The brainstem represents the rostral continuation of the spinal cord. It develops, caudally to cranially, from the myelencephalon (5th CNS vesicle), the metencephalon (4th CNS vesicle) and the mesencephalon (3rd CNS vesicle).
On each of these three levels (myelencephalon, metencephalon, mesencephalon) the general characteristics known from the development of the spinal cord can be identified, namely the central, peri-ependymal perikaryon layer (gray matter) and the accumulated bundles of myelinized neurites (white matter) in the periphery.
However, there are three important differences:
- The enlargement of the cavity system up to the IVth ventricle (rhombencephalon)
- The creation of the core region of the brainstem and the cranial nerves through the emigration of neurons into the white matter at the expense of the peri-ependymal gray matter
- The formation of the cerebellum gray matter in the form of the deep cerebellar nucleus and the surface cerebellar cortex through the emigration of neuroblasts.
The cerebellum arises out the dorsal portion of the metencephalon and thus can be considered part of the brainstem from an embryologic point of view. The cerebellum is in connection with the other portions of the brainstem via the three cerebellar peduncles (pedunculi cerbellares). The lower cerebellar penduncle is drawn toward the myelencephalon, the middle one toward the metencephalon (pons) and the upper toward the mesencephalon
A study of the development of the various brainstem vesicles reveals an increasing cephalization that shows itself in the appearance of suprasegmental structures. These structures represent the switching and association centers at a higher level than the spinal cord. As the interface between the spinal cord and the peripheral nervous system they assure the integration and coordination of neuronal signals. In this it is mainly the diffuse reticular formation, extending from the myelencephalon to the diencephalon and around the red nucleus (stage 18), the substantia nigra, the core region of the lamina quadrigemina and the olivary nucleus in the medulla oblongata, that is involved.
When considering the embryogenesis of the various brain vesicles one should always distinguish between the development of the roof and that of the floor. Analogous to the spinal cord situation, the former develops from the dorsal part of the neural tube while the latter emerges from its ventral portion.