ESSAY discuss mechanisms of planarian regeneration

Question 1

What is regeneration?

Answer 1

the reactivation of development in post embryonic life to restoring missing tissue

Question 2

What are the 4 mechanisms of regeneration?

Answer 2

stem cell mediated regeneration
epimorphosis
morphallaxis
compensatory regeneration

Question 3

Define stem cell mediated regeneraton

Answer 3

a stem cell can give rise to more differentiated cellular progeny by asymmetric cell division (Self renewal)

regrowth of hair
replacement of red blood cells

Question 4

define epimorphosis

Answer 4

dedifferentiation of adult structures to form an undifferentiated mass of cells that become re-specified e.g. regeneration of salamander limbs

Question 5

define morphallaxis

Answer 5

regeneration occurs through repatterning of existing tissue with little new growth e.g. Hydra

Question 6

define compensatory regeneration

Answer 6

differentiated cells divide but maintain their differentiated functions
e.g. mammalian liver

Question 7

How do planarians regenerate?

Answer 7

Combination of mechanisms including neoblasts (Stem cells, epimorphosis, morphallaxis)

Question 8

how long does it take planarians to regenerate?

Answer 8

Planarians exhibit a unique regeneration capacity as any small fragment from almost anywhere can regenerate a new organism in approximately 17 days

Question 9

what happens after amputation?

Answer 9

neoblasts close to the wound proliferate, giving rise to the regenerative blastema, defined as the unpigmented tissues where the missing tissues will differentiate

Question 10

The process of tissue regeneration in the wound region from proliferating neoblasts was termed _____

Answer 10

epimorphosis

Question 11

A repatterning of the whole organism is required to produce a complete planarian, this process of remodelling old tissue was termed ______

Answer 11

morphallaxis

Question 12

Describe terminal regeneration

Answer 12

After transverse sectioning the anterior end will regenerate the missing head, whereas the posterior end will regenerate the missing tail. this indicates that the remaining tissue is polarised and knows what is missing

Question 13

Describe lateral regeneration

Answer 13

after longitudinal sectioning the old tissue regenerates the missing lateral half

Question 14

Describe intercalary regeneration

Answer 14

after joining two distal pieces produced by transverse sections, planarians intercalate the missing region. In that case, cells from each piece participate equally in the production of an intercalary blastema

Question 15

which 2 models can explain the mechanisms of regeneration in planarians?

Answer 15

positional information

polar co-ordinate

Question 16

positional information is established how?

Answer 16

by a gradient of a chemical substance known as a morphogen

Question 17

In the positional information how do cells know their positions?

Answer 17

by sensing concentration levels of the morphogen

Question 18

Cells would know their polarity how?

Answer 18

by sensing the direction of the gradient

Question 19

The positional information model divides organisms how?

Answer 19

into a number of regions, usually along the anterior posterior axis

Question 20

The regions in the positional information model represent what?

Answer 20

different threshold levels of the morphogen

Question 21

In the positional information model cells are thought to respond to the different threshold levels how?

Answer 21

by forming different structures at different thresholds

Question 22

When was the polar co-ordinate model proposed?

Answer 22

1976

Question 23

What does the polar co-ordinate model attempt to explain?

Answer 23

patterns of regeneration in three dimensions

Question 24

what does the polar co-ordinate model state?

Answer 24

that cells recognise their position in the whole because of positional information that each cell receives designating where the cell is in relation to the rest of the organism

Question 25

In the polar co-ordiante model how many pieces of information are required?

Answer 25

2
one that gives positional information along the anterior posterior axis and one that indicates position within the circumference

Question 26

How is the information in the polar co-ordinate model thought to be coded?

Answer 26

Not in diffusible morphogens but in the molecules found on the surfaces of cells

Question 27

how would you test the polar co-ordinate model?

Answer 27

remove a piece of the planarian

rotate the piece and then replace it

Question 28

What should occur if the polar co-ordinate model is correct when you remove a piece and rotate it?

Answer 28

If you rotate it such that the ventral side is now dorsal, you will have inverted the clockface. The cells of the graft now have circumferential positional values that dont match those of its graft site.

the model predicts that there should be regeneration along the edges of the graft, even though technically you haven’t removed any tissue from the animal. If the model is correct, you should get duplication of structures in the region of the graft

Question 29

After the head and tail of a planarian have been cut off what happens?

Answer 29

The anterior end of the remaining fragment regenerates a head, whereas the posterior end regenerates a tail

Question 30

what indicates morphogen activity?

Answer 30

in general, planarians amputated at any level along the AP axis can regenerate a head; however, the rate of head regeneration decreases posteriorly and this indicates morphogen activity

Question 31

Observations indicating morphogen activity led to what?

Answer 31

the so-called head-frequency curves and showed a time graded regeneration rate along the AP axis

Question 32

What indicated that some minimal AP distance in a region of tissue is required to specify polarity?

Answer 32

in a set of classical experiments, narrow bipolar regenerating pieces occasionally regenerated 2 headed planarians

Question 33

What has been shown recently about planarians and gradient activity?

Answer 33

Recently, a gradient of neoblast mitotic activity along the AP axis of intact planarians has been reported. However, a testable mechanistic framework of metabolic gradients in planarians remains to be developed

Question 34

what is the Wnt/beta catenin signalling pathway?

Answer 34

it is an evolutionarily conserved mechanism that confers polarity during embryonic development

Question 35

What leads to the disruption of the beta catenin degradation complex?

Answer 35

The binding of Wnts, the secreted elements of the pathway to the receptors frizzled and co receptors LRP

Question 36

the beta catenin degradation complex is formed of what?

Answer 36

composed by axin, GSK3, CKI, APC

Question 37

Afterwards beta catenin the key intracellular element of the pathway accumulates where?

Answer 37

in the cytoplasm, enters the nucleus, and activates TCF transcription factors, which regulate the expression of multiple genes

Question 38

Two beta catenins have been reported from the planarian Schmidtea mediterranea. Explain this further

Answer 38

the silencing of one of them, Smed-bcatenin1 leads to an extreme phenotype - radial like hypercephalised. they have many eyes all around the planarian body.

this transformation occurs gradually in trunk pieces regenerating both head and tail regions

Question 39

The severity of the phenotype depends not only on the time of regeneration in the RNAi experiment but also on what?

Answer 39

the level of Smed bcatenin1 inhibition as demonstrated after injection of varying amounts of dsRNA

Question 40

what does injecting beta catenin do?

Answer 40

injecting beta catenine double stranded RNA, leads to destruction of the internal beta catenine RNA, which lowers the activity of the gene

Question 41

with the lowest doses what occurs?

Answer 41

the phenotype observed is the inhibition of tail regeneration

Question 42

as the dose increases what happens?

Answer 42

you get to the lower levels of beta catenin that cause heads to be produced hence the many eyed planarian