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Organogenesis MCB141 part III Spring 2012
Neural tube formation:
Lecture 3. spinal cord
1. The neural ectoderm forms the neural tube during neurulation
Gilbert6. Neurulation and somite formation progress from anterior to posterior, so that in one animal,
several stages of somite formation and neurulation can be seen (Gilbert6).
2. Cell behaviors: Wedging (apical constriction), convergence and extension in the neural plate,
medial migration of epidermis following from epiboly, fusion of the neural folds. These behaviors are
particularly clear in the chick (Gilbert6) and the amphibian urodele embryo. Freeze fracture electron
micrographs show the regions of bending in the floor plate and dorsolateral hinges. The urodele is
represented in the figure from Gilbert6 and in this movie (1.6Mbytes). (by RMH)
3. Morphogenetic movements are summarized in this summary figure. The neural plate narrows, by
the convergent extension of cells to the midline. This brings the folds closer together, so that the
folds can reach each other and fuse, as shown in the diagram and the linked movies. The
convergence and extension relies on cells being able to orient their behavior in th eplane of the
tissue. This conserved process is the Planar cell Polarity (PCP) pathway, which uses upstream
components of the Wnt signaling pathway, including Dishevelled, and diverges to use different
proteins downstream, such as the Strabismus/VanGogh protein (Vangl in vertebrates). Mutations in
the Vangl genes cause afilure of the neural tube to close in spina bifida. Even heterozygous
mutations cause defects in some C/E processes, for example in the inner ear where there is a precise
arrangement of sensory cells, and mutations in C/E components often cause deafness.
4. In addition to C/E bringing the neural folds together, the cells at hingepoints become wedge
shaped, by apical constriction. The basis for apical constriction is not well-understood, but in the
neural plate requires the expression of the shroom gene that mediates apical constriction in epithelial
cells. When its function is blocked, neurulation fails, as shown in this paper (by Haigo et al)-see
figure 5.
5. As the neural folds touch, they then fuse, and the neural tube segregates from the epidermis.
Changes in cell adhesion contribute to the segregation of tissues: as mediated by cadherins
(Gilbert6)
Secondary neurulation and cavitation.
6. Primary neurulation is the process of rolling up of the neural tube from a sheet. However, this only
occurs in the head and trunk of most vertebrates. The hollow neural tube forms by secondary
neurulation in the posterior region, often also called the tailbud. Here cells proliferate as an
apparently homogeneous mass, then segregate to form the nerve cord and somites (Gilbert6).
Organization of differentiated cells
7. Neural crest cells form at the boundary of neural tube and epidermis (Gilbert6). These migrate
widely through the embryo. In the head, they form the skull, and a variety of peripheral ganglia. In
the trunk they form melanocytes, perpipheral neurons, and support cells (glia and schwann cells),
some of the smooth muscle, endocrine cells (such as the adrenal medulla).
8. Anatomy of the spinal cord: During neurulation, the medial to lateral organization becomes ventral
to dorsal. (see figure 1 from Chitnis et al). Types of neurons born in the neural tube: neural crest/
dorsal are sensory, intermediate are interneurons, ventral are motor neurons.
9. Larval vertebrates form primary neurons, and select them for differentiation using lateral inhibition,
via the Notch pathway. (G8 161-163, Gilbert6 and discussion of juxtacrine signaling).
Amniotes have the same overall organization, but the sensory component is derived from neural
crest, which migrates out and coalesces into the dorsal root ganglia (Gilbert6)
How is the complexity of the spinal cord established?
10. Embryological experiments implicate the notochord and surface ectoderm as signaling sources.
Both grafting and deletion experiments are used in intact embryos, or in explant culture.
Medial/lateral become Ventral and dorsal signals; Sonic Hedgehog and BMPs are important signals
Organogenesis MCB141 part III Spring 2012 Lecture 3. spinal cord
in establishing the ventral to dorsal pattern.
In the ventral neural tube, a morphogen gradient of SHH establishes different motor neuron types.
Patterning of the spinal cord by countervailing midline/ventral (notochord and floor plate) signals
mediated by SHH and lateral/dorsal signals mediated by BMPs.
1. These signals set up different cell identities, which are important for the integration of neuronal
activity. A diagram (not for memorization) shows the various cell types that arise from these domains
this is also summarized in Gilbert.
2. The patterning of the spinal cord by Sonic Hedgehog (SHH) provides one of the best examples of
a morphogen. The notochord and floorplate provide a localized source of SHH with diffusion of the
ligand. Diffusion sets up a concentration gradient, which elicits multiple different cell fates (see
picture). SHH has been shown to be present in a graded concentration. For a convincing view of SHH
diffusion see figure 5 J, K (from Gritli-Linde et al) This has also been viewed more recently using a SHHGFP fusion protein (Figure 1J of Chamberlain et al. see supplementary figure for a diagram of the
knockout)
Is SHH a morphogen?
3. The signaling pathway is shown in Gilbert 6 , G8 page153. This pathway is unusual in that the
primary receptor, patched, is a negative regulator of the pathway, inactivated by HH binding. In the
last few years, differences in the vertebrate and invertebrate pathway have been found- in
vertebrates, the Primary cilium acts an antenna for Hedgehog signals (see Ribes and Briscoe
4.Patched is also a target of the pathway, so once again, we see that an immediate effect of
activation is the production of a negative feedback.
5. Explant assays using the intermediate neural tube can be used to document morphogen effectsillustrating the principle that SHH might work as a morphogen. Different doses cause development of
floor plate, motor neurons etc.
6. Mutant studies in the mouse prove that SHH is required for ventral spinal cord fates, such as motor
neurons. However, long range signaling from other hedgehog sources (IHH from the gut) contributes
to signaling (adding to the evidence that HH can diffuse and act over a range). Elimination of the
Hedgehog transducer Smoothened eliminates ventral fates. (from Wijgerde et al.)
7. Does SHH work directly as a morphogen, or might it set up a relay of signals?
Use of chimeras between wild-type, and smo-/- SHH non-responsive cells (which should provide a
SHH non-responsive “barrier” to a relay) proves that SHH can move over a distance as a morphogen,
and is not activating a relay system. Dorsal fates occur in cells that cannot respond to SHH.
Importantly, ventral fates result in cells beyond the “barrier” of smo-/-, and can even occur at
distances larger than they usually would- this also shows that normal HH response soaks up the HH
signal. This is because a response to HH is the induction of the HH receptor, patched. This illustrates
the negative feedback property that hedgehog response normally restricts the range of SHH action.
8. Different subtypes of neurons are marked by different constellations of transcription factors, these
act in combination to determine cell fate.
9. SHH causes a switch between gli repressor function and gli activator function (default proteolysis
of the Gli transcription factors to make a repressor is prevented by activity of the Smo receptor
(Gilbert 6) to establish graded activity of repressors in the ventral spinal cord.
10. At the dorsal side of the spinal cord, BMPs activate dorsal programs of gene expression. Just as
in fly segmentation, the opposing action of repressors in the spinal cord establishes sharp
boundaries. e.g. Pax6 vs Nkx2.2, Irx3 vs Olig2 (See p401 Gilbert7, 384 of G8 or the more up-to-date
Figure 2 of this review for summary of cross repressive interactions. In the case of motor neurons,
the domain is defined by olig2 expression. The domain is defined by the broad activator (activated
Gli), and sculpted by localized repressors; dorsally by Irx3 repression, and ventrally by Nkx2.2
repression.