How does the brain grow?

Vishal Dhokia vishal at ddhokia.fsnet.co.uk
Sun Oct 21 09:24:03 EST 2001


The CNS originates from the ectoderm (one of the three primary germ layers
in the embryo).  It is internalised during neurulation to form the neural
tube inside the embryo along the dorsal surface.

The anterior neural tube is divided into 3 primary vesicles (which become
the ventricles) - forebrain, nidbrain, hindbrain.  Spemann and Mangold
showed that underlying mesoderm from different positions along the ant-post.
axis induce structures specific to its region of origin i.e. meural tube is
broadly patterend by signals from underlying mesoderm.  Patterning along the
ant-post. axis involves expression of Hox (homeobox) genes (due to a
retinoic acid gradient) that specifiy positional identity along this axis.
These broad regions then become further subdivided by interaction between
cells of the CNS e.g. hindbrain subdivided into rhombomeres giving rise to
segmentally arranged cranial nerves.

Dorso-ventral axis is specified by signals from adjacant tissue - in the
spinal cord the epidermis induces dorsal identity (BMP4 or BMP7) and the
notocord induces ventral identity (sonic hedgehog).


The embryonic telencephalon consistys of two layers:
marginal (future pial surface)
ventricular zones

Neuroblasts (neuron precursors) divide in the venticular zone, then after
their terminal division migrate outwards to the marginal zone along radial
glia.  The innermost layer  (of the eventual adult striate cortex) is
generated first and is displaced inward by the next layer to form, and so
on.  Thus, late-born cells migrate through the early-born layers.  Neurons
then beginin to elaborate their dendritic and axonic processes after
reaching their correct layer.

Axons make synapses with neurons hundreds of microns away - this navigation
poses a potential problem.  They navigate by responding to guidance cues in
the environment through specific receptors in the membrane: how a growth
cone behaves when it encounters a guidance cue depends on its unique
complement of receptors.  These cues can be long/sort range and
attractive/repulsive depending on the receptor.  Information is mapped
topographically in the CNS - 2 orthogonal cytochemical gradients can provide
cells with precise lattitude and longitude - Eph receptors/ephrins.




"Liar42" <liar42 at aol.com> wrote in message
news:20011004073404.00227.00001731 at mb-fp.aol.com...
> Neuron numbers can change both directions.
>
> Concerning how the brain grows for a mammal I dimly recall some spine area
(I
> hope I recall that right) forming into sort of a tube, and from the upper
part
> then grows the brain, and also there was something with sort of long
"lines"
> (glia?) along which neurons moved to their later brain positions.
>
> If I recall right using an area a certain way a lot and feelings of
success
> might increase cell numbers in the brain (but I forget for what cells that
all
> went), while non-use and feelings like the opposite of success can
decrease.
>
>
> ... But don't nail me down on parts that I wrote; it is a long time that I
read
> about that.





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