lec 11 - axis formation, neural induction, and neural tube patterning

the spemann-mangold organizer is ____ located at ____

a group of mesendodermal progenitor cells

dorsal blastopore lip

during gastrulation, the spemann-mangold organizer migrates

anteriorly

the spemann-mangold organizer gives rise to three distinct tissues

anterior endoderm

prechordal mesoderm

notochord

anterior endoderm contributes to ___ and is from the ____ organizer

head structures

early

prechordal mesoderm contributes to ____ and is from the ____ organizer

anterior structures

early

notochord contributes to _____ and is from the _____ organizer

patterning the overlying spinal cord

mid/late

who discovered the spemann-mangold organizer

hilde mangold and hans spemann

how did mangold and spemann discover the organizer

1. took cells from the dorsal blastopore lip of one xenopus embryo

2. transplant to ventral side of second embryo

3. second complete body axis forms, including second brain and spinal cord

   1. those dorsal cells only formed a bit of the notochord, a transietnt structure

conclusions of the mangold-spemann experiments

1. the dorsal lip cells organize the patterning of the rest of the embryo - ‘organizer’

2. the organizer does its job via cell-cell signaling

organizer grafts - early vs late phenotypes + why

early: full split, complete second head and trunk

late: partial split, only second posterior parts

why?

• organizer induces anterior structures first as it migrates

• signaling molecules that organizer produces change over time

the organizer moves ______ly during gastrulation, _____ as it moves

anteriorly

patterning the AP axis

as the organizer migrates

it induces overlying ectoderm to form the neural plate

organizers inducing properties are achieved by secreting molecules that

inhibit Wnt and BMP signaling pathway activities

early organizer - BMP, Wnt, or both are blocked (and by what)

both are blocked

cerberus is most important - blocks both Wnt and BMP but BMP most

mid organizer - BMP, Wnt, or both are blocked (and by what)

Wnt a little, BMP a lot

cerberus is gone - BMP is blocked by BMP inhibitors chordin and noggin

mid organizer - BMP, Wnt, or both are blocked (and by what)

BMP is heavily blocked - added follistatin into the mix

Wnt is free

t/f - BMP expression is blocked everywhere

false - not blocked ventrally

what is the double gradient model

the organizer patterns the blastula through two perpendicular morphogen gradients

• AP gradient of Wnt activity, from low Wnt anteriorly to high Wnt posteriorly

• DV gradient of BMP activity, from low BMP dorsally to high BMP ventrally

each cell’s position can be read off a map like a cartesian plane

principle BMP is ____ which is expressed ____

BMP4

throughout early gastrula

principle Wnt is ____ which is expressed _____

Wnt8

marginal zone/wheere mesoderm is induced

what is the default state of ectoderm

neural tissue

_____ promotes brain formation in the anterior neural plate

______ promotes spinal cord formation

dual inhibition of Wnt and BMP signaling

BMP inhibition without Wnt inhibition

how is neural fate induced

epidermal fate is suppressed → neural fate induced in ectoderm

• BMP antagonists chordin, noggin, and cerberus expressed in the organizer

• diffuse from organizer and act on overlying ectoderm to suppress epidermal fate

neural induction

making neural plate from ectoderm

head induction requires inhibition of

Wnt and BMP

cerberus unique characteristics

blocks BMP and Wnt signaling

cerberus experiement

hypothesis: cereberus inhibits Wnt8 and BP4 → anteriorization and dorsalization of neural plate → head induction

ectopic cerberus mRNA injection → induced a second head

cerberus experiment conclusions

ectopic cerberus mRNA injection is sufficient for head induction

induction vs intrinsic differentiation

induction: extrinsic, a cell produces a signal that acts on another cell to change its fate

intrinsic: cells divide asymmetrically based on the specific cellular components within each cell

who proposed the activation-transformation model

hans spemann

the activation-transformation model explains

how the neural plate/tube acquires detailed AP patterning

explain the activation/transformation model

activation

• early organizer secretes Wnt and BMP antagonists

• neuroectoderm adopts anterior forebrain fate by default

transformation

• mid → late organizer - Wnt secretion is attenuated (slowly increases)

• posterior neural plate is exposed to multiple gradients of posteriorizing factors (Wnts, RA, FGFs)

• forebrain fate is suppressed; over time, increasingly posteriorized neural fates are induced

• neural plate elongates and transitions into the neural tube

what are the posteriorizing factors

• Wnts

• Retinoic Acid (RA)

• FGFs

what is retinoic acid (RA)

• diffusable small molecule

• defivative of vitamin A

RA acts as a ligand for ____ which activate ____ in gene promoters

retinoic acid receptors

TFs that bind to Retinoic Acid Response Elements (RAREs)

FGFs activate ___ via ___

ETS TFs in the nucleus

cascade of serine-threonine kinases

activation-transformation involves

simultaneous integration of multiple signaling pathways (Wnt, RA, FGFs)

net result of the activation-transformation model

two opposing gradients

• anteriorizing factors - BMP and Wnt antagonists (cerberus, chordin, noggin, dickkopf)

  • promote forebrain fate

• posteriorizing factors - Wnts, RA, FGFs

  • suppress forebrain fate

  • promote increasingly posterior cell fates

hox genes

encode homeodomain TFs that interpret positional information from signaling molecules

once broad AP gradients are set up, TF in the neural plate:

• interpret the signal levels

• activate specific Hox genes at different positions

• Hox genes determine what structure forms where

RA and Hox gene expression

• RA establishes a posterior → anterior gradient

• induces specific patterns of Hox gene transcription along different positions in the axis

• different combos of Hox gene expression in the neural tube specify distinct domains of the spinal cord

hox genes are expressed in _____ order. explain this order

chromosomal

• 3' genes expressed first → more anterior

• 5' genes expressed later → more posterior

describe the stages of neural tube formation

1. flat neural plate

2. formation of neural furrow - by folding of the neural plate along the midline

3. elevation of neural folds - forms the neural groove

4. neural folds bend inward

5. neural folds fuse - closed neural tube + detachment of epidermis

6. neural crest cells migrate away

DV patterning of the neural tube - two opposing signaling systems

• dorsal - BMPs secreted by the dorsal roof plate, induce dorsal neuronal fates

• ventral - Shh secreted by the ventral Floor plate (and notochord), induces ventral neuronal fates

neural tube - BMPs are secreted by

dorsal Roof Plate

neural tube - Shh is secreted by

ventral Floor Plate + notochord

DV neural tube patterning - sequence

• epidermal BMP signaling specifies roof plate; notochord Shh signaling specifies floor plate

• BMPs from roof plate diffuse ventrally and pattern the dorsal half of the neural tube

• Shh from floor plate diffuse dorsally and pattern the ventral half of the neural tube

in the dorsal neural tube, how are neuronal fates patterned

different members of the BMP family are expressed in different domains, and induce different dorsal neuronal fates

• stepwise process

in the ventral neural tube, how are neuronal fates patterned

different concentrations of Shh induce different ventral neuronal fates