Download TSM3 - Development of the CNS and PNS

TSM3: DEVELOPMENT OF THE CNS AND PNS
25/09/08
LEARNING OUTCOMES
Describe key elements in the development of the CNS and PNS and the main topography of the CNS
EMBRYOLOGICAL DEVELOPMENT










Shortly after the second week of embryonic development the dorsal midline ectoderm thickens to
form the nascent neural plate
A subsequent infolding of the plate gives rise to the neural groove and bilateral neural crests which
propagate into the mesoderm
The folds in the neural plate fuse together to form the neural tube which is contained within the
mesoderm and fully formed by the fourth week of development
The neural tube then undergoes further growth and distortion to form the primary vesicles
o The primitive forebrain, midbrain and hindbrain appear shortly after the fourth week in the
rostral part of the tube; the caudal part will later become the spinal cord
o After the fifth week the forebrain has further differentiated into:
 Cerebral vesicles laterally which contain the lateral ventricles (first and second?)
 Optic vesicles inferolaterally which surround the third ventricle
o The midbrain forms an extremely narrow tube called the cerebral aqueduct
o The hindbrain broadens out to form the fourth ventricle
During the fourth and fifth weeks the neural tube folds as the embryo does forming two flexures:
o The cephalic flexure divides the fore- and midbrains inferiorly
o The pontine flexure divides the mid- and hindbrains superiorly
The dorsal cell groupings of the neural tube form the alar lamina – giving rise to sensory regions
The ventral cell groupings of the neural tube form the basal lamina – giving rise to motor regions
In the mature spinal cord, these laminae give rise to the dorsal and ventral horns
Ventral spinal nerve roots arise from outgrowing axons from the basal lamina
Dorsal spinal nerve roots arise from ingrowing axons from the dorsal root ganglia – these also extend
peripherally
THE ADULT BRAIN








The forebrain is the largest part of the adult brain and comprises the two cerebral hemispheres
Inferomedially the corpus callosum connects the cerebral hemispheres
Below the corpus callosum are the thalamus, hypothalamus and pituitary
The midbrain is inferior to the thalamus and posterior to the pituitary
The cerebellum sits posteriorly and inferior to the cerebral hemispheres
Anterior to the cerebellum is the pons which ‘bulges’ further anteriorly
Inferior to the pons is the medulla which joins on to the spinal cord
Collectively the medulla, pons and midbrain are referred to as the brainstem