Transcript Development of the spinal cord

Development of the spinal
cord
• The nervous system develops from an
area of embryonic ectoderm called the
neural plate which appears during week
3.
• The underlying notochord and adjacent
mesoderm induce the formation of the
neural plate.
• The neural tube and the neural crest
differentiate from the neural plate.
• The neural tube gives rise to the
central nervous system (brain and
spinal cord; .
• The neural crest gives rise to the
peripheral nervous system (cranial,
peripheral, autonomic ganglia and
nerves) and Schwann cells, pigment
cells, odontoblasts, meninges, and
bones and muscles of the head .
Central nervous system
• Formation of the neural tube
begins during the early part of
week 4 (22-23 days) in the region
of the 4th to 6th pairs of somites
(future cervical region of the
spinal cord;
• At this stage ,the cranial 2/3 of the
neural plate and neural tube down
to somites #4 represent the brain
and the caudal 1/3 of the neural
tube and plate represent the
spinal cord.
• Neural folds fuse and the neural tube is
temporarily open at both ends,
communicating freely with the amniotic
cavity.
• The rostral neuropore closes around day
25 and caudal neuropore on day 27.
• Walls of the neural tube thicken to form the
brain and spinal cord.
• The lumen of the neural tube is converted
to the ventricular system of the brain and
the central canal of the spinal cord.
• The spinal cord is formed from the neural tube
caudal to somites 4.
• The central canal is formed by week 9 or 10 .
• Pseudostratified, columnar neuroepithelium in
the walls constitute the ventricular zone
(ependymal layer) and give rise to all neurons
and macroglial cells (astroglia and
oligodendroglia) in the spinal cord.
• The outer parts of the neuroepithelial cells
differentiate into a marginal zone which will give
rise to the white matter of the spinal cord as
axons grow into it from neurons in the spinal
cord, spinal ganglia and brain.
• Neuroepithelial cells in the ventricular zone
differentiate into neuroblasts and form an
intermediate zone between the ventricular and
marginal zones. They will give rise to neurons.
• Glioblasts (spongioblasts) differentiate from
neuroepithelial cells after neuroblast formation
has stopped. They migrate from the ventricular
zone into the intermediate and marginal zones.
• Some become astroblasts and then astroglia
(astrocytes). Others become oligodendroblasts
and then oligodendroglia (oligodendrocytes).
The remaining neuroepithelial cells differentiate
into ependymal cells lining the central canal of
the spinal cord
• Microglia are derived from the mesenchymal
cells. They invade the nervous system late in
the fetal period after penetration from blood
vessels.
• Proliferation and differentiation of
the neuroepithelial cells in the
developing spinal cord produce
thick walls and thin roof and floor
plates.
• A shallow longitudinal sulcus
limitans appears in the lateral
walls of the spinal cord and
separates the dorsal alar plate
from the ventral basal plate
• Alar plates: cells form the dorsal
horns and will have afferent
functions.
• Basal plates: cells form the ventral
and lateral horns and will have
efferent functions. Axons grow out of
the spinal cord to form the ventral
roots.
• The dorsal root ganglia are formed
from the neural crest cells. Their
axons enter the spinal cord and form
the dorsal roots.
• Mesenchyme surrounding the
neural tube condenses to form
the primitive meninx.
• The outer layer thickens to
form the dura mater.
• The inner layer remains thin
and forms the pia-arachnoid
• Positional changes of the developing
spinal cord
• In the embryo, the spinal cord extends the
entire length of the vertebral canal and the
spinal nerves pass through the
intervertebral foramina near their levels of
origin.
• This relationship does not persist because
the spine and the dura mater grow more
rapidly than the spinal cord. The caudal
end of the spinal cord comes to lie at
relatively higher levels.
• Positional changes of the
developing spinal cord.
• At month 6 of gestation, the end of
the spinal cord lies at the level of S1.
• In the newborn infant, it lies at L 3
• In the adult, it lies at L 1.
• Lumbar and sacral spinal nerve roots
run obliquely from the spinal cord to
their corresponding intervertebral
foramina inferiorly.
• Congenital malformations:
• are mostly due to the defective closure of
the caudal neuropore at the end of week
4.
• The defects will involve the tissue
overlying the spinal cord (meninges,
vertebral arch, dorsal muscles and skin).
• involving the spinal cord and vertebral
arches are called spina bifida (nonfusion
of the vertebral arches
• Spina bifida occulta.
• is a defect in the vertebral arch (neural arch)
resulting from failure of the halves of the
vertebral arch to grow normally and fuse in the
median plane.
• occurs at L 5 or S 1 vertebra in about 10% of the
population.
• may only be evident as a small dimple with a tuft
of hair.
• produces no clinical symptoms although a small
percentage may have significant defects of the
underlying spinal cord and spinal roots.
• Spinal dermal sinus
• representing the area of closure
of the caudal neuropore at the
end of week 4, may exist.
• It is the last place of separation
between the ectoderm and the
neural tube.
• The dimple may be connected by
a fibrous cord with the dura mater.
• Intramedullary dermoids are
tumors arising from surface
ectodermal cells incorporated
into the neural tube during
closure of the caudal
neuropore.
• Spina bifida cystica
• is a protrusion of the spinal cord and/or
meninges through the defective neural
arch.
• is present in 1/1000 births.
• may result in loss of sensation in
corresponding dermatome, complete or
partial skeletal muscle paralysis, sphincter
paralysis (with lumbar
meningomyeloceles) and saddle
anesthesia.
• Spina bifida
• with meningocele: only meninges and
cerebrospinal fluid in the sac.
• with meningomyelocele : spinal cord and nerve
roots included with meninges and CSF in the
sac, covered by skin or thin membrane. There
are marked neurological deficits inferior to the
sac, due to incorporation of the neural tissue into
the wall of the sac.
• with myeloschisis (with myelocele: open spinal
cord due to failure of neural folds to fuse. The
spinal cord in this area is a flattened mass.
• cystica and/or meroanencephaly (absence
of part of the brain; is suspected in utero
when there is a high-level of alphafetoprotein in the amniotic fluid or in the
maternal blood serum.
• Amniocentesis or ultrasound should be
performed at about week 10 when the
vertebral column becomes visible.
• The telencephalon is the most rostral of
the secondary vesicles.
• Two buds emerge from either side of its
rostral portion to form the two
telencephalic vesicles.
• These two vesicles grow rapidly to form
the two cerebral hemispheres.
• First they grow back over the
diencephalon, then they grow down to
cover its sides.
•
• Another pair of vesicles will also sprout
from the ventral surface of these cerebral
hemispheres to become the olfactory
bulbs and other structures that contribute
to the sense of smell.
• Various structures will then emerge from
the walls of the telencephalon while the
white matter that connects these
structures develops as well.
• The neurons of the telencephalon wall
proliferate to form three distinct regions—
the cerebral cortex, the basal
telencephalon, and the olfactory bulb.
• The axons of these neurons will also gradually
elongate to make connections with the other
parts of the nervous system.
• Some of these axons will constitute the cortical
white matter that arises from and projects to
neurons in the cortex.
• Others will form the corpus callosum, the band
of nerve fibres that connects the two
hemispheres of the brain. Still others—those of
the internal capsule—will connect the cortical
white matter to the brain stem, generally by way
of the thalamus.
• For example, the axons arising from the motor
cortex will pass through the internal capsule to
connect to the motor neurons in the spinal cord.
• In the the remaining space between the
telencephalon and the diencephalon on
either side, the two cerebral ventricles
(also known as the lateral ventricles or the
first and second ventricle) form, while the
third ventricle forms in the space at the
centre of the diencephalon.
The diencephalon also differentiates into
distinct areas: the thalamus and the
hypothalamus.
•
• On either side of the diencephalon, two
secondary vesicles also develop—the
optic vesicles.
• The optic vesicles lengthen and fold
inward to form the optic peduncles and
optic cups, which will give rise to the
retinas and the optic nerves.
• The retinas and the optic nerves are
therefore not part of the peripheral
nervous system, but rather they are
integral parts of the brain!
• Compared with the prosencephalon
(telencephalon and diencephalon), the
mesencephalon undergoes far less
transformation.
• Its dorsal surface forms the tectum, while
its floor forms the tegmentum.
• While these structures are differentiating,
the cavity that separates them shrinks to a
narrow channel called the cerebral
aqueduct.
• The rostral portion of this aqueduct opens
into the third ventricle of the diencephalon.
• The mesencephalon serves as the passageway
for the bundles of fibres that connect the cortex
to the spinal cord—both those that arise from the
sensory system and those that descend to
participate in movement control.
The tectum differentiates into two structures.
One, the superior colliculus, receives
information directly from the eye and controls
eye movements.
• The other, the inferior colliculus, receives
information from the ear and serves as an
important relay in the auditory pathways.
• The tegmentum is one of the most
colourful areas of the brain.
• It contains the substantia nigra (“black
matter”) and the red nucleus, two
structures that are involved in controlling
voluntary movement.
• Other groups of cells in the
mesencephalon project their axons
diffusely into large areas of the brain and
influence a wide variety of functions, such
as consciousness, mood, pleasure and
pain.
• Caudal to the mesencephalon lies the
metencephalon, which is the rostral
portion of the hindbrain and differentiates
into two major structures: the cerebellum
and the pons.
• The cerebellum arises from the thickening
of the tissue covering the lateral walls of
the neural tube at this location.
• The two masses thus formed ultimately
fuse dorsally to form the cerebellum.
• During this time, a swelling develops on
the ventral side of the metencephalon and
forms the pons.
• This structure is an important information
pathway between the brain, the
cerebellum, and the spinal cord.
• In the the myelencephalon (the caudal
portion of the hindbrain) the changes are
less spectacular.
• The ventral and lateral regions of this
structure swell to form the medulla
oblongata.
• Along the ventral aspect of the medulla,
the two medullary pyramids will also
develop, formed by the passage of the
corticospinal bundles responsible for
voluntary movement.
• Lastly, the central canal, which persists
while the medulla is forming, becomes the
fourth ventricle
• The entire portion of the neural tube
that lies caudal to the five secondary
vesicles becomes the spinal cord
through a fairly direct process of
differentiation consisting in the
thickening of the tube walls.
• This thickening gradually reduces the
diameter of the neural tube until it
becomes the very narrow spinal
canal.
• As the cross-section shown here
illustrates, the cell bodies of the neurons in
the spinal cord are concentrated in the
grey matter at the centre (the butterflyshaped area), while the white matter at
the periphery is composed of bundles of
axons.
• The grey matter of the spinal cord is in
turn divided into the dorsal horn, which
receives sensory inputs, and the ventral
horn, whose neurons innervate the
skeletal muscles.
• Likewise, within the white matter, there
develop dorsal columns composed of
sensory axons that ascend to the brain
and lateral columns composed of
corticospinal axons that descend to
transmit signals for controlling movement.
• Between the dorsal and ventral horns, a
large number of interneurons also develop
that are involved in various types of
reflexes as well as in establishing
networks that perform initial processing of
the information received in the spinal cord.