copy of development of the spinal cord

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Development of the spinal

cord



• The nervous system develops from an

area of embryonic ectoderm called the

neural plate which appears during week3.

• The underlying notochord and adjacent

mesoderm induce the formation of theneural 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 andnerves) 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 ofweek 4 (22-23 days) in the regionof the 4th to 6th pairs of somites

(future cervical region of thespinal cord;

• At this stage ,the cranial 2/3 of the

neural plate and neural tube downto somites #4 represent the brainand the caudal 1/3 of the neuraltube and plate represent thespinal cord.



• Neural folds fuse and the neural tube is

temporarily open at both ends,

communicating freely with the amnioticcavity.

• 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 convertedto the ventricular system of the brain and

the central canal of the spinal cord.



• The spinal cord is formed from the neural tubecaudal to somites 4.

• The central canal is formed by week 9 or 10 .

• Pseudostratified, columnar neuroepithelium inthe walls constitute the ventricular zone(ependymal layer) and give rise to all neuronsand macroglial cells (astroglia and

oligodendroglia) in the spinal cord.• The outer parts of the neuroepithelial cellsdifferentiate into a marginal zone which will giverise to the white matter of the spinal cord asaxons grow into it from neurons in the spinal

cord, spinal ganglia and brain.• Neuroepithelial cells in the ventricular zone

differentiate into neuroblasts and form anintermediate 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 ventricularzone 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 producethick walls and thin roof and floor

plates.

• A shallow longitudinal sulcus

limitans appears in the lateral

walls of the spinal cord andseparates 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 ofthe spinal cord to form the ventral

roots.

• The dorsal root ganglia are formedfrom the neural crest cells. Their

axons enter the spinal cord and form

the dorsal roots.



• Mesenchyme surrounding the

neural tube condenses to formthe primitive meninx.

• The outer layer thickens toform 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 theentire length of the vertebral canal and the

spinal nerves pass through the

intervertebral foramina near their levels oforigin.

• This relationship does not persist because

the spine and the dura mater grow morerapidly 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 totheir corresponding intervertebral

foramina inferiorly.



• Congenital malformations:

• are mostly due to the defective closure of

the caudal neuropore at the end of week4.

• 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 (nonfusionof the vertebral arches



• Spina bi f ida occu l ta.

• is a defect in the vertebral arch (neural arch)

resulting from failure of the halves of thevertebral 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 smallpercentage may have significant defects of the

underlying spinal cord and spinal roots.



• Spinal dermal sinus

• representing the area of closureof the caudal neuropore at theend of week 4, may exist.

• It is the last place of separationbetween the ectoderm and theneural tube.

• The dimple may be connected bya fibrous cord with the duramater.



• In t ramedu l lary dermoids are

tumors arising from surfaceectodermal 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 neuralarch.

• is present in 1/1000 births.

• may result in loss of sensation in

corresponding dermatome, complete or

partial skeletal muscle paralysis, sphincter

paralysis (with lumbarmeningomyeloceles) 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 thesac, due to incorporation of the neural tissue into

the wall of the sac.

• with myeloschisis (with myelocele: open spinalcord 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 alpha-fetoprotein in the amniotic fluid or in the

maternal blood serum.

• Amniocentesis or ultrasound should beperformed 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 itsrostral portion to form the two

telencephalic vesicles.

• These two vesicles grow rapidly to formthe two cerebral hemispheres.

• First they grow back over the

diencephalon, then they grow down tocover its sides.

•



A th i f i l ill l t



• Another pair of vesicles will also sprout

from the ventral surface of these cerebral

hemispheres to become the olfactory

bulbs and other structures that contributeto the sense of smell.

• Various structures will then emerge from

the walls of the telencephalon while thewhite matter that connects these

structures develops as well.

• The neurons of the telencephalon wallproliferate to form three distinct regions—

the cerebral cortex, the basal

telencephalon, and the olfactory bulb.



• The axons of these neurons will also graduallyelongate to make connections with the otherparts of the nervous system.

• Some of these axons will constitute the corticalwhite matter that arises from and projects toneurons in the cortex.

• Others will form the corpus callosum, the bandof nerve fibres that connects the twohemispheres of the brain. Still others—those ofthe internal capsule—will connect the corticalwhite matter to the brain stem, generally by wayof the thalamus.

• For example, the axons arising from the motorcortex will pass through the internal capsule toconnect to the motor neurons in the spinal cord.



• In the the remaining space between thetelencephalon and the diencephalon oneither side, the two cerebral ventricles(also known as the lateral ventricles or thefirst and second ventricle) form, while thethird ventricle forms in the space at the

centre of the diencephalon.

The diencephalon also differentiates intodistinct areas: the thalamus and the

hypothalamus. •

http://www.med.unc.edu/embryo_images/unit-nervous/nerv_htms/nerv017.htm

http://www.med.unc.edu/embryo_images/unit-nervous/nerv_htms/nerv017.htm



• 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 theretinas and the optic nerves.

• The retinas and the optic nerves are

therefore not part of the peripheralnervous system, but rather they are

integral parts of the brain!



• Compared with the prosencephalon(telencephalon and diencephalon), themesencephalon undergoes far lesstransformation.

• Its dorsal surface forms the tectum, whileits floor forms the tegmentum.

• While these structures are differentiating,the cavity that separates them shrinks to anarrow channel called the cerebral

aqueduct.• The rostral portion of this aqueduct opens

into the third ventricle of the diencephalon.



• The mesencephalon serves as the passagewayfor the bundles of fibres that connect the cortexto the spinal cord—both those that arise from the

sensory system and those that descend toparticipate in movement control.

The tectum differentiates into two structures.

One, the superior colliculus, receivesinformation directly from the eye and controlseye movements.

• The other, the inferior colliculus, receives

information from the ear and serves as animportant relay in the auditory pathways.



• The tegmentum is one of the mostcolourful areas of the brain.

• It contains the substantia nigra (“blackmatter”) and the red nucleus, twostructures that are involved in controllingvoluntary movement.

• Other groups of cells in themesencephalon project their axonsdiffusely into large areas of the brain and

influence a wide variety of functions, suchas consciousness, mood, pleasure andpain.

• Caudal to the mesencephalon lies the



Caudal to the mesencephalon lies themetencephalon, which is the rostralportion of the hindbrain and differentiatesinto two major structures: the cerebellum and the pons.

• The cerebellum arises from the thickeningof the tissue covering the lateral walls ofthe neural tube at this location.

• The two masses thus formed ultimatelyfuse dorsally to form the cerebellum.

• During this time, a swelling develops on

the ventral side of the metencephalon andforms the pons.

• This structure is an important informationpathway between the brain, the

cerebellum, and the spinal cord.



• In the the myelencephalon (the caudalportion of the hindbrain) the changes areless spectacular.

• The ventral and lateral regions of thisstructure swell to form the medullaoblongata.

• Along the ventral aspect of the medulla,the two medullary pyramids will alsodevelop, formed by the passage of the

corticospinal bundles responsible forvoluntary movement.

• Lastly, the central canal, which persistswhile 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 cordthrough 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 thegrey matter at the centre (the butterfly-

shaped 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



Likewise, within the white matter, theredevelop dorsal columns composed ofsensory axons that ascend to the brain

and lateral columns composed ofcorticospinal axons that descend totransmit signals for controlling movement.

• Between the dorsal and ventral horns, alarge number of interneurons also developthat are involved in various types ofreflexes as well as in establishing

networks that perform initial processing ofthe information received in the spinal cord.

copy of development of the spinal cord

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