Embryology: Germ Layers, Neural Induction and Neurulation

What does a Xenopus blastula explant experiment test?

-Remove a region and culture it separately

-Observe which tissues it differentiates into without its normal neighbours

-This tests whether its developmental fate has already been specified

What does an intact animal-cap explant normally become?

- Epidermis, an ectodermal derivative.

- It does not normally form neural tissue when cultured as an intact sheet.

Which derivatives form from a dorsal marginal-zone explant with adjacent ectoderm?

- Dorsal mesoderm produces notochord and muscle.

- The neighbouring ectoderm can form neural tissue.- Dorsal mesoderm produces notochord and muscle.

- The neighbouring ectoderm can form neural tissue.

Which derivatives form from a ventral marginal-zone explant with adjacent ectoderm?

- Ventral mesoderm produces blood and mesenchyme.

- The associated ectoderm forms epidermis.

What is the normal fate of vegetal blastula cells?

- Endoderm.

What do regional blastula explants show about the timing of specification?

- In the illustrated explant experiment, isolated vegetal cells remain relatively undifferentiated in culture.

- Regional differences in developmental fate are present before gastrulation movements begin.

- Gastrulation subsequently moves and arranges these tissues.

How are the germ layers arranged in an embryo after gastrulation?

- Ectoderm is outside.

- Mesoderm occupies the middle.

- Endoderm lies internally, beneath the mesoderm.

Where is the notochord relative to the neural tube?

- It is an axial mesodermal structure immediately beneath the neural tube.

- It extends along the midline and acts as a signalling centre.

What are the main subdivisions of mesoderm, from the midline outwards?

- Axial mesoderm, including the notochord.

- Paraxial mesoderm.

- Intermediate mesoderm.

- Lateral plate mesoderm.

What does paraxial mesoderm produce?

- Somites in the trunk and mesodermal tissues in the head.

What does intermediate mesoderm produce?

- Major components of the kidneys and gonads.

What are major derivatives of lateral plate mesoderm?

- Components of the circulatory system, including the heart and blood vessels.

- Body-wall and visceral connective tissues, and linings of body cavities.

- Some extraembryonic mesodermal tissues.

What are major endodermal derivatives?

- Epithelial components of the digestive and respiratory tracts.

- Epithelial components of associated organs, including the liver and pancreas.

- The whole organ is not necessarily endodermal: supporting tissues can have other origins.

What are the three major ectodermal groups discussed in this lecture?

- Surface ectoderm, including epidermis.

- Neural tube, which forms the central nervous system.

- Neural crest, which contributes to the peripheral nervous system and other tissues.

Which surface-ectoderm derivatives are shown in the lecture?

- Epidermis, hair, nails and sebaceous glands.

- Olfactory and oral epithelia; lens and corneal epithelium.

- Anterior pituitary and tooth enamel.

Which neural-crest derivatives are shown in the lecture?

- Peripheral nervous-system components, including Schwann cells and autonomic ganglia.

- Adrenal medulla and melanocytes.

- Craniofacial cartilage and dentine-producing cells.

Which neural-tube derivatives are shown in the lecture?

- Brain and spinal cord, including motor neurones.

- Retina.

- Neural pituitary, meaning the posterior pituitary.

How does progressive refinement of ectoderm illustrate generative development?

- Broad ectodermal identity is established first.

- Later signals distinguish epidermal, neural and neural-crest fates.

- Further signals specify individual regions and cell types, such as retinal cells or motor neurones.

What does a fate map show?

- Where cells and their descendants normally contribute during development.

- It describes normal fate, rather than proving that a cell cannot change fate.

How was the 32-cell Xenopus fate map constructed?

- Label a selected blastomere with a traceable dye.

- Allow development and locate its labelled descendants.

- Repeat for different blastomeres to build the whole map.

Why must mesoderm induction and mesoderm patterning be distinguished?

- Induction gives cells mesodermal identity.

- Patterning then specifies the type of mesoderm, such as notochord, muscle or blood.

What is combined in the classic mesoderm-induction experiment?

• Animal-cap cells are placed in contact with vegetal cells. - The marginal zone is removed, excluding pre-existing prospective mesoderm.

Why is the marginal zone removed in the tissue-combination experiment?

• It prevents existing mesoderm from explaining the result. - Mesoderm appearing in the recombination therefore provides evidence of induction.

What does the animal-cap and vegetal-cell recombination demonstrate?

- Vegetal tissue supplies signals that induce mesoderm in responding animal-cap cells.

- Cell fate can change through interactions between tissues.

How do dorsal and ventral vegetal cells differ in their inductive effects?

- Dorsal vegetal cells induce dorsal mesodermal derivatives, including notochord and muscle.

- Ventral vegetal cells induce more ventral derivatives, including blood and mesenchyme.

What are the main events in the three-signal model of mesoderm induction?

- Vegetal signals induce mesoderm in the marginal zone.

- Stronger dorsal vegetal induction establishes dorsal mesoderm and the organiser.

- Signals from the organiser pattern neighbouring mesoderm along the dorsal-ventral axis.

Where do the maternal factors VegT and Vg1 act?

- In the vegetal region of the Xenopus embryo.

- They help establish endoderm and induce mesoderm-inducing Nodal-related signals.

How do VegT and Nodal differ as signalling components?

- VegT is a transcription factor acting within the cells that contain it.

- Nodal is a secreted signalling protein that can influence neighbouring cells.

What does Xnr stand for?

- Xenopus Nodal-related.

- Xnr proteins are Nodal-related signals involved in germ-layer induction.

How do vegetal cells induce mesoderm in the marginal zone?

- Vegetal factors promote Nodal-related signalling.

- Secreted signals act on cells in the adjacent marginal zone.

- These responding cells acquire mesodermal identity.

How does dorsal beta-catenin contribute to organiser formation?

- Beta-catenin is part of the Wnt signalling pathway and activates dorsal gene expression, including Siamois.

- It cooperates with vegetal factors to promote strong dorsal Nodal-related signalling.

- These inputs help establish the organiser.

Why is Nodal-related signalling strongest on the dorsal vegetal side?

- Vegetal inputs and dorsal beta-catenin-dependent inputs overlap there.

- Their combined effects promote higher Nodal-related activity than ventrally.

How does BMP activity help pattern mesoderm?

- Higher BMP activity promotes ventral mesodermal fates, including blood.

- BMP antagonists from the organiser reduce BMP activity dorsally.

- Different signalling environments produce different mesodermal derivatives.

Why can the same signalling molecule have different developmental effects?

- Its effect depends on developmental time, location and the responding cell.

- For example, BMP inhibition favours early neural induction, but BMP signalling has later roles in nervous-system development.

Which main signalling systems recur throughout embryonic development?

- TGF-beta superfamily signals, including BMPs and Nodal-related proteins.

- Fibroblast growth factors, Hedgehog factors and Wnts.

- Retinoic acid and Notch/Delta signalling.

Which antagonists illustrate inhibition of developmental signalling?

- BMP antagonists: Chordin, Noggin and Follistatin.

- Nodal-related antagonists include Lefty and Cerberus.

- Wnt antagonists include secreted Frizzled-related proteins, or sFRPs.

How does the level of Nodal-related signalling influence germ-layer fate?

- High signalling in the vegetal region promotes endoderm.

- Lower signalling in the marginal zone promotes mesoderm.

What is the core endoderm-induction pathway required in this lecture?

- Maternal VegT and Vg1 promote Nodal-related signalling.

- The lecture includes maternal and zygotic Nodal-related inputs.

- High Nodal activity promotes the endodermal programme, including zygotic Sox17.

Why is Sox17 useful experimentally?

- Sox17 is a zygotically expressed endodermal gene.

- Its expression can be used as a marker of endodermal identity.

Does ectoderm induction require instructive signals, inhibitory signals or both?

- Both.

- Instructive signals establish ectodermal identity.

- Inhibitory signals prevent mesoderm- and endoderm-inducing signals from overriding it.

What instructive pathway establishes ectodermal identity in Xenopus?

- Foxi2.

- Xema (Foxl1e).

Why is the Foxi2-to-Xema pathway insufficient on its own to preserve ectoderm?

- It supplies the instructive ectodermal programme.

- Inhibitors are also needed to prevent mesendoderm-inducing signals from overriding that programme.

Why must mesoderm-inducing signals be restricted in the animal pole?

- Vegetal Nodal-related signals can influence neighbouring cells.

- Inhibitors restrict the effective signalling response in the animal region.

- Without this restriction, animal-pole cells could acquire mesodermal or endodermal rather than ectodermal fates.

Which inhibitors help preserve ectodermal identity in the animal region?

- Coco, Norrin and Ectodermin are examples discussed in the lecture.

- They restrict inappropriate mesendoderm-inducing signalling in the animal region.

How are all three germ-layer territories established together?

- Strong vegetal Nodal-related activity promotes endoderm.

- Lower Nodal-related activity in the marginal zone promotes mesoderm.

- Animal-region ectodermal factors and inhibition of mesendodermal signalling preserve ectoderm.

- Maternal Foxi2 promotes expression of zygotic Xema, also called Foxile.

- Xema/Foxile helps establish ectodermal fate in the animal region.

What is the amphibian organiser?

- The dorsal blastopore lip region of the early gastrula.

- It is a signalling centre that helps organise the body axis and induce neural tissue.

Which structures are equivalent organiser regions in chick and mammals?

- Hensen's node in the chick.

- The node in mammals, including humans.

How does proximity to the organiser relate to ectodermal fate?

- Ectoderm near organiser-derived neural-inducing signals can become neural tissue.

- More distant ectoderm retains stronger BMP activity and tends towards epidermis.

What did Spemann and Mangold transplant in their classic experiment?

- The dorsal blastopore lip from an early gastrula.

- They grafted it to the ventral side of another gastrula.

What was the result of an early organiser graft to the ventral side?

- A secondary body axis developed.

- The embryo formed a twinned axis alongside its original axis.

Why were differently pigmented newts used in the organiser experiment?

- Pigmentation distinguished graft-derived cells from host-derived cells.

- This allowed the origin of the secondary tissues to be traced.

Which tissues in the induced secondary axis came from the graft and the host?

- The graft contributed mainly axial mesoderm, including notochord.

- Much of the induced axis, including neural tissue, came from the host.

Why does the secondary axis demonstrate induction?

- The graft did not simply grow into the entire second embryo.

- It caused neighbouring host cells to adopt new developmental fates.

- One tissue therefore changed another tissue through signalling.

How do early and late organiser grafts differ?

- Early organiser tissue can induce an axis with anterior and posterior structures.

- Late organiser tissue mainly induces posterior structures.

Why does a late organiser have reduced anterior-inducing activity?

- During gastrulation, early anterior-inducing tissues have already involuted and moved forwards.

- The remaining organiser region is associated more with posterior development.

What does transplanting a quail node into a chick embryo show?

- The graft can induce a secondary axis.

- The organiser function of the node is conserved in birds.

Why is a quail-specific antibody useful in the quail-to-chick graft?

- It labels quail donor cells but not chick host cells.

- It shows that donor cells contribute to axial mesoderm while host cells contribute much of the induced axis.

What do organiser grafts between different vertebrate species demonstrate?

- Organiser signals can act across species boundaries.

- The underlying inductive mechanisms are evolutionarily conserved.

How does the age of a chick node affect its inductive ability?

- An early node can induce an axis with anterior neural structures.

- A late, regressed node mainly induces posterior tissue.

- Timing matters in chick as well as amphibians.

Why was a hindbrain marker used in the early-versus-late chick node experiment?

- It helped identify which neural regions were induced.

- Its absence from the late-induced tissue supported a posterior identity.

What is neural induction?

- The process by which competent ectoderm acquires neural identity.

- Organiser-derived signals play a key role in this process.

What happens when Xenopus animal-cap cells are dissociated and cultured separately?

- They can differentiate into neural cells.

- Dissociation reduces the effective concentration of secreted epidermis-promoting signals.

What hypothesis explains the difference between intact and dissociated animal caps?

- A signal within the intact cap promotes epidermis and suppresses neural fate.

- Dissociation dilutes this signal, allowing neural differentiation.

Which signalling proteins were implicated by the animal-cap experiments?

- Bone morphogenetic proteins, or BMPs.

- BMP activity favours epidermal over neural differentiation in this experimental context.

What happens when BMP protein is added to dissociated animal-cap cells?

- The cells differentiate towards epidermis rather than neural tissue.

- Restoring the signal reverses the effect of dissociation.

Why is adding BMP back to dissociated cells an important experimental control?

- It tests whether loss of BMP signalling explains neural differentiation.

- Restoration of epidermal fate supports a causal role for BMP activity.

What is the neural default model?

- Competent ectoderm can adopt neural fate when BMP signalling is sufficiently reduced.

- Active BMP signalling instead promotes epidermis.

- The model derives from amphibian experimental evidence and is a simplified account of neural induction.

Why does the organiser need to produce BMP antagonists?

- BMP activity would otherwise oppose neural differentiation in nearby ectoderm.

- Secreted antagonists reduce BMP signalling and permit neural tissue to form.

Which three BMP antagonists should be learnt from this lecture?

- Chordin.

- Noggin.

- Follistatin.

Where are Chordin, Noggin and Follistatin expressed in the induction model?

- In the organiser.

- They are secreted and inhibit BMP activity in neighbouring tissues.

Why does organiser signalling not turn all ectoderm into neural tissue?

- Its effective influence is spatially limited.

- Dististant ectoderm retains sufficient BMP signalling to form epidermis.

What happens when key organiser BMP antagonists are removed experimentally?

- Neural tissue is reduced or lost.

- This supports their requirement for normal neural induction.

How do organiser BMP antagonists affect both ectoderm and mesoderm?

- In ectoderm, reduced BMP activity favours neural induction.

- In mesoderm, reduced dorsal BMP activity helps establish dorsal rather than ventral fates.

Why can removal of a single antagonist have a limited effect?

- Other antagonists can have overlapping functions and compensate.

- This is functional redundancy.

How do neural induction and neurulation differ?

- Neural induction establishes neural cell identity.

- Neurulation is the morphogenetic formation of the neural tube from neural tissue.

What is the neural plate?

- A sheet of neural ectoderm formed during and by the end of gastrulation.

- It bends and closes to form the neural tube.

What are the main stages of primary neurulation?

- Neural plate forms and bends.

- Neural folds rise on either side of a neural groove.

- The folds converge and fuse.

- The neural tube separates from the overlying surface ectoderm.

Which major structures develop from anterior and posterior neural tube?

- Anterior regions form the brain.

- Posterior regions form the spinal cord.

Where are neural crest cells specified?

- At the border between neural and non-neural ectoderm.

- They are associated with the neural folds and dorsal neural tube.

What happens to neural crest cells during and after neural-tube formation?

- They leave the dorsal neural region and migrate into the embryo.

- They contribute to the peripheral nervous system and several other tissues.

What is the relationship between the notochord and neurulation?

- The notochord lies beneath the neural plate and neural tube.

- Its signals help pattern neural tissue, including ventral neuronal fates.

How is chick neural-tube formation related to primitive-streak regression?

- The primitive streak and node regress posteriorly.

- Anterior neural tissue develops while more posterior regions continue forming.

- Neural-tube closure therefore progresses along the developing axis.

What are the hinge points involved in neural-plate bending?

- The median hinge point at the midline.

- Dorsolateral hinge points on either side.

- Local cell-shape changes at these sites help the plate fold.

What are intrinsic and extrinsic forces in neurulation?

- Intrinsic forces arise within the neural tissue, including cytoskeletal contraction and cell-shape changes.

- Extrinsic forces arise from surrounding tissues and influence neural-plate bending and closure.

Which cellular processes contribute to neurulation?

- Constriction and bending at hinge points.

- Cytoskeletal contraction.

- Regulated cell proliferation and cell death.

- Neural-crest emigration accompanies neural-tube development.

What separates the neural tube from the outside after closure?

- Surface ectoderm closes over it and contributes to epidermis.

- The neural tube is internal, with the notochord beneath it.

Why is neural-tube closure essential?

- Failure of closure leaves developing neural tissue abnormally exposed and disrupts development.

- Different affected regions give rise to different neural-tube defects.

What is anencephaly?

- A severe cranial neural-tube defect in which much of the brain and skull fails to develop normally.

- Exposed cranial neural tissue degenerates.

How are exencephaly and anencephaly related?

- Failed cranial closure initially leaves neural tissue exposed, termed exencephaly.

- Degeneration of that exposed tissue leads to anencephaly.

What is spina bifida?

- A spectrum of defects affecting closure or development in the spinal region.

- Severity varies greatly according to the type and extent of the defect.

What is craniorachischisis?

- A severe defect in which the neural tube remains open across extensive cranial and spinal regions.

Why should the mouse closure pattern not be assumed to be identical to the human pattern?

- Mouse embryos have characteristic closure initiation sites.

- The number and pattern of human closure sites are less certain and have been debated.

- The lecture emphasises the defects rather than a definitive human site count