer 19

Primary osteon tunnel formation happens around existing blood vessels in the active periosteum, which happens when bone is growing. Whereas secondary osteons the tunnels are created inside existing bone to help repair/ increase vasculature.

Question 20

What are the 5 steps of forming a secondary osteon

Answer 20

1. Osteoclasts form and gather in an area that needs to be remodelled. They start boring its way through the existing bone.
2. Osteoblasts move in behind the cutting cone and line tunnel wall as new endosteum and start depositing osteoid.
3. Osteoid calcifies to form new lamella and a blood vessel will grow into the tunnel to supply the cells.
4. Osteoblasts deposit layer upon layer of new concentric lamellae on the wall of the tunnel to fill it in. Some osteoblasts get trapped and become osteocytes
5. When the tunnel is reduced to size of normal Haversian canal, remaining osteoblasts die or become osteogenic cells part of the resting endosteum.

Question 21

What is the cement line

Answer 21

This is the junction between the closests to outer surface lamella of the new osteon and the pre existing older bone. It is filled with GAGs which are glue between old and new bone.

Question 22

What is the closing cone

Answer 22

The active area of osteoblasts depositing new layers of concentric lamellae behind the cutting cone front of osteoclasts.

Question 23

Compare blood supply of spongy and compact bone and therefore nutrient flow

Answer 23

Spongy bone has blood vessels in medullary cavity so nutrient flow is outside to in but in Compact bone, blood vessels are within Haversian and Volkmanns canals. so flow is from inside to out.

Question 24

Compare the units of spongy bone and compact bone and their direction of growth

Answer 24

Spongey bone units are trabeculae that grow outwards. Whereas compact bone unit is haversian system/ osteon and grows inwards.