Download The Nervous System

Central Nervous System
Development
David L. McWhorter, Ph.D.
Origin of the Nervous System
• Develops from the neural
plate:
– Dorsal thickened area of
embryonic ectoderm
• Neural folds:
– elevated lateral margins of
neural plate
– longitudinal midline
depression of neural plate
Dorsal view, ~ 17 days,
amnion removed
• Neural tube differentiates
into:
– CNS (brain and spinal cord)
• Neural crest:
– Some cells from apices of
neural folds become
separated to form dorsolateral
groups and gives rise to:
• Cells that form most of PNS
and ANS (cranial, spinal, and
autonomic ganglia)
Dorsal view, ~22 days
Transverse sections
• Neural groove:
Transverse section
Neurulation
• Formation of neural plate and subsequent
formation of neural tube and neural crest
• Begins at 22-23 days of development:
– in region of fourth to sixth pairs of somites
– At this stage:
• cranial two-thirds of neural plate and tube (as
far caudal as fourth pair of somites) represent
future brain
• caudal one third of the neural plate and tube
represents future spinal cord
• Fusion of neural folds forms neural tube
– proceeds in cranial and caudal directions
– small areas of tube remain open at both ends
• Lumen is called neural
canal
– communicates freely with
amniotic cavity
– forms:
Neural Tube
• ventricular system of
brain
• central canal of spinal
cord
• Cranial opening is called
rostral neuropore
– closes on approximately
day 25
Dorsal view, ~23 days
Lateral view,
~24 days
• Caudal opening is called
caudal neuropore
– closes about 2 days later
• Closure of Neuropores
– Coincides with
establishment of blood
vascular circulation for
neural tube
Lateral view, ~ 27 days
Walls of Neural Tube
•
Thicken to form:
– brain and spinal cord
•
Three primary brain
vesicles:
1. Forebrain
2. Midbrain
3. Hindbrain
•
Five secondary brain
vesicles:
1.
2.
3.
4.
5.
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
lateral view, ~28 days
Transverse section
6-week embryo,
showing
secondary brain
vesicles
Development of the Spinal Cord
Introduction
• Neural tube
caudal to fourth
pair of somites
develops into
spinal cord
Lateral Walls of Neural Tube
• Thicken, gradually reducing size of neural
canal
– only a minute central canal is present at 9 to 10
weeks
• Initially, neural tube wall is composed of:
– pseudostratified, columnar neuroepithelium
Transverse
section, ~ 23
days
9 Weeks
Neuroepithelial Cells
Constitute The Ventricular
Zone (Ependymal Layer)
• Gives rise to:
– all neurons and macroglial
cells in spinal cord
Two Other Layers of Developing Spinal
Cord Wall
1. Outermost layer, marginal
zone:
– gradually becomes white
matter (substance) of spinal
cord
• axons grow into it from nerve cell
bodies in the spinal cord, spinal
ganglia, and brain
2. Intermediate zone (mantle
layer) consists of
neuroepithelial cells that
differentiate into primordial
neurons called neuroblasts:
– Become neurons as they
develop cytoplasmic processes
Glioblasts (Spongioblasts)
• Primordial supporting
cells of CNS
• Differentiate from
neuroepithelial cells
– mainly after neuroblast
formation has ceased
• Migrate from ventricular
zone into:
– intermediate and marginal
zones
• Some glioblasts become:
– astroblasts and later
astrocytes
• Other glioblasts become
oligodendroblasts and
eventually
oligodendrocytes
Microglial Cells (Microglia)
• Small cells derived
from mesenchymal
cells
– Scattered throughout
gray and white matter
• Invade CNS in late
fetal period after it has
been penetrated by
blood vessels
• Originate in bone
marrow
– are part of
mononuclear
phagocytic cell
population
Development Of The Spinal Cord
• Dorsal and ventral midline
portions of neural tube form:
– thin roof- and floor-plates that
serve as:
• pathways for nerve fibers crossing from one
side to the other
• Lateral wall thickening produces
a shallow longitudinal groove on
each side called sulcus
limitans that separates:
– dorsal part, alar plate later
associated with afferent or sensory
functions
– ventral part, basal plate later
associated with efferent or motor
functions
– Disappears in adult spinal cord
– Retained in rhomboid fossa of
brain stem (4th ventricle floor)
Alar (Sensory) Plates
• Cell bodies in the alar plates
form dorsal gray horns that
consist of:
– Afferent nuclei
• As alar plates enlarge, dorsal
median septum forms
Basal (Motor) Plates
• Cell bodies in the basal
plates form ventral and
lateral gray horns that
contain:
– Efferent nuclei
• Basal plate axons grow out of
spinal cord and form:
– ventral roots of spinal
nerves
• As basal plates enlarge, they
form a deep longitudinal
groove:
– ventral median fissure
Development of the Spinal Ganglia
• Unipolar neurons in the spinal
ganglia (dorsal root ganglia) are
derived from neural crest cells
• Axons of spinal ganglia cells are at
first bipolar, but the two processes
unite in a T-shaped fashion:
– Both processes have structural
characteristics of axons
• Peripheral process is functionally a
dendrite:
– conduction from sensory endings in
somatic or visceral structures toward
cell body
• Central process enter spinal cord
and constitute:
– dorsal roots of spinal nerves
Positional Changes of Spinal Cord
• Initially, embryo spinal cord
extends entire length of
vertebral canal
– Spinal nerves pass through
intervertebral foramina opposite
their levels of origin
• Vertebral column and dura
mater grow more rapidly than
the spinal cord, changing
positional relationship to
spinal nerves
– Caudal end of spinal cord
gradually comes to lie at
relatively higher levels
• 6-month-old fetus, spinal cord
lies at first sacral vertebra level
8 weeks
24 weeks
increasing inclination of the root of the first
sacral nerve
Newborn and Adult Spinal Cord
• Newborn spinal cord:
– Terminates second or third lumbar
vertebra level
• Adult spinal cord:
– Usually terminates at inferior border of
first lumbar vertebra:
• caudal end may be as superior as 12th
thoracic vertebra or as inferior as third
lumbar vertebra
– As a result, spinal nerve roots (especially
lumbar and sacral segments) run:
• obliquely from spinal cord to
corresponding level of vertebral column
– Nerve roots inferior to end of cord are
called medullary cone (L., conus
medullaris)
• form a bundle of spinal nerve roots called
cauda equina (L., horse's tail)
• dura mater and arachnoid mater usually
end at S2 vertebra in adults
– pia mater ends at conus medullaris
Newborn
Adult
Caudal End of Spinal Cord
• Distal to caudal end,
pia mater forms a long
fibrous thread:
– terminal filum (L.,
filum terminale):
• indicates original caudal
end of embryonic spinal
cord
• extends from medullary
cone and attaches to
periosteum of first
coccygeal vertebra
Development of the Brain
Neural Tube
•
•
Cranial to fourth pair of somites
develops into brain
Fusion of neural folds in the
cranial region and closure of the
rostral neuropore form:
–
three primary brain vesicles from
which the brain develops…
•
During Fifth Week: Five Secondary
Brain Vesicles
Forebrain partly divides into:
1. Telencephalon
2. Diencephalon
•
•
Midbrain does not divide
Hindbrain partly divides into:
1. Metencephalon
2. Myelencephalon
Brain Flexures (slide 1 of 2)
• During fourth week, embryonic
brain grows rapidly and bends
ventrally with head fold,
producing:
– midbrain (cephalic) flexure in the
midbrain region:
• located between prosencephalon
and rhombencephalon
– cervical flexure at junction of
hindbrain and spinal cord
4-5 weeks
• Later, unequal growth of brain
between cephalic and cervical
flexures produces:
– pontine flexure in the opposite
direction
• Located between metencephalon
and myelencephalon
• Results in thinning of hindbrain roof
7-8 Weeks
Brain Flexures (slide 2 of 2)
• Initially, primordial brain has
same basic structure as
developing spinal cord
• Brain flexures produce:
– considerable variation in outline of
transverse sections at different
levels of the brain
– position of the gray and white
matter (substance)
• Sulcus limitans extends
cranially to:
– junction of midbrain and forebrain
• Alar and basal plates recognizable
only in midbrain and hindbrain
Hindbrain
• Cervical flexure
demarcates:
– hindbrain from spinal cord
• Later, this junction is defined
as:
– level of superior rootlet of first
cervical nerve
» located roughly at
foramen magnum
• Pontine flexure divides
hindbrain into:
– caudal (myelencephalon)
• becomes medulla oblongata
(often called medulla)
– rostral (metencephalon)
• becomes pons and
cerebellum
• Cavity of hindbrain becomes:
– fourth ventricle
– central canal in the medulla
Brainstem Development
Brainstem Primary
Vesicles
Brainstem
Secondary Vesicles
Derivatives in
mature brain
Mesencephalon
Mesencephalon
MIDBRAIN
Rhombencephalon
(Hindbrain)
Metencephalon
Ponds and
cerebellum
myelencephalon
medulla
Match Nerve Fiber Types with Functions
1. General Somatic
Afferent (GSA) fibers
2. General somatic
efferent (GSE) fibers
3. General visceral
afferent (GVA) fibers
4. General visceral
efferent (GVE) fibers
A. Transmit impulses to
smooth and cardiac
muscle and glandular
tissues
B. Transmit reflex or pain
from mucous
membranes, glands, and
blood vessels back to
CNS
C. Transmit sensations
from body to spinal cord
(eg., pain, temperature,
touch, & pressure)
D. Transmit impulses to
skeletal muscles
Seven Types of Nerve Fibers
Associated with Cranial Nerves
•
Motor (3):
1. General Somatic Efferent (GSE)
2. Special Visceral Efferent (SVE)
3. General Visceral Efferent (GVE)
•
Sensory (4):
1.
2.
3.
4.
General Visceral Afferent (GVA)
General Somatic Afferent (GSA)
Special Visceral Afferent (SVA)
Special Somatic Afferent (SSA)
Cranial Nerves: Motor Fibers (3)
•
Two subtypes of motor fibers to voluntary (striated)
muscle based on embryologic origin:
1. General Somatic Efferent (GSE) axons innervate
muscles derived from sources other than embryonic
pharyngeal arches:
–
ocular muscles, tongue, external neck muscles
(sternocleidomastoid and trapezius)
2. Special Visceral Efferent (SVE) axons innervate muscles
derived from the embryonic pharyngeal arches (branchial
motor):
–
face, palate, pharynx, and larynx
3. General Visceral Efferent (GVE) axons innervate
involuntary (smooth) muscles or glands:
–
cranial outflow of parasympathetic division of ANS
Cranial Nerves: Sensory Fibers (4)
1. General Visceral Afferent (GVA) axons carry
sensation from the viscera:
–
Carotid body and sinus, pharynx, larynx, trachea, bronchi,
lungs, heart, gastrointestinal tract
2. General Somatic Afferent (GSA) transmit general
sensation from the skin and mucous membranes:
–
•
Touch, pressure, heat, cold
Two subtypes of fibers transmitting unique
sensations:
3. Special Visceral Afferent (SVA) axons convey taste and
smell
4. Special Somatic Afferent (SSA) axons convey vision,
hearing and balance
Caudal
Myelencephalon
• Caudal part (Closed part of
medulla) resembles spinal cord
(developmentally and structurally)
• Neural canal of neural tube forms:
– small central canal of
myelencephalon
• Neuroblasts from alar plates migrate
into marginal zone and form isolated
areas of gray matter:
– gracile nuclei, medially
– cuneate nuclei, laterally
• Gracile and cuneate nuclei are
associated with correspondingly
named tracts that enter medulla
from spinal cord
• Ventral area of medulla contains:
– pair of fiber bundles called the
pyramids
• consist of corticospinal fibers
descending from developing
cerebral cortex
C
O
Rostral Myelencephalon
• Rostral part (“Open" part of
medulla) is wide and rather flat
(especially opposite pontine
flexure)
• Pontine flexure causes:
– lateral walls of medulla to move
laterally (like pages of an open book)
• As a result, its roofplate is stretched and
greatly thinned
• Cavity of this part (future fourth
ventricle) becomes somewhat
rhomboidal (diamond-shaped)
• As walls of medulla move laterally
– alar plates come to lie lateral to
basal plates
• As positions of plates change, motor
nuclei generally develop medial to
sensory nuclei
Neuroblasts in Basal Plates of Medulla
•
•
Develop into motor neurons
Form nuclei (groups of nerve
cells) and organize into three
cell columns on each side,
from medial to lateral:
1. General somatic efferent (GSE)
column:
• represented by neurons of
hypoglossal nerve that supply tongue
muscle
2. Special visceral efferent (SVE)
column:
• represented by neurons innervating
striated muscles derived from
pharyngeal arches (CN V, VII)
3. General visceral efferent (GVE)
column:
• represented by some vagus and
glossopharyngeal neurons that supply
involuntary muscles of respiratory
tract, intestinal tract and heart
Neuroblasts in Alar Plates of Medulla
•
Form neurons arranged in four columns (on each
side), from medial to lateral:
1. General visceral afferent (GVA) column, receiving
impulses from gastrointestinal tract and heart
2. Special visceral afferent (SVA) column, receiving taste
buds of tongue
3. General somatic afferent (GSA) column, receiving
impulses from surface of head
4. Special somatic afferent (SSA), receiving impulses from
ear
Some Neuroblasts From Alar Plates
of Medulla
• Migrate ventrally and form:
– neurons in the olivary nuclei
• Connections with cerebellum and involved in control of
movement
•
Walls of
metencephalon
form:
–
–
•
Cavity of
metencephalon
forms:
–
•
pons
cerebellum
superior part of
fourth ventricle
As in rostral part of
myelencephalon,
pontine flexure
causes:
–
divergence of lateral
walls of pons that
spreads gray matter
in floor of fourth
ventricle
Metencephalon
Basal Plate Neuroblasts In Metencephalon
•
Develop into motor nuclei and organize into three
columns on each side:
1. General somatic efferent (GSE) column gives rise to abducens
nerve nucleus
2. Special visceral efferent (SVE) column contains nuclei for
trigeminal and facial nerves that innervate first and second
pharyngeal arch musculature
3. General visceral efferent (GVE) column whose axons supply
submandibular and sublingual glands
Alar Plate Neuroblasts In Metencephalon
•
Develop into sensory nuclei and organize into four
columns on each side:
1.
2.
3.
4.
•
General visceral afferent (GVA) column contains neurons from CN VII
(sensation from viscera)
Special visceral afferent (SVA) column contains taste fibers from CN VII
General somatic afferent (GSA) column contains neurons of trigeminal
nerve (skin…touch, pressure, temperature)
Special somatic afferent (SSA) column contains neurons from CN VIII
(hearing and balance)
Cells from alar plates also give rise to pontine nuclei:
–
Consist of cerebellar relay nuclei
Metencephalon: Cerebellum
• Develops from thickenings of
dorsal parts of alar plates called
cerebellar swellings (rhombic
lips):
– project into fourth ventricle
• Cerebellar swellings enlarge and
fuse in median plane:
– overgrow rostral half of fourth
ventricle
– overlap pons and medulla
• Some neuroblasts in the
intermediate zone of alar plates
migrate to marginal zone and
differentiate into:
– neurons of the cerebellar cortex
• Other neuroblasts from alar plates
give rise to:
– central nuclei, the largest of which
is:
• dentate nucleus
Midbrain (mesencephalon)
• Undergoes less change
than any other part of
developing brain
– except for caudal part of
hindbrain
• Neural canal narrows and
becomes:
– cerebral aqueduct:
• channel that connects third
and fourth ventricles
Neuroblasts From Alar Plates of
Midbrain
• Migrate into tectum (L.,
roof) and aggregate to
form:
– four large groups of
neurons (corpora
quadrigemina):
• paired superior colliculi
(concerned with visual
reflexes)
• paired inferior colliculi
(concerned with auditory
reflexes)
Neuroblasts From Basal Plates of
Midbrain
• Give rise to groups of neurons in the
tegmentum (L., covering structure) of
midbrain:
– red nuclei gives rise to rubrospinal tract
(control over tone of limb flexor muscles)
– nuclei of third and fourth cranial nerves
– reticular nuclei central region of brainstem,
occupying most of tegmentum of midbrain,
pons, and medulla:
• involved in virtually every activity from visceral
functions to consciousness
• core integrating structure of the brain
• Substantia nigra, a broad layer of gray
matter adjacent to cerebral peduncle:
– associated with functions of basal ganglia:
• associated with regulation of motor functions
– some authorities believe it is derived from
alar plate cells in basal plate that migrate
ventrally
Forebrain Parts
• Rostral or anterior part, including primordia of
cerebral hemispheres, is telencephalon
• Caudal or posterior part of forebrain is diencephalon
• Cavities of telencephalon and diencephalon contribute
to:
– formation of lateral ventricles third ventricle, respectively
Telencephalon
• Arise at beginning of fifth
week as bilateral
evaginations of lateral wall
of prosencephalon
• Consists of two lateral
outpocketings called
cerebral or telencephalic
vesicles:
• primordia of cerebral
hemispheres and their
cavities (lateral
ventricles)
Walls of Developing Cerebral
Hemispheres
•
Initially show three typical
zones of neural tube:
1. Ventricular
2. Intermediate
3. Marginal
•
•
Later a fourth one appears,
subventricular zone
Intermediate zone cells migrate
into marginal zone and give rise
to:
–
cortical layers
•
–
Gray matter is thus located
peripherally
axons from its cell bodies pass
centrally to form:
•
large volume of white matter,
medullary center
Cerebral Hemispheres
A.
Dorsal surface of forebrain, indicating how ependymal roof of
diencephalon is carried out to cerebral hemispheres
Developing cerebral hemispheres grow from lateral walls of forebrain and
expand in all directions until they cover diencephalon
B.
–
–
C.
arrows indicate some directions hemispheres expand
rostral wall of forebrain, lamina terminalis, is very thin
Ependymal roof is carried into temporal lobes as a result of C-shaped
growth of cerebral hemispheres
• Cavities of cerebral hemispheres are called lateral ventricles:
– communicate with lumen of diencephalon through interventricular
foramina (of Monro)
Surface of Cerebral
Hemispheres
• Initially, smooth
• As growth proceeds, the following
develop:
– gyri are rounded surface elevations
– sulci are grooves or furrows between gyri
• Sulci and gyri permit a considerable
increase in surface area of cerebral
cortex without requiring an extensive
increase in cranial size
Diencephalon
• Three swellings develop
in lateral walls of third
ventricle and later
become:
– Thalamus
– Hypothalamus
– Epithalamus
• Thalamus is separated
from epithalamus and
hypothalamus by:
– epithalamic sulcus
– hypothalamic sulcus
• not a continuation of sulcus
limitans into forebrain
• does not divide sensory and
motor areas