Axes specification

Question 1

What is the essential difference between basics of axis specification between anamniotes and amniotes?

Answer 1

anamniotes: contribution of maternal determinants
amniotes: interplay between embryonic and extraembryonic tissues

Question 2

In xenopus: how does the organizer arise (what predetermines where the future dorsal site will form)?

Answer 2

1. oocyte polarization along animal-vegetal axis (localization of maternal determinants)
2. sperm entry site defines microtubule polymerization
3. maternal determinants are transported along these microtubules to the future dorsal site
4. B-catenin on dorsal site
   
   • > high Nodal(-related) protein
   • > low BMP at the site of organizer; high Chordin, Noggin, Folistatin

Question 3

What do cells of the dorsal blastopore lip become?

Which characteristic transcription factor is present there?

Answer 3

DBL -> dorsal mesoderm

Chordin

Question 4

What are the major functions of the organizer?

Answer 4

• initiation of the gastrulation movements
• generation of a self-regulation morphogen along DV
• dorsalisation

Question 5

What is dorsalisation?

Answer 5

organizer (DBL) -> dorsal mesoderm
dorsal mesoderm -> prechordal plate, notochord
also:
- mesoderm next to it becomes paraxial mesoderm (later: somites)
- ectoderm next to it forms neural tube

Question 6

On which site along DV axis is BMP high?

Which TFs are expressed in Spemann’s organizer?

Answer 6

BMP high on ventral side

Spemann:

• Chordin (later in dorsal mesoderm)
• Noggin
• Follistatin

Question 7

How is the BMP gradient maintained?

Answer 7

Organizer expresses Chordin

-> Chordin forms tertiary complex with Twisted gastrulation gene and BMP dimer

Question 8

BMP gradient patterns all 3 germ layers along DV axis.

Which structures develop in regions where its concentration is low?

Answer 8

low = dorsal

ecto: neural tube
meso: notochord
endo: pharynx

Question 9

BMP gradient patterns all 3 germ layers along DV axis.

Which structures develop in regions where its concentration is high?

Answer 9

high = ventral

ecto: epidermis
meso: heart, blood
endo: intestine

Question 10

Which 2 things specify AP axis?

Answer 10

1. timing (anterior = early)
2. position:
   - Wnt expressed in organizer when the latter is formed, later spreads out ventrally
   - Wnt is inhibited anteriorly in the head (Frizzled, Dickkopf, Cerebrus)

Question 11

What (regarding the morphogen gradients) do all bilaterians have in common and what all vertebrates?

Answer 11

bilaterians: perpendicular Wnt and BMP
vertebrates: Wnt and BMP dependent on Nodal (high in D)

Question 12

What is the correlation between Wnt and BMP signaling?

Answer 12

If Wnt is present: Dsh is active

• inhibits Gsk3 = B-catenin is free = can localize to nucleus
• Smad1 is stabilized = promotes BMP signaling

Question 13

What is different about establishing the axes in amniotes?

Answer 13

Role of extra-embryonic tissues:

• primitive streak is positioned by ANTERIOR VISCERAL ENDODERM (away from extraembryonic VE; low Nodal)
• • Wnt and Nodal inhibitors in rostral parts to inhibit caudal fate