8/9/2019 How the Brain is Made
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COMPILED BY MEGAN OGILVIE GRAPHICS BY CATHERINE FARLEY/TORONTO
STAR
By the 16th day of development, theneural plate has emerged from
theoutermost layer of the embryo.This single sheet of cells will
giverise to the entire nervous system.At around 21 days, the neural
plate
buckles in the middle and the sheetof cells curves together to
form a
tube.
Neural tube formsAfter a sperm has fertilized an egg,the cell
divides until it becomes amulberry-shaped cluster of cells.As cells
continue to divide, somewill form an inner grouping, whichwill
become the embryo. Othercells will form an outer grouping,which
will become the embryo's
support tissue, including theplacenta.
Cluster of cellsDuring this rapid cell division, threevesicles
emerge from the frontend of the neural tube. The firstand third
vesicles divide to create atotal of five vesicles. These willbecome
the major portions of thebrain.The embryo now measures about
20 mm in length and all essentialorgans have begun to form.
Vesicles developAfter the tube has closed, thesingle layer of
cells begins torapidly divide. A newborn's brainhas 20 billion
neurons, whichmeans the fetal brain mustgenerate some 250,000
neuronsevery minute to meet thatdemand.
Rapid cell divisionThe tube then closes along themidseam, like a
zipper, followed bythe closure of the front end, thenthe rear
end.If the rear end of the neural tube
does not properly close, the fetuswill have some form of spina
bifida.
The tube closes
Neurons are being produced at arate of up to 50,000 per
second.The neurons, which are produced inthe middle of the brain,
migrateoutwards towards the surface andself-organize into different
brainstructures.The head makes up nearly half of
the fetus' size. Any movement seenin an ultrasound image is
merelyreflex and not deliberate exploring.
Neurons proliferateThe peripheral nervous system hasjust
connected with the cerebralcortex, the brain region responsiblefor
higher thought processesincluding memory, awareness
andlanguage.Fetal experts say it is now possible
for a fetus to have some level of
awareness. This is when a fetusbegins to respond to pain.
Nervous systemNeurons continue to generate,migrate and make
connections. Butin the last months of fetal life, thebrain starts
to prune back excessneurons, eliminating cells andconnections that
are weak orduplicate.By 32 weeks, a fetus has defined
sleep states, including deep, REMsleep. At this time, a preterm
infanthas developed the ability to see.
Sculpting neural pathwaysA newborn's brain has 20 billionneurons
and a trillion synapticconnections. Most pronounced inthe first 12
months, brain growthand development continues foryears after birth.
The brain is littlemore than one-quarter of its adultsize and the
higher regions of the
brain, including the cerebral cortex,are still primitive.
Birth
Growth of the embryo
Development of the fetus
Cell division
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Between the 20th and 24th weeks,neurons that have migrated to
theappropriate place now extendaxons and dendrites nerve fibresthat
send and receive electricalsignals to connect with otherneurons.
This forms the complexneural circuitry that allows
communication between cells.
Neurons make connections
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Neuraltube
Neural plate
