Central Nervous System
Time period: day 22 to postnatal
development
Ectoderm is induced by the
notochord to form neuroectoderm during neurulation (see Chapter 17). This
neuroectoderm in turn produces the neural tube and neural crest cells from
which the central nervous system develops. The central nervous system comprises
the brain and spinal cord.
The caudal end of the neural tube
continues to elongate and form the spinal cord. A lumen through the centre of
the spinal cord, the neurocoel (or neural canal), forms by week 9 and will
become the central canal. The neurocoel is lined with thickening layers of neuroepithelia
known as the ventricular zone (Figure 44.1) or ependymal layer.
Cells of the ventricular zone
differentiate into neuroblasts and glioblasts (or spongioblasts). Glioblasts
will form supporting cells, or neuroglial cells. Neuroblasts become neurons and
migrate out to form the mantle (or intermediate) layer of cell
bodies, which will form the grey matter. Surrounding this layer is the
outer marginal layer that carries the axons from the neurons in the
mantle layer, and will become white matter (Figure 44.1).
Dorsal root ganglia (also known
as spinal ganglia) outside the neural tube formed from neural crest cells extend
central processes which grow into the neural tube. Some form synapses with
neurons in the mantle layer (grey matter) while other central processes ascend
within the marginal layer (white matter).
Further distinction of the mantle
layer occurs as the ventral area becomes the basal plates and the dorsal
part becomes the alar plates, forming the ventral motor horns and dorsal
sensory horns, respectively (Figure 44.1). They are divided by a groove, the sulcus
limitans. Between these horns the intermediate horn develops
containing neurons of the sympathetic nervous system between spinal levels T1
and L2 (or L3).
Spinal nerves form as the axons
of dorsal and ventral roots from the spinal cord combine, with input from
peripheral processes of the dorsal root ganglia and, where level appropriate,
input from the autonomic nervous system.
In the embryo the spinal cord
reaches to the caudal end of the vertebral column. Towards the end of the
embryonic period the coccygeal vertebrae are reduced and the vertebral column
grows rapidly causing the tail of the spinal cord to extend to the level of the
L3 vertebra by birth, and by adulthood commonly only reaches to between L1 and
L2 (Figure 44.2).
Three vesicles develop in the
cranial end of the neural tube (Figure 44.3). The distinct alar and basal
plates we saw in the spinal cord are retained as the hindbrain and midbrain
develop, whereas in the forebrain the dorsal alar plates expand and the ventral
basal plates degenerate.
The vesicles become the prosencephalon
(forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain).
As the vesicles grow the tube folds (Figure 44.4). In the fifth week the three
vesicles become five, with the forebrain and hindbrain both splitting into two,
forming the telecephalon and diencephalon from the forebrain, and
from the hindbrain the metencephalon and the myelencephalon form
(Figure 44.5).
The dorsal telencephalon of the
forebrain will grow rapidly to form the cerebral cortex, and the ventral
telencephalon will become the basal ganglia.
The diencephalon will form the optic
cup and stalk, pituitary gland, thalamus, hypothalamus and pineal
body.
The caudal part of the hindbrain,
the myelencephalon, becomes the medulla oblongata. The metecephalon
develops into the pons ventrally and the cerebellum dorsally. The
midbrain, together with the pons and medulla oblongata form the brainstem.
Ventricles of the brain
form from the lumen of the neural tube. Ventricles contain cerebrospinal fluid
produced by choroid plexuses. Lateral ventricles of the
telencephalon link with the third ventricle of the diencephalon, which
is connected to the fourth ventricle of the myelencephalon through the
cerebral aqueduct of the mesencephalon. The fourth ventricle is continuous with
the central canal of the spinal cord.
Neural crest cells migrate to
both sides of the spinal cord and form the dorsal root ganglia (sensory
ganglia) of the spinal nerves. In the hindbrain area they contribute to cranial
nerve ganglia, sensory ganglia of CN V, VII, VIII, IX and X.
Of the connective tissue layers
that surround the brain, the pia mater and arachnoid mater are
mesoderm and neural crest cell derived, whereas the dura mater is
only mesoderm derived.
Encephalocoele is the
herniation of dura, potentially containing brain tissue, through a midline
skull defect, caused by incomplete closure of the neural tube. Repaired
surgically, recovery depends upon whether or not neural tissue was enclosed in
the encephalocoele.
Anencephaly describes
incomplete closure at the cranial end of the neural tube, resulting in a
complete failure of forebrain formation. This is lethal and most infants with
this condition are stillborn.
Hydrocephalus is a
condition resulting from excess cerebrospinal fluid (CSF) resulting from a
blockage of flow, failure to reabsorb or increased production. Conditions
include increasing head circumference and vomiting, but ventricular dilation
can be identified before symptoms become visible. Treatment options include
inserting a shunt to open a t the CSF to an area where it can be reabsorbed.
