Overview
- Positional information
- Recapitulation mesoderm induction
- Mesoderm patterning in frog and mouse
o Organizer concept
o Opposing gradients
o Different types of mesoderm
- Neuroectoderm induction and patterning
o Organizer concept
o Neural plate/groove/tube closure
o A-P and D-V patterning
- Neural crest lineage, fate map, plasticity
- Congenital defects
Patterning
- Interpretation of positional information
o Hox-code (class 7)
o French flag model (morphogens and threshold levels)
o Extracellular antagonists of morphogens
- ‘sorting out of cells’
- Lateral inhibition can give a spacing pattern
- Patterning
o Mesoderm
o Neuroectoderm
o Neural crest
- Syndromes
Patterning is very important, the Hox-code genes play an important role here. The French flag model
explains how the gradients work and how the patterning happens. There are extracellular antagonists
of morphogens. Patterning:
- Mesoderm
- Neuroectoderm
- Neural crest
1
,EMT = Epithelial to Mesenchymal Transition, it is
important for mesodermal induction.
How do different types of mesoderm form within
the mesoderm?!
Cells become dorsal mesoderm (prechordal plate mesoderm, notochord,
paraxial mesoderm situated beneath dorsal ectoderm – important for
neural induction).
➔ Signal in the vegetal side of the
embryo that will induce the dorsal
blastopore lip, the cells that induce this are
called the Spemann organizer. The
mesoderm that is formed here will become
dorsal mesoderm (notochord!)
o Dorsal mesoderm very important the
for neural induction!
Sandwich experiments:
Development of mesoderm
will differ depending on the
type of combination with
vegetal pole that was made.
➔ There are different
signals in dorsal and
ventral pole of
vegetal pole!
2
, Mesoderm patterning in Xenopus laevis (frog) (very comparable to zebrafish)
➔ See drawing of gene expression in blastocyst on slides.
➔ What is lacking in these sandwich
experiments?
Never any intermediate (kidney/gonads) or lateral
plate (heart) mesoderm developed ?!
How come we don’t get development of the full
array of mesoderm?
The marginal zone is regionalized
Marginal zone = equatorial zone
They used fluorescent dye and labelled cells → they found out which
cells would further develop into intermediate mesoderm, see fate
map, although they did not get intermediate mesoderm when testing
this in the sandwich experiments.
They performed in-situ hybridization for different genes: Watch out
for orientation of dorsal and ventral and localize that with fate map!
- Some genes specifically expressed in the ventral mesoderm
o Xvent-1, Xvent-2, xMyoD
- Some genes specifically expressed in the dorsal mesoderm
o noggin, xMyf-5
Gene expression already in some sort of pattern!
How to establish intermediate mesoderm? Which can not be formed by
the sandwich experiments.
➔ New experiment:
o Only taking dorsal and ventral sides of mesoderm, when co-culturing these then all of
a sudden pronephrons (urogenital mesoderm) and cardiomyocytes
Dorsal and ventral mesoderm talk to each other! And by that conversation the intermediate
region gets specified. But then the question becomes which signals are being used?
o DMZ (dorsal marginal zone, mesoderm) and VMZ (ventral marginal zone, mesoderm)
regulate the patterning in mesoderm!
Found out that the DMZ secretes something that is necessary to induce that intermediate mesoderm.
The organizer comes into play!
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