Download THE NEURAL TUBE AND ITS SUBDIVISIONS

DEVELOPMENT OF THE
CNS
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INTRODUCTION
• The central nervous system (CNS) appears
at the beginning of the third week as a
slipper-shaped plate of thickened
ectoderm, the neural plate, in the
middorsal region in front of the primitive
node. Its lateral edges soon elevate to form
the neural folds
• With further development, the neural folds
continue to elevate, approach each other in
the midline, and finally fuse, forming the
neural tube
• The notochord (a midline rod of cells)
stimulates the overlying ectoderm to
differentiate into neuroectoderm through
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a process known as induction.
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THE NEURAL TUBE…..
• NOTE: Stages of formation of
neural tube do not proceed
simultaneously all over the
length of the neural plate.
• The middle part is the first to
become tubular, so for some
time the neural tube is open
cranially and caudally to form
(neuropores)
• Once fusion is initiated, the open
ends of the neural tube form the
cranial and caudal neuropores
that communicate with the
overlying amniotic cavity G.LUFUKUJA
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THE NEURAL TUBE…..
• Closure of anterior (cranial) neuropore occurs on 25th day, where
as the posterior (caudal) neuropore closes on 27th day.
• Even before the neural tube has completely closed, it is divided
into an enlarged cranial part and a caudal tubular part.
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CONGENITAL ANOMALIES OF THE SPINAL CORD
(Spina bifida)
Spina bifida (Latin: "split spine") is a developmental congenital
disorder caused by the incomplete closing of the embryonic
neural tube.
1.spina bifida occulta,
2.spina bifida cystica and
3.spina bifida rachischisis.
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Spina bifida occulta
• Occulta is Latin for "hidden". Many people with this type of spina
bifida do not even know they have it, as the condition is
asymptomatic in most cases
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Spina bifida cystica
• This is a developmental defect of the central nervous system in
which a hernial cyst containing meninges (meningocele), spinal
cord (myelocele), or both (myelomeningocele) protrudes through
a congenital cleft in the vertebral column
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Spina bifida with myeloschisis
(rachischisis).
• This is often a severe or complete defect involving the entire spine
from the cervical region through to the sacrum
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CONGENITAL ANOMALIES
• ANENCEPHALY- Is failure of closure of the anterior
neuropore. The brain substance is exposed to the
surface as an irregular degenerated mass of nervous tissue
with no bony covering. The appearance is characterized
by; prominent eyes which bulge forwards and the chin is
continuous with the chest due to the absence of the neck.
The condition is incompatible with life
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THE SPINAL CORD
• NEUROEPITHELIAL,
MANTLE,
AND
MARGINAL
LAYERS
• The spinal cord is developed from the caudal part of the
neural tube, caudally to the fourth pair of somites.
• The wall of a recently closed neural tube consists of
neuroepithelial cells. These cells extend over the entire
thickness of the wall and form a thick pseudostratified
epithelium (neuroepithelium).
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THE SPINAL CORD…
• Once the neural tube closes, neuroepithelial cells begin to give
rise to another cell type characterized by a large round nucleus
with pale nucleoplasm and a dark-staining nucleolus. These are
the primitive nerve cells, or neuroblasts. They form the mantle
layer, a zone around the neuroepithelial layer . The mantle
layer later forms the gray matter of the spinal cord.
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THE SPINAL CORD…
• The outermost layer of the spinal cord, the marginal layer,
contains nerve fibers emerging from neuroblasts in the mantle
layer. As a result of myelination of nerve fibers, this layer takes on
a white appearance and therefore is called the white matter of the
spinal cord
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THE SPINAL CORD…
• BASAL, ALAR, ROOF, AND FLOOR PLATES
• As a result of continuous addition of neuroblasts to the mantle
layer, each side of the neural tube shows a ventral and a dorsal
thickening. The ventral thickenings, the basal plates, which
contain ventral motor horn cells, form the motor areas of the
spinal cord; the dorsal thickenings, the alar plates, form the
sensory areas
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THE SPINAL CORD…
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THE SPINAL CORD…
• GLIAL CELLS
• The majority of primitive supporting cells, the gliablasts, are
formed by neuroepithelial cells after production of neuroblasts
ceases. Gliablasts migrate from the neuroepithelial layer to the
mantle and marginal layers. In the mantle layer, they differentiate
into protoplasmic astrocytes and fibrillar astrocytes
• In the second half of development, a third type of supporting cell,
the microglial cell, appears in the CNS. This highly phagocytic
cell type is derived from mesenchyme.
• Another type of supporting cell possibly derived from gliablasts is
the oligodendroglial cell. This cell, which is found primarily in
the marginal layer, forms myelin sheaths around the ascending
and descending axons in the marginal layer.
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THE SPINAL CORD…
• When neuroepithelial cells cease to produce neuroblasts and
gliablasts, they differentiate into ependymal cells lining the
central canal of the spinal cord.
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THE SPINAL CORD…
• Neural Crest Cells & Myelination of PNS
• During elevation of the neural plate, a group of cells appears along
each edge (the crest) of the neural folds. These neural crest cells
are ectodermal in origin and extend throughout the length of the
neural tube. Crest cells migrate laterally and give rise to sensory
ganglia (dorsal root ganglia) of the spinal nerves and other cell
types
• Schwann cells myelinate the peripheral nerves. These cells
originate from neural crest, migrate peripherally, and wrap
themselves around axons, forming the neurilemma sheath.
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THE SPINAL CORD…
• Spinal Nerves
• Motor nerve fibers begin to appear in the fourth week, arising from
nerve cells in the basal plates (ventral horns) of the spinal cord. These
fibers collect into bundles known as ventral nerve roots.
• Dorsal nerve roots form as collections of fibers originating from cells
in dorsal root ganglia (spinal ganglia). Central processes from these
ganglia form bundles that grow into the spinal cord opposite the
dorsal horns.
• Distal processes join the ventral nerve roots to form a spinal nerve.
Almost immediately, spinal nerves divide into dorsal and ventral
primary rami. Dorsal primary rami innervate dorsal axial
musculature, vertebral joints, and the skin of the back. Ventral primary
rami innervate the limbs and ventral body wall and form the major nerve
plexuses (brachial and lumbosacral).
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POSITIONAL CHANGES OF THE CORD
• In the third month of development the spinal cord extends the
entire length of the embryo, and spinal nerves pass through the
intervertebral foramina at their level of origin.
• With increasing age, the vertebral column and dura lengthen
more rapidly than the neural tube, and the terminal end of the
spinal cord gradually shifts to a higher level.
• Note: The dura remains attached to the vertebral column at the
coccygeal level
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Positional Changes of the Spinal Cord
8- weeks – Entire length of
vertebral canal
24-weeks – S1
Newborn – L3
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Adult – L1/L2
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Positional Changes …
• In the adult, a threadlike
extension of the pia mater forms
the filum terminale, which is
attached to the periosteum of the
first coccygeal vertebra and
which marks the tract of
regression of the spinal cord.
• Nerve fibers below the terminal
end of the cord collectively
constitute the cauda equina.
When cerebrospinal fluid is
tapped during a lumbar puncture,
the needle is inserted at the lower
lumbar level, avoiding the lower
end of the cord.
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THE BRAIN (primary brain vesicles)
• The brain is developed from the anterior end of the neural tube
cranial to the fourth pair of somites.
• Fusion of the neural folds in the cranial region and closure of the
rostral neural pore form the three primary brain vesicles from
which the brain develops.
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Development of the brain…
• During the fifth week, the fore brain partly divides into two
secondary brain vesicles, the telencephalon and diencephalons;
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Brain flexures
• During the fourth week the embryonic brain grows rapidly and
bends ventrally with the head fold. This produces the three brain
flexures which are; mid brain (cephalic) flexure in the mid brain
region, pontine flexure, and the cervical flexure at the junction of
the hind brain and spinal cord.
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HOLOPROSENCEPHALY
• The forebrain does not properly divide into two hemispheres
as it should. A child born with holoprosecephaly will have a single
lobed brain with severe defects of the skull and face. Babies with
this condition have abnormalities of the eyes nose and upper
lip.
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Myelencephalon/ medulla oblongata
• The myelencephalon is a brain vesicle that gives rise to the
medulla oblongata. It differs from the spinal cord in that its
lateral walls are everted. Alar and basal plates separated by the
sulcus limitans can be clearly distinguished.
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Myelencephalon…The basal plate
• The most lateral is the somatic afferent group: receives impulses
from the ear and surface of the head via the staticoacoustic (VIII)
and bulbospinal part of the trigeminal (V) nerves
• The intermediate is the special visceral afferent group: receives
impulses from the taste buds of the tongue and from the palate,
oropharynx, and epiglottis. These neurons later form the nucleus of
the solitary tract
• The medial is the general visceral afferent group: represented by the
dorsal sensory nucleus of the vagus (X) nerve with its neurons
receiving interoceptive information from the heart and
gastrointestinal tract.
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Myelencephalon …The alar plate
• The roof plate of the myelencephalon consists of a single layer of
ependymal cells which is later covered by vascular mesenchyme, the
pia mater. Together they make up the tela choroidea. As a result
of active proliferation of vascular mesenchyme, the tela choroidea
forms a series of saclike invaginations that project into the
underlying ventricular cavity in the region of the pontine flexure,
forming the choroid plexus.
• At about month 4, areas of the roof plate of the rhombencephalon
thin out, bulge outward, and finally disappear. The apertures formed
are the 2 lateral foramina of Luschka and a median foramen of
Magendie which allow the cerebrospinal fluid to move freely
between the ventricles and the surrounding subarachnoid space
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Metencephalon
• The metencephalon is the embryonic part of the hindbrain that
differentiates into the pons and the cerebellum. It contains a
portion of the fourth ventricle and the trigeminal nerve (CN
V), abducens nerve (CN VI), facial nerve (CN VII), and a portion
of the vestibulocochlear nerve (CN VIII)
• The marginal layer of the basal plates of the metencephalon
expands as it makes a bridge for nerve fibers connecting the
cerebral cortex and cerebellar cortex with the spinal cord. Hence
this portion of the metencephalon is known as the pons (bridge).
• In addition to nerve fibers, the pons contains the pontine nuclei,
which originate in the alar plates of the metencephalon and
myelencephalon
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Metencephalon…
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Cerebellum
• The dorsolateral parts of the alar plates bend medially and form
the rhombic lips. In the caudal portion of the metencephalon,
the rhombic lips are widely separated, but immediately below
the mesencephalon they approach each other in the midline.
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Cerebellum…
• A transverse fissure soon separates the nodule from the vermis
and the lateral flocculus from the hemispheres. This
flocculonodular lobe is phylogenetically the most primitive part
of the cerebellum.
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MESENCEPHALON (midbrain)
• The mesencephalon is morphologically the most primitive of
the brain vesicles. Its basal and alar plates, separated by the
sulcus limitans.
• Each basal plate contains 2 groups of motor nuclei
• A medial somatic efferent group is represented by the
oculomotor (III) and trochlear (IV) cranial nerves, which
innervate the preoptic (eye) muscles. A small general visceral
efferent group is represented by the Edinger-Westphal nucleus,
also associated with the oculomotor (III) nerve, and innervates
the sphincter pupillary muscle
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MESENCEPHALON…
THE ALAR OR ROOF PLATE AND THE COLLICULI
• The alar plates initially appear as 2 longitudinal elevations separated by
a shallow midline depression. The (superior) and a posterior (inferior)
colliculus. The colliculi are formed by waves of neuroblasts produced
by the neuroepithelial cells that migrate into the overlying marginal
zone and become arranged in stratified layers
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Diencephalon
• The diencephalon develops from the median portion of the
prosencephalon. The alar plates form the lateral walls of the
diencephalon. A groove, the hypothalamic sulcus, divides the
plate into a dorsal (thalamus ) and a ventral region
(hypothalamus).
• The hypothalamus, forming the lower portion of the alar plate,
differentiates into a number of nuclear areas that regulate the
visceral functions, including sleep, digestion, body temperature,
and emotional behavior.
• One of these groups, the mamillary body, forms a distinct
protuberance on the ventral surface of the hypothalamus on
each side of the midline
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Diencephalon…
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DEVELOPMENT OF THE PITUITARY
GLAND
• Just anterior to the buccopharyngeal membrane, a midline diverticulum
known as the Rathke pouch develops in the oral ectoderm of the roof
of the primitive oral cavity.
• This evaginating pouch comes in contact with a pouch developing from
the floor of the diencephalon. Further development of these two
opposed structures gives rise to the pituitary gland.
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Hypophyseal Defects
• Occasionally a small portion of Rathke’s pouch persists in the
roof of the pharynx as a pharyngeal hypophysis.
• Craniopharyngiomas arise from remnants of Rathke’s pouch.
They may form within the sella turcica or along the stalk of the
pituitary but usually lie above the sella. They may cause
hydrocephalus and pituitary dysfunction (e.g., diabetes insipidus,
growth failure).
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HYDROCEPHALUS
• Hydrocephalus is a significant enlargement of the head due to
excessive accumulation of CSF within the skull.
• Hydrocephalus results from obstruction of CSF passage through
which it circulates or interference with the absorption of the CSF.
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Telencephalon
• The telencephalon, the most rostral of the brain vesicles, consists
of two lateral out-pocketings, the cerebral hemispheres, and a
median portion, the lamina terminales.
• The cavities of the hemispheres, the lateral ventricles,
communicate with the lumen of the diencephalon through the
interventricular foramina of Monro
• The cerebral hemispheres arise at the beginning of the fifth
week of development as bilateral evaginations of the lateral wall
of the prosencephalon. Simultaneously side to side expansion of
the cerebral cortex results into greatly increased surface area as a
result, the cerebral cortex becomes folded on itself.
• The sulci and gyri are formed as a result of this folding.
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CRANIUM BIFIDUM
• This is a defect in the formation of the cranium. Large protrusion
from the occipital region of the skull
• Cranium bifidum with meningocele- in this condition the gap in
the occipital bone is small, only the cranial meninges protrude or
herniate filled with CSF.
• Cranium bifidum with meningo-encephalocele- in this
condition the gap in the occipital bone is large, making the
protrution to consist the portion of the cerebellum that is covered
by the meninges and the skin.
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