neural development 2

Question 1

what end of the neural tube closes first

Answer 1

the head

Question 2

how does the neural tube change shape by elongation

Answer 2

it folds at the median hinge point and slides to elevate and converge at the dorso lateral hinge point
when the tube closes neural cells migrate

Question 3

how does cell shape drive the primary neural tube closure

Answer 3

cells elongate so neural plate thickens = chaping
local cell shape changes around the median hinge point, cells anchor to the notochord and change their shape, presumptive epidermal cells move centrally = folding
elevation of neural folds involves cells shortening at the apical side, microfilaments appear in cells as this takes place and are required for apical constriction, this facilitates bending the tissue - can be blocked by actin/myosins inhibitors = elevation
dorso-lateral hinge points become wedge-shaped and promote closure = bending/ closure

Question 4

what is secondary neurulation

Answer 4

in caudal regions a tail bud forms and sacral neural tube form not by a neural plate rolling up but from a solid rod of mesenchyme cells
posterior neural tube beyond the lumbo-sacral region develops from stem like cells in the tailbud which forms a solid rock of mesenchymal cells and undergoes mesenchymal to epithelial transition and develops an interior cavity or lumen that connects with that of the anterior neural tube

Question 5

differences between primary and secondary neurulation

Answer 5

primary is rolling of a flat neuroepithelium/neural plate for form a tube
secondary involved cavitation of central core of mesenchymal cells which epithethelialise to form a neural tube
primary accounts for most formation of the CNA secondary accounts for caudal most region and lumbo-sacral nervous system

Question 6

when are where does neural tube closure occur

Answer 6

in humans closure occurs at the cranio rachischisis and the lumbosacral spine bilida
in mice it occurs at these 2 points and at anenoephaly

Question 7

neural tube defects

Answer 7

1-2 in 100 are effected, defects are established during pregnancy and are the 2nd most common birth defects
failure of cranial neurulation, exencephaly, anencephaly, failed spinal neurulation, early spina bifida and myeoceli

Question 8

example of exencephaly

Answer 8

KO of the gene Sycthe that controls apoptosis results in over proliferation and resistance to apoptosis in the developing brain

Question 9

gene functions impacting neurulation

Answer 9

NTDs are multi-factoral with genes and environment interacting to determine individual risk of malformation
over 200 mutant genes cause open NTDs in mice but much less is known about what causes human NTDs
recent evidence have implicated genes for planar cell polarity signalling pathways as a cause in a proportion of cases
other signals implicated include Shh signalling pathway which regulates neural fold bonding

Question 10

why do planar cell polarity pathway mutants fail to undergo convergent extension movements at neural plate stage

Answer 10

wide neural plate prevent neural folds contacting eachother = craniorachischisis
individual mutation of any of these key PCP pathway genes leads to the most profound neural tube defects

Question 11

Shh is a major signalling pathway in regulating neural development

Answer 11

Shh secreted protein expressed by notochord and floor plate/ median hinge point

Question 12

evidence for the envolvement of Shh signalling in neurulation

Answer 12

loss of Shh does not generate NTDs but embryos lack ventral midline structures, however these are not critical for mechanical closure of the neural tube
over active Shh - neural tube becomes ventralised and without dorsal region it fails to close = open spinal chord and brain (excencephaly)
Shh also promotes proliferation and regulates apoptosis so these actions may also contribute to this closure defect
Shh signalling additionally inhibits formation of dorsolateral hinge point - this may reflect changes of cell-cell adhesion and matrix interactions

Question 13

environmental factors contributing to neural tube defects

Answer 13

diabetes: maternal hypoglycaemia - neuro epithelial cell death
hyperthermia: maternal fever in weeks 3-4 during pregnancy
obesoty: maternal obesity - mechanism unknown
micronutrient deficiency - zinc - mechanism unknown , inositol - disturbance in phosphorylation downstream of PKC, folate - disturbance in folate related metabolism, vitB12 - disturbance in folate related metabolism

Question 14

prevention of neural tube defects

Answer 14

since fortification of flour the NTDs in the US has decreased by 20% overall
it is most likely sporadic human NTDs result from 2 or more heterozygous genetic risk factors coexisting in individuals who are also exposed to adverse environmental influences such as folate deficiency

Question 15

neurogenesis - generation neurons

Answer 15

lateral inhibitions ensures cells do not differentiate

1. proneural cluster - group of cells in the neuroepithelium in direct contact with eachother
2. lateral inhibition - central cell inhibits neighbouring cells from differentiating
3. neuronal precursor - central cell differentiates into a neuron, neighbouring cells may undergo further division

Question 16

molecular basis of lateral inhibition

Answer 16

proneural cluster - proneural gene Ngnr-1 defines cells that can form neurons
lateral inhibition - Ngnr-1 induces expression of delta-1 but one cell expresses this gene quicker than its neighbours
neuronal precursor - delta-1 presented on the surface of the cells by signalling through the notch receptor - notch signalling inhibits differentiation through a feedback look

Question 17

lateral inhibition feedback loop

Answer 17

at first cells express neurogenin, notch receptord and low levels of delta-1
notch signalling inhibits neurogenin which normally induces delta-1
when one cell comes to express slightly more delta 1 this increases notch signalling in neighbouring cells
cells with high notch signalling will have less neurogenin and less delta-1
the central cell will therefore receive less inhibition and will therefor have increasing levels of neurogenin and delta-1
neurogenin also induces neuronal differential genes such as neuroD

Question 18

gene hierarchy leading to neuronal differentiation

Answer 18

the proneural gene neurogenin not only induces delta-1 but it also promotes expression of neuronal differentiation genes such as neuroD
frog embryo used to test gene function - neuroD expressed in differentiating neurons, neurogenin injected side expressed ectopic neuroD
neuroD transforms epidermis into neurons

Question 19

how is the cortex built

Answer 19

symmetric and asymmetric division
radial glial cells can divide asymmetrically expanding the progenitor pool
the can also divide asymmetrically to generate a neuron and a progenitor - direct neurogenesis
radial glial cells can divide asymmetrically to generate a progenitor and an intermediate progenitor - divides to make 2 neurons - indirect neurogenesis
cerebral cortex is a layered structure

Question 20

generation of neuronal layers in mammalian cortex

Answer 20

neurons born at early stages of cortical development migrate to the layers closest to their site of birth
neurons born later end up further away in more superficial layers
newer neurons must migrate through the older ones on their way to the correct position - this gives rise to layers and columns of cell bodies
the exception to this is the first formed layer which remains as the outer layer

Question 21

mice lacking in reeling do not form cortical layers, why?

Answer 21

disrupts cortical neuronal migration
have poor motor coordination as cortical layers are in reverse
reeler gene encodes for an extracellular matrix molecule reelin which is normally expressed in the first formed outer layer of the cortex
mutations in reeler that cause loss of reelin proteins disrupt radial neuronal migration with successive waves of neurons being unable to pass through the layer formed by their predecessors
brain neurons also migrate tangentially within the wall of the neural tube in a process independent of radial glial cells

Question 22

mechanisms of controlling neuron migration in the cortex

Answer 22

1. go signals - BDNF, GABA, TGFalpha
2. interaction with radial glia - Astn1, alpha 3, beta 1
3. movement - Lis 1, Dcx, Filamin2
4. laminar arrangement and detachment - reelin, VLDLR, LRP8, Dab1, Cdk5, p35/39