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Spinal Cord
Development
Spinal Cord Development
A. The neural tube caudal to the fourth pair of somites develops into the
spinal cord.
Neural canal
Dorsal nerve
rootlet
Mesoderm
Primordia of
spinal
ganglion
Neural
tube
Ventral
nerve
rootlet
Spinal Cord Development
A. Neural tube
1. Derived from future-neural tube portion of neural ectoderm
2. Forms the central nervous system (brain & spinal cord)
B. Dorsal nerve rootlet
1. Derived from neural crest portion of neural ectoderm
2. Forms portion of peripheral nervous system
C. Primordia of spinal (dorsal root) ganglion
1. Derived from neural crest portion of neural ectoderm
2. Forms portion of peripheral nervous system
D. Ventral nerve rootlet
1. Derived from neural crest portion of neural ectoderm
2. Forms portion of peripheral nervous system
Spinal Cord Development
A. Composition of Neural Tube
Internal limiting membrane
Neuroepithelial cells
Sclerotomal somitic mesoderm
External
limiting
membrane
Spinal Cord Development
A. Composition of Neural Tube
Internal limiting membrane
Internal limiting membrane and
external limiting membrane are “true”
(extracellular) acellular basement
membranes – composed of type-IV
collagen, laminin, entactin, nidogen,
and cold insoluble fibronectin.
Neuroepithelial cells
Sclerotomal somitic mesoderm will
form bony vertebrae, annulus
fibrosis of intervertebral disc (IVD)
surrounding forming spinal cord
(nucleus pulposus of IVD is formed by
notochord), and meninges (dura
mater, arachnoid mater, and pia
mater)
Sclerotomal somitic mesoderm
External
limiting
membrane
Spinal Cord Development
A. Initially, the wall of the neural tube is
composed of a thick, pseudostratified
columnar neuroepithelium.
Ventricular Zone
B. These neuroepithelial cells constitute
the ventricular zone (ependymal
layer) of the neural tube.
C. The ventricular layer gives rise to:
1. All neurons
2. Macroglial cells
a. Astrocytes
1. Protoplasmic
2. Fibrous
b. Oligodendrocytes
3. Ependymal cells
Neuroepithelial cells
Spinal Cord Development
Marginal zone
D. Soon a marginal zone, composed
of the outer portions of the
neuroepithelial cells, appears.
E. This zone gradually becomes the
white matter of the spinal cord.
F. Axons grow into this area from the
spinal cord, spinal ganglia, and
brain.
Neuroepithelial Cells
Spinal Cord Development
Intermediate (Mantel) zone
G. Some dividing neuroepithelial cells
in the ventricular zone differentiate
into neuroblasts (primordial
neurons).
H. The cells form the intermediate
(mantel) zone.
I. Neuroblasts become neurons as
they develop cytoplasmic
processes.
J. The intermediate (mantel) zone
becomes the gray matter of the
spinal cord.
Neuroepithelial cells
Spinal Cord Development
K. Neuroepithelial cells in the
ventricular zone differentiate into
glioblasts.
Ventricular zone
L. The glioblasts migrate into the
marginal and mantel zones from
the ventricular zone.
M. Some glioblasts differentiate into
astroblasts, then astrocytes
(protoplasmic and fibrous).
N. Some glioblasts differentiate into
oligodendroblasts, then
oligodendrocytes.
Neuroepithelial cells
Spinal Cord Development
Ventricular zone
O. When the neuroepithelial cells
finish producing neuroblasts and
glioblasts, they differentiate into
ependymal cells.
P. Ependymal cells form the
ependyma, which lines the central
canal of the spinal cord.
Neuroepithelial cells
Neuroepithelial cells
Review Slide of
neural tube
differentiation
Neural tube
Ependyma
Simple
cuboidal
epithelium
covering
choroid
plexus
Neuroblast
(Apolar)
Glioblast
Astroblast
Oligodendroblast
Neuroblast
(Bipolar)
Neuroblast
(Unipolar)
Astrocyte
(Protoplasmic)
Astrocyte
(Fibrous)
Oligodendrocyte
Neuron
Spinal Cord Development – Review Slide
Intermediate (mantle) zone
(gray matter)
Ventricular zone
(ependymal layer)
Marginal zone
(white matter)
Developing
spinal
meninges
(dura mater,
arachnoid
mater, pia
mater)
Spinal Cord Development
Microglial Cells
A. Are derived from hematopoietic cells located in the bone marrow
B. Formed after week 12 of development
C. Migrate to nervous system via the vasculature
D. Take up residence in both white matter and gray matter of the CNS
E. Are macrophages that function to remove
1. Cell debris
2. Dead and dying cells
Spinal Cord Development
Myelination of Nerve Fibers
A. Myelin sheaths surrounding nerve processes in the CNS
1. begin to form during the late fetal period
2. Continue during the first postnatal year.
3. Formed by oligodendrocytes
a. Derivative of neuroepithelial cells
B. Myelin sheaths surrounding nerve processes in the PNS
1. Begin about 20 weeks gestation
2. Continue till puberty
3. Formed by Schwann cells
a. Derivative of neural crest
4. Motor roots are myelinated before sensory roots
Nerve Fiber Coverings
Schwann cell nucleus (PNS) or
Oligodendrocyte nucleus (CNS)
Basement
membrane
Nerve
fiber
Unmyelinated axons (5)
Each Schwann cell is able
to house multiple axons
occupying invaginations of
its cytoplasm.
Myelinated axon (1)
Multiple wrapping of Schwann
cell plasma membrane (“jellyroll”) around single axon.
Myelinated (Motor) Axon
Schwann cell
Schwann cell nucleus
Axon
Myelin sheath
Node of Ranvier
Schwann cell basement membrane
Schwann cell process
Spinal Cord Development
Roof plate
A. Proliferation and
differentiation of
neuroepithelial cells in the
developing spinal cord
produce thick walls and
thin roof plate and floor
plate.
B. Differential thickening of
the lateral walls produces
a shallow longitudinal
groove on either side, the
Sulcus limitans
Sulcus limitans
Floor plate
Spinal Cord Development
1. Signals from sonic hedgehog
(Shh) in notochord induces
floor plate development.
Roof plate
BMPs-4 & -7
BMPs-4 & -7
Dorsal
2. BMP-4 & BMP-7 from adjacent
ectoderm maintains Pax-3 &
Pax-7 expression, dorsally.
3. Sonic hedgehog, produced by
the floor plate, suppresses the
expression of the dorsal Pax
genes (3 & 7) and induces the
expression of Pax-6 in the
ventral half of the spinal cord.
Pax-6 induces formation of
motor neurons.
Pax-3
Pax-7
Pax-3
Pax-7
Pax-6
Pax-6
Shh
Sulcus limitans
Ventral
Shh
Floor plate
N
Spinal Cord Development
Roof plate
Alar plate
(sensory)
C. The Sulcus limitans
separates the dorsal part,
alar plate, from the ventral
part, basal plate.
Sulcus limitans
Basal plate
(motor)
D. The alar plate is associated
with afferent (sensory)
function
E. The basal plate is
associated with efferent
(motor) function.
Floor plate
Spinal Cord Development
F. The alar and basal
plates form longitudinal
bulges extending
through most of the
length of the spinal
cord.
Marginal Zone
Alar Plate
Sulcus
limitans
Basal Plate
Spinal Cord Development
A. Marginal Zone
1. Derivative of neural tube
2. White matter portion of central nervous system
B. Alar plate
1. Derivative of neural tube
2. Portion of central nervous system
3. Associated with afferent (sensory) function
C. Basal plate
1. Derivative of neural tube
2. Portion of central nervous system
3. Associated with efferent (motor) function
D. Sulcus limitans
1. Derivative of neural tube
2. Portion of central nervous system
3. Separates alar plate from basal plate
Spinal Cord Development
G. Cell bodies in the alar plate form the dorsal (gray) columns that extend the entire
length of the spinal cord. These columns are the dorsal horns of the spinal cord.
H. Neurons in the dorsal columns constitute afferent nuclei.
Afferent nuclei
Spinal
(dorsal
root)
ganglion
Dorsal columns
Dorsal rootlet
Afferent
sensory
neuroblasts
in spinal
ganglion
Dorsal
roots of
spinal
nerve
Spinal Cord Development
A. Dorsal Columns
1. Derivative of alar plate (neural tube)
2. Portion of central nervous system
3. Neurons in the dorsal columns constitute afferent nuclei
B. Dorsal rootlet
1. Derivative of neural crest
2. Portion of peripheral nervous system
3. Composed of Schwann cells and cell processes for sensory
neuroblasts in spinal (dorsal root) ganglion
C. Afferent sensory neuroblasts in spinal (dorsal root) ganglion
1. Derivative of neural crest
2. Portion of peripheral nervous system
3. Associated with afferent (sensory) function
D. Dorsal roots of spinal nerve
1. Derivative of neural crest
2. Portion of peripheral nervous system
3. Cell processes for sensory neuroblasts in spinal (dorsal root)
ganglion
Spinal Cord Development
I. Cell bodies in the basal plates form the ventral and lateral (gray) columns.
J. In the spinal cord these are the ventral horn and the lateral horn, respectively.
K. Axons of the ventral horn cells grow out of the spinal cord and form the ventral
roots of the spinal nerves.
Efferent
motor
neuroblasts
Lateral
column
Ventral rootlet
of spinal nerve
Ventral roots of
spinal nerve
Ventral columns
Spinal Cord Development
A. Ventral Columns
1. Derivative of basal plate (neural tube)
2. Portion of central nervous system
3. Neurons in the dorsal columns constitute efferent nuclei
B. Lateral Columns
1. Derivative of basal plate (neural tube)
2. Portion of central nervous system
3. Neurons in the lateral columns constitute efferent nuclei
C. Ventral rootlet of spinal nerve
1. Derivative of neural crest
2. Portion of peripheral nervous system
3. Composed of Schwann cells
D. Ventral roots of spinal nerve*
1. Derivative of neural tube
2. Portion of central nervous system
3. Cell processes for motor neuroblasts in ventral horn
Spinal Cord Development
Efferent motor
neuroblasts
Afferent sensory Sulcus
neuroblasts
limitans
Roof Plate
Marginal Zone
Dorsal horn
Afferent
sensory
neuroblasts
in spinal
ganglion
Lateral
horn
Dorsal
root of
spinal
nerve
Floor Plate
Ventral horn
Ventral root of spinal nerve
Spinal Cord Development
L. As the dorsal marginal zone and alar plates enlarge, the dorsal median
septum forms.
alar plate
dorsal marginal zone
Dorsal median septum
Spinal Cord Development
M. As the basal plates enlarge, they bulge ventrally on each side of the median plane.
N. As this occurs, a deep longitudinal groove, the ventral median fissure, develops
on the ventral surface of the spinal cord.
Basal
plate
Ventral median septum
Spinal Cord Development
O. The pseudo-unipolar neurons in the spinal (dorsal root) ganglia are derived
from neural crest cells.
P. Peripheral processes of spinal ganglion cells pass in the spinal nerves to
sensory endings in somatic or visceral structures.
Q. The central processes enter the spinal cord and constitute the dorsal roots of
the spinal nerves.
Axon
Dorsal
rootlets
Spinal
(dorsal
Pseudoroot)
unipolar
ganglion
neurons
Dorsal
Roots of
spinal
nerves
Dendrite
(conduction
towards cell
body)
Spinal Cord Development
R. Axons of the ventral horn cells grow out of the spinal cord and form the
ventral roots of the spinal nerves.
S. Peripheral processes of ventral horn cells pass in the spinal nerves to
motor end plates in somatic or visceral structures.
Ventral
horn
Ventral roots of spinal nerves