neural tube
Recall the way in which most
of the neural tube is formed by folding and closure of the neural
plate, a region of thickened ectoderm. This process is called
neurulation. Remember how the neural crest cells migrate away from
the site of closure, and the important part they play in-amongst
other things - subsequent development of the peripheral nervous
system.
Recent evidence has confirmed
an earlier observation that the remainder of the neural tube - the
most caudal tip of the future spinal cord - develops by a different
process: the neural plate closes at its caudal end (caudal neuropore),
and a solid cord of cells is built onto the end of the existing
neural tube. A central lumen then develops within this cord
secondarily. There is a complex transitional region where the neural
plate method of development gives way to the second method of neural
tube development, and it is in this transitional region that a
defect called spina bifida most commonly develops.
Embryonic disc, with sections
through the primitive streak and neural plate
Folding of the neural plate
Folding of the neural plate
Neural tube and somites
Stages in primary
neurulation
brain vesicles
Even as the future brain
region of the neural plate is closing, three dilated regions are
apparent. After closure of the cranial neuropore, these enlarge
rapidly to form the three primary brain vesicles:
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forebrain vesicle |
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midbrain vesicle |
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hindbrain vesicle |
differentiation of the
neural tube
The neural plate has an
epithelial organisation. Most of the cells are wedge-shaped and
elongated (remember the microtubules and microfilaments that produce
these changes in shape) and extend from the basal surface of the
epithelium to the apical surface. The exceptions are cells that are
dividing - these round up to the apical surface to divide, and the
daughter cells then elongate until they contact the basal surface.
After closure of the neural plate, the neural tube retains its
epithelial organisation. The sidewalls are much thicker than the
roof and floor plates. Cell division continues, and then a change in
organisation becomes apparent: newly-formed cells are released from
the neuroepithelium and begin to form a new layer around the
periphery of the neural tube. The cells in this new layer - the
mantle layer - are neuroblasts, which means that they will later
differentiate into neurons (nerve cells). The mantle layer is not of
equal thickness throughout, but is arrnaged in four longitudinal
columns - two dorsal, and two ventral. The dorsal columns are called
the alar plates and the ventral columns the basal plates. As the
neurons within these columns differentiate, they form the grey
matter of the central nervous system.
Differentiation of the wall of the
neural tube into alar and basal plates
The processes (axons) which
grow out from the neuroblasts in the basal plates pass out into the
body as the motor components of the peripheral nerves, or pass to
higher or lower levels to link with other neurons. The neuroblasts
of the alar plates receive incoming fibres from the dorsal root
ganglia, and will send their axons to other regions of the central
nervous system. As more and more fibres develop, a third layer is
built up around the periphery of the neural tube - this is the
marginal layer. later, when the fibres become myelinated, the
marginal layer will form the white matter of the neural tube. The
spinal cord retains this simple arrangement of inner grey matter and
outer white matter, but in the brain the organisation becomes more
complex.
neural crest
Neural crest cells -
originally part of the ectodermal layer - migrate away from the site
of fusion of the neural folds. They differentiate ultimately into
many different types of cells:
|
some neural crest cells
remain close alongside the neural tube and form segmental
clusters - the future dorsal root ganglia of spinal nerves and
ganglia associated with cranial nerves. The cells in these
ganglia differentiate into sensory neurons whose processes
extend out into the body and also back to the neural tube to
make contact with developing neurons there. In this way, the
sensory components of peripheral nerves are formed. (The motor
components of peripheral nerves are formed by axons growing out
from the basal plate of the neural tube) |
|
other neural crest cells
migrate more ventrally and aggregate to form sympathetic
ganglia, differentiating as postganglionic neurons whose
processes innervate smooth muscle and glands. A special group of
neural crest cells in this category forms the adrenal medulla,
which can be thought of as an aggregate of modified
postganglionic sympathetic cells secreting their transmitter
substance directly into the blood stream |
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some neural crest cells
differentiate into Schwann cells and myelinate the peripheral
processes of nerve cells |
|
many neural crest cells
end up in the skin where they form melanocytes, which synthesise
the pigment melanin |
|
it is now clear that
neural crest cells also play an important part in development of
skeletal elements and other structures in the head and neck
region |
Clearly neural crest cells
have diverse fates within the embryo, and contribute to many
important structures.
ventricles of the brain,
cerebrospinal fluid
The central nervous system is
developed from a hollow, tubular structure - the neural tube. The
central lumen persists throughout development: in the spinal cord it
becomes the tiny central canal, but in the brain region it becomes
modified into broad and narrow portions - the ventricular system of
the brain. There are two lateral ventricles, a third ventricle, and
a fourth ventricle. These communicate with each other and with the
central spinal canal.
The ventricular system is
filled with cerebrospinal fluid which is secreted by thin, highly
vascularised membranes called choroid plexuses. Initially, this
fluid is moved within the ventricular system by cilia on the inner
surface of the neural tube, but during the 4th month of development
perforations appear in the roof of the 4th ventricle, and a true
circulation of cerebrospinal fluid is set up. The cerebrospinal
fluid formed within the brain passes out through the perforations
into the subarachnoid space around the brain, and is eventually
returned to the bloodstream within the venous dural sinuses. (A
failure to establish this circulation of cerebrospinal fluid
prenatally can result in hydrocephalus.)
abnormalities of the
nervous system
Click on the following link
to find out about abnormalities of the nervous system:
neural tube defects
Questions on development
of the nervous system:
1 Describe the process
of cell division in the neural plate and neural tube.
2 Which organelles are
thought to take part in folding of the neural plate?
3 List the types of
cells that can develop from neural crest cells.
4 Name the three
layers that can be recognised in the neural tube as it
differentiates.
5 What is the
embryological origin of postganglionic parasympathetic neurons?
6 Name the three
primary brain vesicles. Which part of the brain are derived from
each of these?
7 Name two congenital
abnormalities of the central nervous system that are thought to be
the result of non-closure of the neural plate.
8 What is
hydrocephalus? What might be the consequences of this condition for
brain function?
9 When does
circulation of cerebrospinal fluid begin? |
