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Neuroblasts of the brainstem develop in a manner similar to the spinal cord. From the medulla through the midbrain, alar and basal plates form motor and sensory columns of cells that supply cranial nerves. However, the organization of alar and basal plates differ from of the spinal cord in that, 1) in the medulla and pons the alar plate lies lateral to the basal plate, not dorsal to it, since the 4th ventricle is “open”, 2) there are migrations of neuroblasts of both plates from the ventricular floor to other locations, and 3) “special” sensory and motor structures of the head require new/different cell groups for innervation. Rostral to the midbrain i.e., diencephalon and cerebral hemispheres develop from the alar plate. The cerebellum also develops from alar plate.
In the 6 mm embryo the thin
ependymal roof
of the neural tube, viz.-a-viz. the spinal cord, becomes even thinner as the
ventricle of the neural tube begins to widen in the early stages of the
development of the 4th ventricle. With continued development,
alar
and
basal
plates
shift laterally and become located in the floor of the
ventricle.
The sulcus limitans continues to be identifiable helping to mark the boundary
between sensory and motor areas. The basal plate forms the motor nuclei of the
cranial nerves, medial to the
sulcus limitans
in the ventricular floor. Lateral to the sulcus, the alar plate forms sensory
relay nuclei; portions of the alar plate migrate ventrally and form the inferior
olivary nucleus, a cerebellar relay nucleus. Medullary pyramids consist of
fibers from the cerebral cortex and develop on the ventral surface near the
midline.
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