Lecture 4 - Dorsal ventral patterning of the neural tube Flashcards
How are avian embryos more beneficial to use in developmental experiments than mammal embryos?
- they are very similar in morphology and embryogenesis to mammals
- but they are much easier to observe and manipulate experimentally
- can be manipulated in culture or in ovo
When does gastrula occur in the avian embryos
After the primitive streak stages
How can the early mesoderm and the notochord be viewed in gastrulation in the avian embryo?
By marking it with brachyury
How does gastrulation proceed in the avian embryo?
The node regresses posteriorly, and the notochord and the somites form anterior to the node
What type of development do avian embryos show in neurulation/gastrulation?
Rostral claudal progression of development
Outline the Rostral claudal progression of development in avian embryos?
- After the notochord forms, the neural plate induced from the overlying ectoderm
- the anterior part of the neural tube folds, rolls and forms a tube first, then neural tube closure proceeds in a head to tail (rostral claudal) direction
- somites, which give rise to skeletal muscle, bone and dermis, also form in a rostral claudal manner
How is avian embryo development unlike frog development?
In frog development, the neural tube forms all at once, in avian embryos the anterior part of the neural tube folds, rolls and forms a tube first, then neural tube closure proceeds in a head to tail (rostral claudal) direction
-somites, which give rise to skeletal muscle, bone and dermis, also form in a rostral claudal manner
What is the effect of the rostral claudal progression of development in avian embryos?
The anterior part of the embryo is developmentally more advanced than the posterior
-in the trunk region, somites will have formed and neurulation will be underway but in the posterior region behind Hensons node, the notochord and somites will not yet have formed
How is the neural tube patterned along the AP axis?
- by signals from the dorsal mesoderm and the posterior mesoderm
- Hox genes provide a molecular basis for the positional identity along the AP axis of the neural tube
Where is the effect of Hox genes patterning the neural tube particularly clear?
-in the hindbrain, where distinct hox genes have definitive anterior boundaries of expression that correspond to rhombomere boundaries
What experiments have demonstrated the important of hox genes in coding the positional identity along the AP axis?
Posterior prevelence of Ubx, AbdA, AbdB
Bithorax mutant
What is the dorsal ventral patterning of the neural tube from the dorsal side?
Sensory neurons Interneurons Motor neurons Floor plate Notochord
When is the pattern of regional cell identity apparent along the DV axis during neurulation?
As the neural plate rolls up to form the neural tube, a pattern of regional cell identity becomes apparent in the DV axis
What is the pattern of regional cell identity in the DV axis of the neural tube?
Where distinct populations of progenitor cells give rise to post mitotic neurons with distinct functional properties
- cells in the dorsal region of the neural tube generate sensory neurons
- cells in the ventral region of the neural tube generate motor neurons that coordinate motor input
- patterning underlies functioning neural circuits
How do the pattern of cells along the DV axis of the neural tube form in respect to the ventral midline?
Distinct cells form in a bilaterally symmetrical way with respoect to the ventral midline
- At the ventral midline is the floorplate
- Motor neurons form ventrolaterally
- Sensory neurons and the neural crest form dosally
- Roofplate forms at the dorsal midline
What are the genes that are specifically expressed in different neural progenitor populations in the neural plate and where?
Dorsally to Ventrally
V1, V2, MN, V3
What are the features of the genes that are specifically expressed in different neural progenitor populations of the neural tube?
- code for TF
- cross regulate each others’ expression boundaries
- define the specific neural progenitor populations
What technique can be used to identify structures like the floorplate and MN?
Immunostaining
-Ab recognise particular proteins specific to particular cells of the floorplate/motor neurons/notochord
What did the experiment where an extra notochord was added to a developing neural tube structure show?
-Add extra notochord, resulted in two floorplates induced with motor neurons
-If put notochord in a different place, get motor neurons in that vicinity (ectopic location)
SHOWS:
that the underlying dorsal mesoderm structure (the notochord) induces floorplate and is important in patterining other cell types along the DV axis of the NT (the motor neurons)
Aside from the notochord, what else was found to induce floorplate when it was ectopically inserted?
- Floorplate itself was found to induce floorplate
- Removal of the notochord prevents floorplate differentiation
What did experiments with intermediatenerual plate show?
- Using explants of intermediate neural plate that would not normally give rise to motor neurons and combining this tissue with notochord, floorplate or floorplate induced media led to the expression of motor neurons
- this suggests that MN inducer is diffusible
What is islet1 and how was it used in experiments on intermediate neural plate?
- islet1 is an antibody that recognises only motor neurons
- detects motor neurons induction
What did experiments on the intermediate neural plate and the ectopic expression of floorplate and notochord illustrate?
That the notochord and the floorplate have an instructive role in establishing the identify and position of cell types along the DV axis of the NT
What is Sonic Hedgehog?
- drosophila sonic hedgehog is a signalling molecule
- member of the family of hedgehog genes
- involved in organising pattern within the parasegment of the developing embryo
- functions later to develop the patterning of the limbs and the eye of the fly
How does shh show conservation between vertebrates and flys?
- as well as being involved in patterning in the drosophila embryo
- all of the vertebrate shh have been shown to be expressed in the notochord and the floorplate
What is the evidence that shh can act as a floorplate inducer?
- Injection of shh mRNA into fly embryos leads to ectopic floorplate
- COS cells expressing shh can induces NP explants to express the floorplate marker F3
What are the ligands, receptors and effectors of shh signalling?
Ligands: shh
Receptors: Ptc/Smo
Effectors: Gli acitvator/repressor of transcription
What genes did mutagenesis screens reveal in the hh pathway?
Patched (ptc)
Smoothened (smo)
Cubitus interrupitus (ci)
What is the process of signalling via shh?
- in the absence of hh, patched represses smo and the multiprotein complex associated with microbtubules casues Ci to be processed into a smaller form called Ci75 which represses hh target genes
- hh signalling results in the stabilisation of a full length Ci which can move to the nucleus and function as a transcriptional activator
What are the vertebrate homologues of cubitus interrupitus in vertebrates?
Gli genes
Outline the french flag model of how morphogens act
- signal source at one end of the array of cells (each of which has the potential to develop into different types)
- diffusion of the signal establishes a gradient of morphogen
- morphogen gradient allows cells to aquire positional information about where they are related to the source
- cell respond approprately to the serial threshhold of morphogen concentration
- the concentration gradient could be interpreted in many ways, could generate different patterns or more threshold responses
What evidence is there to support the idea that shh is a morphogen (it’s not, by the way, next lecture)
- in the ventral neural tube, motor neurons and interneurons are generated by the graded activity of shh
- cultured explants respond to different concentrations of shh protein; form more ventral neurons in reponse to high concentrations and more dorsal neurons in reponse to lower levels of shh
- which reflects their in vivo position along the DV axis of the neural tube
Outline the model for the role of graded shh signalling in the establishment of ventral cell fates in the vertebrate spinal cord
- graded shh signalling establishes the ventral expression domains of a number of homeodomain transcriptional regulators in neural progentior population
- distinct boundaries are established on the basis of shh mediated repression (class I) or induction (class II) at different concentration thresholds
What is the lateral motor column?
A continuous medial motor column spanning all rostroclaudal (AP) levels and contains motor neurons projecting to the axial muscles
Where does the lateral motor column develop?
- develops at precise positions along the AP axis.
- spans the brachial and lumbar levels of the spinal cord but only where forelimbs and hindlimbs develop (as a result of a specific hox code)
What does the lateral motor column contain?
the limb-innervating motor neurons
How is the lateral motor column further divided?
What is motor neuron specification down the AP axis of the neural tube dependent upon?
the AP expression of hox genes
- where HoxC6 is highest, the pool of motor neuron progenitors are specified as LMC (Hox C6 is highest in the brachial region)
- HoxC9 expression is highest in the thoracic region where another type of motor neuron will develop.
