L7 - Regeneration

Question 1

Which cells live as long as the organism?

Answer 1

Neurones

Heart muscle

Question 2

Which cells are replaced continuously by stem cells?

Answer 2

Blood

Epithelia

Question 3

Which cells are replaced when tissue is damaged?

Answer 3

Skeletal muscle

Question 4

What is regeneration?

Answer 4

Possibility of fully developed organism to replace organs by growth or repatterning

• Not linked to complexity of the organism
• Seen in flatworms and hydra

Question 5

What are the two different types of regeneration?

Answer 5

Morphallaxis – repatterning without growth

Epimorphosis – regeneration by regrowth

Question 6

What sort of regeneration do hydra show?

Answer 6

Morphallaxis
Simple organism with only two germ layers ectoderm and endoderm
Hydra grows continuously, therefore cells have to change their positional values or die
- Occurs at the tip of the tentacles, at the basal disc and by asexual budding
Repatterning also occurs during reproduction

Question 7

What are the two gradients created during head regeneration in hydra?

Answer 7

Gradient in positional value which determines
- Head inducing ability
- Resistance to head inhibitor
Gradient in head inhibitor
When the head is removed it is reformed by respecifying existing cells

Question 8

What experiment was used to show head inhibitor in hydra?

Answer 8

Shown by transplanting a piece of region 1 tissue to an intact hydra
Head inhibitor from existing head will prevent graft from forming a second head
If the donor head is removed the source of head inhibitor is lost and the graft can form a second axis

Question 9

What experiment was used to show the effect of positional value/head inducing capacity in hydra?

Answer 9

If the head of a hydra is removed and after 6 hours region 1 is transplanted into the hosts body it can induce a head - it has gained a stronger head inducing capacity
Region 5 cannot do this - need to wait 30 hours before this region has a similar head inducing capacity

Question 10

What happens if the head of hydra is removed?

Answer 10

If the head is removed the source of inhibitor is lost and will result in an increase in positional values

• The leftover tissue with the highest positional value will reach the head-value first
• It will start producing inhibitor restablishing the original situation only in a smaller body

Question 11

How are Wnt and BetaCatenin signalling involved in hydra positional values?

Answer 11

Wnt signaling involved in head formation
- Wnt expressed in hydra head and in regenerating tip
Inhibition of GSK3β leads to upregulation of nuclear βcatenin – activating Wnt pathway
- If done in hydra or Xenopus all regions of the body acquire characteristics of head organizer

Question 12

What are Urodele amphibians and what can they regenerate?

Answer 12

Urodele (tailed) amphibians can regenerate their dorsal crest, limbs, retina, lens, jaw and tail

Question 13

Where does regeneration occur in urodele amphibians?

Answer 13

Regeneration occurs from a level that is appropriate to where the cut was made

• A distal amputation only regenerates distal structures
• A proximal amputation also regenerates proximal structures

Question 14

What kind of regeneration is seen in the newt?

Answer 14

Epimorphic

Question 15

Urodele limb regeneration method

Answer 15

1. After amputation epithelial cells migrate over wound surface
   - Regeneration is dependent on this
   - Generation/regeneration have a different scale – morphogens involved need to work over a 10x larger range
2. Cells below the epithelium dedifferentiate forming a blastema
   - Derived from the dermis, cartilage and muscle
   - Multinucleate muscle cells can revert to mononucleate cells under the influence of thrombin
   - Dedifferentiation of muscle cells involves
   
   • Expression of msx - homeobox transcription factor
   • Inactivation of Rb gene - inhibits proliferation of muscle in normal circumstances

Question 16

What is the exception to the very little transdifferentiation seen in urodele limb regeneration?

Answer 16

Dermis –> cartilage

Question 17

What are the rules of regeneration?

Answer 17

Limb regeneration is always distal to the wound according to positional value at site of the cut
- Not just replacing missing parts

Question 18

What is the evidence that limb regeneration is always distal to the wound?

Answer 18

If the distal limb of a newt is amputated and the stump is inserted into the flank and the limb is cut again, the anterior limb will regenerate the missing distal parts
The remaining stump will also regenerate a distal limb

Question 19

What is the role of the wound blastema?

Answer 19

Reads the local positional value and generates more distal positional values
Has autonomy - once formed it can regenerate the structures lost in a different location
- If a distal blastema is transplanted to a proximal wound – entire limb will form

Question 20

What is the cellular basis of positional value?

Answer 20

Proximal and distal blastema cells may sort via differential adhesion
If a distal is combined with a proximal blastema the proximal blastema will engulf the distal blastema
- A sign to the distal cells to stick more tightly to one another
Distally transplanted cells do not mix with proximal blastema cells that will form as a result of the intercalary regeneration

Question 21

How is positional information encoded in the blastema?

Answer 21

Retinoic acid can reset a positional value to a more proximal value via

• Rarδ2
• Meis homeobox genes
• Upregulation of prod1 - expressed at the highest level in the proximal regenerating blastema

Question 22

What is regeneration dependent on?

Answer 22

In a normal limb nerve innervation is required for regeneration to occur
- Except if the limb had no nerve in the first place

Question 23

What is the newt anterior gradient?

Answer 23

A protein that can replace the nerve in supporting outgrowth, binds prod1
Expressed in nerve sheath in response to wounding
Innervation leads to downregulation of epidermal nAG

Question 24

What does a neurogenic limb express?

Answer 24

Persistent epidermal nAG expression

Question 25

What is the mechanism of insect regeneration?

Answer 25

Sensing discontinuities in positional values

Missing positional values are then filled in irrespective of the overall structure

Question 26

In insects segments of leg contain?

Answer 26

Similar positional values

Graft of a mid tibia to mid femur will not cause regeneration of intermediate structures

Question 27

What can mammals regenerate?

Answer 27

Young children/mice can regenerate tips of their digits

Question 28

Can regeneration occur in the PNS?

Answer 28

In the PNS axons can be regenerated
If the neuron itself is lost no regeneration occurs
Regeneration occurs along the original pathway of the axon - stimulated by Schwann cells

Question 29

Can regeneration occur in the CNS?

Answer 29

In the CNS very little regeneration
Non permissive environment created by astrocytes and oligodendrocytes
Implanted Schwann cells in the CNS can promote axon growth
The CNS could be disrupted easily if new axonal could form without difficulty

Question 30

What other mammalian organs can be regenerated?

Answer 30

Liver and ribs

As long as the periosteum (membrane that surrounds the ribs) is intact

Question 31

Cardiac regeneration in mammals method?

Answer 31

Cardiomyocyte are present but don’t divide
Progenitors present but not used when tissue is damaged
Necrotic tissue that forms after infarction is replaced by scar tissue
Scar/maladaptive hypertrophy – may increase the risk of further infarction

Question 32

Ventricular regeneration in zebrafish method?

Answer 32

Not identical to embryonic heart development
- E.g. msxB and C expressed
Regeneration dependent on dedifferentiating muscle cells
Endocardium and epicardium are involved
Neuregulin a signal from the epicardium induces proliferation in myocardium

Question 33

Regeneration of the heart method

Answer 33

1. After resection a blood clot will form
2. The endocardium will be activated and start expressing certain genes
3. The epicardium will expand and start to cover the wound
4. Newly forming muscle expresses FGF
5. Epicardial cells respond to FGF by invading the regenerate and reforming blood vessels
6. The epicardium expresses Neuregulin, which can stimulate cardiomyocyte cell division
   - In uninjured zebrafish hearts Neureglin can drive cardiomyocyte proliferation and result in cardiomegaly

Question 34

Can neonatal mice regenerate their hearts?

Answer 34

Neonatal mice can regenerate their heart similarly to zebrafish but loose this ability soon after birth
- Linked to reduction in erbb2 expression which is required for Neu signal transduction

Question 35

How was the link between mice loosing their ability to regenerate their heart and a reduction in erbb2 expression tested?

Answer 35

A transgenic mouse with a doxyxcyline inducible- dominant active form of erbb2
This transgene was then induced for some time after an infarct was created in these mice
Able to induce cardiomyocyte proliferation and improve function