bio lecture 12 - nervous system: neural tube formation and patterning

establishing neural cells

a portion of the dorsal ectoderm is specified to become neural ectoderm

- this region of the embryo is called the neural plate

- the neural plate with form the neural tube - an early nervous system

- the process by which these changes occur is called neurulation and the embryo at this stage is called a neurula

formation of the neural tube

there are 2 major ways of converting the neural plate into a neural tube (neurulation):

1. primary neurulation

2. secondary neurulation

primary neurulation

cells around neural plate direct the neural plate cells to proliferate, invaginate, and pinch off from the surface to form a hollow tube

secondary neurulation

neural tube arises from the coalescence of mesenchyme cells into a solid cord that subsequently forms a hollow tube

primary neurulation divides the original ectoderm into 3 sets of cells:

1. internally positioned neural tube that will form the brain and spinal cord

2. externally positioned ectoderm that will form the skin

3. the neural crest cells that will form PNS and other structures (adrenal medulla, melanocytes, dentine of teeth)

primary neurulation divided into 4 events that overlap spatially and temporally:

1. extension and folding of the NP

2. bending of the NP

3. convergence of the neural folds to form the groove

4. closure of the neural folds to form a tube

extension and folding of the neural plate

- the neural plate is shaped by the movements of epidermal and NP regions

- the NP lengthens along the A-P axis and narrows by convergent extension

- bending of the NP involves hinge regions: medial hinge point, dorsolateral hinge point

- hinge cells take on a wedge shape-creates furrow-buckling point

Convergence and closure of the neural plate

- pushing from epidermal cells also provides force

- neural tube closes as NFs are brought together through convergence at the DLHP

- closure of NT does not occur simultaneously, rather it zips up both anteriorly and posteriorly

- open ends are called the anterior and posterior neuropores

- closure failure results specific neural tube. NT defects occur in about 1:1000 births

Neurulation in the human embryo

neural tube defects include:

- exencephaly

- spina bifida

- anencephaly

changes in cadherins are necessary for separation of neural tube form presumptive epidermis

- cells that will become NT originally express e-cadherin, but change their expression as NT forms

- this allows separation of surface ectoderm from NT- they won't stick to each other any long

Folate is a critical factor for NT closure

- expression of folate receptor in the mouse embryo

Secondary neurulation

mesenchyme --> condensation to form medullary chord --> cavitation to form several lumens --> coalesce into a single tube