Lecture 8- Patterning of the Nervous System: Anterior-Posterior Patterning

Question 1

What is the progression of development in the first 24 hours in a chick embryo?

Answer 1

• 15-16h: Formation of neural plate- full streak stage embryo, the anterior and posterior is beggining to be set up, Hensen’s node is visible
• 19-22hr: Bending of neural plate, formation of underlying notochord- the anterior part is set up and the streak has started regressing (mostly due to the anterior expanding), neural furrow is formed and the neural folds begin to get close to one another
• 23-24hr: Closure of neural tube- somites start to appear, the embryo anterior-posterior pattern is clearly visible, hinge points developed in the closing neural tube
• the anterior-posterior patterning and specification occurs before neural tube closure

Question 2

What are the lines pointing to?

Answer 2

-this is a chick embryo

Question 3

What are the lines pointing to?

Answer 3

-chick embryo structures

Question 4

What are the primary brain structures in the 3 vesicle stage and what do they develop into?

Answer 4

1. Forebrain- into telencephalon and diencephalon
2. Midbrain- into Mesencephalon
3. Hindbrain- into Metencephalon and Myelencephalon

Question 5

What are the brain structures in the 5 vesicle stage and what do they develop into in the adult brain?

Answer 5

1. Telencephalon (also called cortex): into olfactory lobes, hippocampus and cerebrum
2. Diencephalon: into optic vesicle, epithalamus, thalamus and hypothalamus
3. Mesencephalon: into midbrain
4. Metencephalon: into cerebellum and pons
5. Myelencephalon: into medulla

Question 6

What is the region in pink, green, blue and yellow?

Answer 6

-embryo in the 3 vesicle stage

Question 7

What is the region in pink, green, blue and yellow?

Answer 7

-embryo in the 5 vesicle stage

Question 8

What principle does this picture represent?

Answer 8

• the underlying principle of how are cells affected to become one cell type or another (specification)
• organizer is a group of cells, these can function within themselves, and they secrete a morphogen or a growth factor, organizers can be derived from endoderm
• morphogen has a gradient effect, the specific effect is mediated via transcription factors to determine cell fate

Question 9

When does specification of the forebrain occur? (anterior neural plate)

Answer 9

-specification of the forebrain occurs first, during earliest stages of neural induction that is established at gastrulation

(specification of the anterior forebrain= anterior neural plate)

Question 10

What does the anterior forebrain become?

Answer 10

-Telencephalon and anterior half of Diencephalon

Question 11

What are the transcription factors involved in anterior neural plate specification? (5)

Answer 11

1. Sox2
2. Otx1
3. Otx2
4. Pax6
5. Six3

Question 12

What are the two primary organizers involved in specification of the anterior neural plate?

Answer 12

1. Anterior visceral endoderm (AVE=mouse) (called hypoblast in a chick)
2. Gastrula organiser (GO)
   - these play a role even before the neural plate is set up

Question 13

What does the anterior visceral endoderm (AVE) secrete? (3)

Answer 13

• FGF8
• Cereberus (Wnt and BMP inhibitor)
• Lefty (Nodal inhibitor)

Question 14

What is the anterior visceral endoderm important for?

Answer 14

• prepares the ectoderm region to be responsive to signals released from EPO
• later plays important role in maintaining anterior neural plate
• inhibits primitive streak formation in the anterior region of the embryo

Question 15

What is the gastrula organizer called at different stages of development?

Answer 15

• pre-node (gives rise to the Node)
• early-gastrula organizer (EGO) during early primitive streak stage
• mid-gastrula organizer (MGO) during mid-primitive streak stage
• secretes chordin and noggin

Question 16

What is the prechordal plate?

Answer 16

• derived from primitive streak and GO (gastrula organizer)
• underlies anterior neural plate
• secretes Wnt inhibitors to maintain the anterior neural plate identity

Question 17

What is the hypoblast’s role in formation of anterior neural plate in a chick embryo? (picture)

Answer 17

-

Question 18

What does chick gastrulation look like?

Answer 18

-inhibition of Wnt and BMP signals is the reason why the anterior neural plate is set up

Question 19

What does the chick neurulation look like and what do the primitive streak, node and prechordal plate secrete to make it happen?

Answer 19

• primitive streak: secretes FGF, RA (retinoic acid), Wnt and BMP
• node: secretes Noggin (BMP inhibotor) + Chordin
• prechordal plate: secretes Cerberus (Wnt and BMP inhibitor)

Question 20

What does the mouse embryo look like from day 6.0 to 8.5, how does neuralation progress?

Answer 20

• yellow= anterior visceral endoderm (or hypoblast in chicks)
• orange= primitive streak
• red= gastrula organiser/node/notochord
• darkest blue= forebrain
• middle blue= midbrain/hindbrain
• lightest blue= spinal cord

Question 21

What does the diagram representing mouse neuralation: formation of anterior neural plate look like?

Answer 21

• yellow= anterior visceral endoderm (or hypoblast in chicks)
• orange= primitive streak
• red= gastrula organiser/node/notochord
• blue= forebrain

Question 22

What is the summary of anterior neural plate induction?

Answer 22

1. AVE (hypoblast) secretes factors to induce pre-neural plate
2. Gastrula organiser (GO) secretes BMP inhibitors to induce neural plate indentity
3. Antreior neural plate region is maintained by Wnt inhibitors secreted by AVE and underlying prechordal plate (primitive streak derivative)
4. Expression of Oxt1/2, Pax 6 and SIx3 transcription factors specifies anterior neural plate

Question 23

When does formation of caudal neural plate occur?

Answer 23

-from mid to late primitive streak stage

Question 24

What is the mesodermal formation that is crucial in caudal neural plate formation?

Answer 24

• mesoderm arising from primitive streak now forms underlying notochordal plate (different to prechordal plate)
• precursor of the notochord

Question 25

What is the notochordal formation position at the beginning of caudal neural tube formation?

Answer 25

- notochord formation, arising from node, extends anteriorly to the border of the prechordal plate, under the neural plate midline - important for dorsal-ventral patterning

Question 26

What are the posteriorizing signals and where are they released from?

Answer 26

- posteriorizing signals: BMPs, FGF, Wnts and Retinoic acid (RA) - released from primitive streak and posterior mesoderm

Question 27

What signals are involved in the formation of caudal neural plate?

Answer 27

- node secretes BMP inhibitors (noggin, chordin) to induce neural plate - anterior neural plate supresses caudalizing agents because of AVE and prechordal plate - caudal neural plate is not exposed to Wnt antagonists and is further displaced from AVE, it therefore responds to Wnt, FGF and RA signals to become caudal/posterior (mid-diacephalon, midbrain, hindbrain and spinal cord)

Question 28

What happens at later primitive streak stage? (caudal neural plate formation)

Answer 28

- the streak is regressing as the anterior region is expanding - still have cells migrating into the node, becoming mesoderm= they become lateral mesoderm (later form muscle etc.) - lateral mesoderm unlike the prechordal plate does not release Cerebrus - so don't have Cerebrus but have the node (so have Wnt that is uninhibited)

Question 29

What are the anterior/posterior gradients in the developing embryo? (earliear stage)

Answer 29

- balance of factors released from the anterior part (prechordal plate and AVE) and posterior part (node and primitive streak) - anterior mainly= Cerebrus and Dickkopf (inhibitor of Wnt) - posterior mainly= Wnt, FGFs, RA, Noggin - physical distance is important, posterior part is far from AVE and thus can respond to Wnts,FGF and RA

Question 30

What are anterior/posterior gradients in a developing embryo and what does each part become? (picture)

Answer 30

-

Question 31

What does inhibition of Dkk(Dickkopf) and/or Noggin in an embryo result in?

Answer 31

- results in head defects - if you delete both dickkopf and noggin the embryo has no head - rest of the neural tube is fine - happens even when only one of Dkk or Noggin is knocked out, the effects are slightly less severe then

Question 32

What are the transcription factors specifying anterior-posterior axis of neural tube?

Answer 32

- Otx2 expression determines forebrain and midbrain - Gbx2 expression determines hindbrain and spinal cord

Question 33

What happens to Otx2 knockout mouse?

Answer 33

-no formation of forebrain and midbrain as Otx2 expression determines these regions

Question 34

What is special about the Hox genes?

Answer 34

- Hox (Homeobox) gene family of transcription factors - Hox gene clusters are aligned on the chromosome - anterior most gene is on 3' - posterior most gene is on 5' - highly conserved across species - where the Hox gene on the chromosome that is where it is in the animal - mapped out on the chromosome

Question 35

What are the secondary organizers of the anterior-posterior axis? (3)

Answer 35

1. Anterior Neural Ridge 2. Zona limitans intrathalamica 3. Isthmic Organizer

Question 36

What are the characteristics of the anterior neural ridge?

Answer 36

- patterns the anterior forebrain (Telencephalon), sets up the most anterior part of the brain - corresponds to region that was AVE - secretes FGF8 (morphogen) (picture: red=Fgf8, ANR= anterior neural ridge)

Question 37

What are the characteristics of Zona limitans intrathalamica?

Answer 37

- divides the middle of diencephalon (P2 and P3) (divides the anterior and posterior neural plate) - corresponds to boundary betweenn prechordal plate and notochord - secretes sonic hedgehog (morphogen) (picture: ZLI-zona limitans intrathalamica, blue= sonig hedgehog)

Question 38

What are the characteristics of the Isthmic organizer?

Answer 38

- divides midbrain and hindbrain - secretes: Wnt1= morphogen (midbrain, maintains Otx2 expression), and FGF8=morphogen (hindbrain, maintains Gbx2 expression) (picture: IsO= Isthmic Organizer, green= Wnt1, red=FGF8)

Question 39

What are the regions of the brain separated like? (picture)

Answer 39

- forebrain: P1-P6 - midbrain: SC and IC - hindbrain: r1-r8

Question 40

What does transplantation of Isthmic Organizer into developing forebrain regions result in?

Answer 40

-results in development in ectopic midbrain/hindbrain tissue in the forebrain region - when you do this is important, can influence cell fate in the early phases of morphgenesis - can only do it in profilitative state

Question 41

What is this?

Answer 41

- shows a summary of all the involved organizers, morphogens and transcription factors - look through this carefully, remember we did not talk about all of these, only look at the ones we mentioned in class - can see the Gbx2 (light green) expression in the hindbrain and spinal region - Otx2 expression (the light blue) in the midbrain and forebrain - Pax6(yellow), Six3(more orange beige), Otx2 in the forebrain, expression of which determines the anterior neural plate