Neurulation Flashcards
What is neurulation?
- Folding of neural plate which bends up and later fuses to form a neural tube that will differentiate into brain and spinal cord
- Occurs at 4th week of human gestation
- The CNS is derived from neuroectoderm
What does the neural tube give rise to?
- Brain
- Spinal cord
- Cranial and spinal nerves
- Eyes and other sensory organs
- Neural crest
-Neural tube arises from ectoderm
What are the steps of neurulation?
- Specification of neuroectoderm
- Neural plate formation
- Neural plate folding
- Neural tube closure
- Neural crest closure
- Neural crest migration
What is the establishment of the anteroposterior axis?
- The AP axis is set out in parallel and before the formation of neuroectoderm
- The CNS is derived from the neuroectoderm
- Neuroectoderm specification is preceded by the establishment of the embryonic cranial-caudal axis
- The appearance of the primitive streak marks the future caudal end of the embryo and establishes the cranio-caudal and left-right axes —> this allows establishment of neuroectoderm
What is the molecular control of body axes establishment in Xenopus?
- There are maternal signals which are initially confined to the vegetal pole which then translocate to the dorsal part of the embryo
- This induces the accumulation of nuclear B-catenin
- This activates the genes required for the organiser function
- The organiser becomes a source of signals such as Nodal, Chordin and Noggin (inhibit serine/threonine kinase pathway), that establish the axes of the developing embryo
- They are antagonists for BMP pathway (serine/threonine pathway inhibition)
- All these signals are secreted and diffuses through the embryo, generating gradients
-These signals are also essential for neuroectoderm specification
What is the molecular control of the AP axis establishment in mouse?
- Amniotes are tetrapods vertebraes including reptiles and mammals
- The topology of the amniote embryo is ‘inside-out’, with the embryo being cylindrical
- Just prior to gastrulation, the embryo is formed by the epiblast surrounded by a layer of cells called the visceral endoderm/hypoblast
- The first indication of axis is establishment of distal visceral endoderm (DVE) at the distal part of visceral endoderm
- DVE expresses Lefty1 at its moment of specification
- Cells from DVE migrate from proximally and they recruit other cells that also start expressing Lefty1 (antagonist TGFB superfamily/Nodal - serine/threonine) and also DKK1 (antagonist of Wnt pathway)
- These cells migrate to the anterior visceral endoderm (AVE) - this is the future anterior region of embryo
- Hence as a result of AVE secreting Lefty1 and DKK1, there is repression of Wnt and Nodal in the surrounding tissue. As a result, Wnt and Nodal activity are confined to the opposite pole (posterior edge) which induces the formation of the primitive streak
- It’s important to mention that the first indication of the AP axis in the mouse embryo is the presence of AVE, not primitive streak.
- AVE is a reliable landmark for anterior pole
-BMP is expressed in the most proximal part of the developing embryo throughout the formation of DVE all the way to the primitive streak formation
- BMP is very important for DVE specification
- BMP facilitates Wnt and Nodal for specification of primitive streak at the posterior edge
- This early BMP4 activity promotes Nodal but represses Lefty1
- Nodal induces Lefty1 but BMP inhibits Lefty1
- This regulatory interaction leads to Lefty1 expression in the most distal portion of the visceral endoderm
What is neural induction in Xenopus and fish?
- Organiser is in the dorsal portion —> this the source of signals: chordin, noggin, follistatin (these are secreted antagonists of BMPs)
- BMPs are expressed in all ectoderm of developing embryonic
- BMP induce the formation of epidermis from ectoderm
- Chordin, Noggin, Follistatin repress BMP which leads to specification of neural fate in the region next to the organiser
- This is called the ‘default model’ of neural induction because the ectoderm defaults to an epidermal fate
- Neural induction requires antagonism of BMP4 by chordin, noggin and follistatin
- This is molecularly conserved from flies all the way up to mammals
What is neural induction in the mouse?
-Neural induction in the mouse also requires BMP antagonism
- A region of epiblast (distal) are specified as neuroectoderm
- This requires antagonism of BMP activity by chordin released from the gastral organiser (MGO) at the tip of the primitive streak
- This prevents the distal region of the epiblast being confined to epidermal fate due to BMP activity
What is neural induction in humans?
-The molecular signals from earlier are conserved
- The human embryo is flat, with a primitive streak extending from the caudal end to roster all end as gastrulation progresses
- When it reaches the rostral end, it starts to regress and lays down a structure called the notochord just below the ectoderm
- Notocord process (tip of notochord) and primitive streak are sources of neural inducers (BMP antagonists - chordin, noggin)
What are the responsibilities of the vertebrae ectoderm?
- The vertebrae ectoderm has 3 major responsibilities
1. One part of ectoderm becomes neural plate (presumptive neural tissue induced by prechordal plate and notochord). The neural plate involutes into the body to form the neural tube (precursor of CNS)
2. One part forms the epidermis
3. Presumptive neural crest is found between the compartments forming the epidermis and CNS. Neural crest cells migrate away to generate PNS, melanocytes and parts of the heart
What is ectoderm specification?
- Ectoderm specification occurs during gastrulation through regulation of BMP levels in ectodermal cells
- High levels of BMP specify ectodermal cells to become epidermis
- Intermediate levels of BMP specify ectodermal cells to become neural crest cells
- Low levels of BMP specify ectodermal cells to become neural plate (neuroectoderm)
- The processes by which the 3 ectodermal regions are made are distinct from each other - this is neurulation
- Embryo undergoing neurulation is called neurulation
- Neurulation directly follow gastrulation
What is Sox expression in the Neural plate?
- Cells of neural plate are characterised by expression of Sox family of transcription factors
- These factors activate genes that specify the cells to become the neural plate and inhibit the formation of epidermis and neural crest cells by blocking the transcription and signalling of BMPs
-The expression of Sox transcription factors establishes the neural plate and neural precursors that can form all the cell types in the CNS
What is the neural plate?
- 1st stage or neurulation is formation of neural plate
- Neural plate is thickened ectoderm along dorsal midline
- Gives rise to neural tube and neural crest
- Neural plate lies on surface of embryo but the future nervous system will be inside the mature body. Hence, the neural plate has to move inside the embryo and form the neural tube
- This is done through neurulation
-There is primary and secondary neurulation
What is primary neurulation and secondary neurulation?
Primary neurulation:
-Cells surrounding the neural plate direct the neural plate cells to proliferate, invaginate into the body and separate from the surface ectoderm to form a follow underlying tube
Secondary neurulation:
-Neural tube arises from aggregation of mesenchyme into a solid cord that can then form cavities which then forms a hollow tube
- Neurulation is complete when 2 separately formed tubes unite
- Size of transition zone is thoracolumbar in humans
- In many vertebrates, primary neurulation forms anterior neural tube; secondary neurulation forms posterior neural tube
- In mammals, secondary neurulation occurs at sacral level
What is primary neurulation in detail?
- After formation of neural plate, its edges thicken and move upwards to form the neural folds.
- This forms a U shaped groove in the centre of the plate, diving the future left and right sides of the embryo
- The neural folds migrate towards the midline of the embryo, fusing to form the neural tube beneath the overlying ectoderm
- There are 4 stages which overlap:
1) elongation and folding of neural plate=
- cell divisions within neural plate are preferentially on the AP axis
- this fuels continued axial elongation that was associated with gastrulation
2) Folding of neural plate
- formation of hinge regions where the neural plate contacts surrounding tissues
- in birds and mammals, cells at the midline of the neural plate form the median hinge point (MHP)
- MHP cells are firmly anchored to the underlying notochord, forming a hinge which enables the creation of a furrow which is the neural groove in the dorsal midline
3) convergence of the neural folds
- after the neural groove is created, 2 dorsolateral hinge points (DLHPs) are induced by and anchored to surface ectoderm
- The plate bends around these hinge regions
- each hinge point acts as a pivot, directing the rotation of cells around it
- continued convergence of surface ectoderm pushes towards midline of the embryo, providing another force that bends the neural plate, causing the convergence of the neural folds
- This movement of the presumptive epidermis and the anchoring of the neural plate to underlying mesoderm may also be important for ensuring that the neural tube invaginates inwards, not outwards
4) Closure of neural tube
- The neural tube closes as the paired neural folds come into contact in the dorsal midline
- the folds adhere to each other and the neural and surface ectoderm cells from one side fuse with their contralateral counterparts
- During this fusion, cells at the apex of neural folds delaminates to become the neural crest cells
How are hinge points formed and regulated?
Formation:
- MHP and DLHPs are 3 regions in the neural plate where epithelial cell shape changes occur
- The epithelial cells in these regions adopt a ‘wedged-shaped’ morphology along the apicobasal axis -they are wider basally than apically
- Similar to the bottle cells in gastrulation, localised contraction of actomyosin complexes at the apical border of the neuroepithelial cells of the hinge points causes apical constriction
- To facilitate this, nuclei are retained in the basal portion of cells
Regulation:
- We know that notochord induces cells of MHP to become wedge shaped
- Shh is produced in notochord and is required for the induction of floor plate cells in the neural plate, which is turn form the MHP
- In DLHP, Noggin appears to be critical for hinge formation
- In mice, loss of Noggin results in severe failure of neural tube closure
- Noggin is expressed in the neural folds and this expression alone is sufficient to induce DLHPs
- Noggin is also expressed in notochord
-Shh is produced by notochord which prevents ectopic hinges from forming
What is the role of BMP for hinge point formation?
- BMP inhibits hinge joint formation
- BMP is expressed by dorsal surface of ectoderm
- Noggin is an antagonist so it may seem reasonable to think that BMP inhibition results in changes in cell shape that lead to DLHP
- Experiments have shown that when BMP is constitutionally active, it binds to its receptors and prevents hinge joint formation; when BMP is completely repressed, there is ectopic and exaggerated MHP and DLHP formation
- This shows that intermediate levels of BMP is required for normal hinge formation. Neural plate cells at this level of BMP will undergo apical constriction and basal thickening - this occurs through a modification of the recruitment of proteins that stabilise junctions proteins and maintain size equality between the apical and basal membrane, preventing folding
- Lack of BMP (caused by Noggin) leads to relaxation of these junctions allowing apically restricted actomyosin contractions and a shortening of apical membrane
-Shh from notochordal plate prevents ectopic hinges from forming in the neural plate
How does the neural tube close?
- Closure of neural tube doesn’t occur simultaneously
- Best in amniotes whose body axis is elongated prior to gastrulation
- Neural tube closure in mammals is initiated at several points along AP axis (5 sites)
- Rostral closure site (1) is located at the junction between the spinal cord and hindbrain. It closes by bidirectional zipping of neural folds
- Closure site 2= midbrain/forebrain boundary. Zipper like mechanism and dynamic cell extension occurs
- Closure site 3= rostral forebrain. The DLHPs appear to be fully responsible for neural tube closure
How does the neural folds zip up?
- During DLHP bending, dynamic cell processes extend from the tips of the neural folds
- Inhibition of myosin prevents zipper advancement
What is fusion and separation of neural tube?
- The neural tube eventually forms a close cylinder that separates from the surface ectoderm
- This separation is mediated by expression of different cell adhesion molecules
- Neuroectodermal cells initially express E-Cadherin, though they stop expressing it as the neural tube forms
- They switch to N-Cadherin instead
- This means the surface ectoderm and neural tube tissues no longer adhere to each other
- The Grainyhead transcription factors are important to switch was E-cadherins to N-cadherins
-Mice with mutated Grainyhead-like2 and 3 have severe neural tube defects including exencephaly and spina bifida
What are neural tube defects?
- Failure of closure of posterior neuropore results in spina bifida - the severity depends on how much of the spinal cord remains exposed
- Failure to close sites 2 and 3 in the rostral neural tube keep the anterior neurones open, resulting in anencephaly which is usually lethal
- In anencephaly, the forebrain remains in contact with the amniotic fluid and subsequently degenerates. The fetal forebrain ceases development and the cranial vault fails to form
-Failure of the entire neural tube to close causes craniorachischisis
What factors affect neural tube closure?
- Can result from both genetic and environmental causes
- More than 300 genes play a role in neurulation
- Mutations in genes like Pax3, Shh, Grainyhead, Tfap2 and Openbrain show that these genes are essential for the formation of the mammalian neural tube
- Environmental factors like drugs, cholesterol and folate, diabetes, obesity and toxins can all affect neural tube closure
What is the role of folate?
- The early use of folic acid antagonists led to fetuses with neural tube defect
- Since then, many trials have shown clear correlations of neural tube defects with folic acid deficiency
- Folic acid is recommended for pregnant women
- Folic acid is important nutrient for regulating DNA synthesis during cell division in the brain and is critical in regulating DNA methylation
- Epigenetic mechanisms are essential for proper neural tube development
What is secondary neurulation?
-Secondary neurulation takes place in the most posterior region of embryo during tailbud elongation
- It involves production of mesenchymal cells from the prospective ectoderm and mesoderm, followed by condensation of these cells into a medullary cord beneath the surface ectoderm
- After mesenchymal to epithelial transition, the central portion of the medullary cord undergoes cavitation to form several lumens
- The lumens then coalesce into a single central cavity
Control of secondary neurulation:
- After Hensen’s node has migrated to the posterior end of embryo, the caudal region of the epiblast contains a precursor cell population that gives rise to both neural ectoderm and paraxial (somitic) mesoderm as the embryo’s trunk elongates
- The ectodermal cells that will form the posterior (secondary) neural tube express the Sox2 gene, whereas the ingressing mesodermal cells (which no longer encounter high levels or BMP as they migrate beneath the epiblast) don’t express Sox2.
- Instead, the ingressing mesodermal cells express Tbx6 and form somites
- Tbx6 transcription factor represses neural-inducing Sox2, preventing too many neural tubes from forming
-During primary neurulation, the surface ectoderm and neural ectoderm are intimately connected through the process of neural tube closure and fusion, whereas in secondary neurulation, these 2 tissues are essentially uncoupled and develop independently of each other