Development of the myelencephalon (5th brain vesicle or medulla oblongata or bulbus spinalis)
The myelencephalon represents the caudal part of the rhombencephalon. In adults it forms the medulla oblongata or bulbus spinalis. The myelencephalon accommodates most of the nucleus regions of the cranial nerves as well as the centers that monitor breathing, cardiac rhythm, swallowing, coughing, and vomiting, among others. It is a transition region between the spinal cord and the brain, so that numerous structural homologies exist to the spinal cord, namely in the caudal region of the myelencephalon.
Development of the caudal part of the myelencephalon
In the "tube-shaped" caudal part of the myenlencephalon the neuroblasts emigrate out of the aral plate into the marginal zone whereby the gracile (medial) and cuneate (lateral) nuclei are continuously being formed. Here the interfaces of the proprioceptive and epicritical sensitivity are involved that project to the cerebellum and thalamus. On the other hand, from the 4th month the pyramidal tract (or corticospinal tract as the pathway of the voluntary motor functions) passes through the ventral part of the myelencephalon. The intersection of the pathways for both sides of the body, the pyramidal decussation, marks the boundary between the myelencephalon (medulla oblongata) and the spinal cord (medulla spinalis).
Development of the rostral part of the myelencephalon
In the "open", rostral part of the myelencephalon, the formation of the flexure on its dorsal, concave side leads to a rhombus-shaped broadening of the roof of the ventricle system (formation of the secondary brain vesicle and the cerebral flexures). The lateral edges of the neural tube move away from each other (like opening a book) whereby the widening of the IVth ventricle occurs.
Due to this pulling apart, the roof of the myelencephalon becomes exceedingly thin, creating the caudal medullary velum with the choroid plexus of the IVth ventricle. With the latter, a single layer of specialized ependyma cells (lamina epithelialis) is involved that becomes underlayered by the vessel-rich mesenchyma of the pia mater (tela choroidea). Through the proliferation of these elements at their lateral borders the choroid plexus arises. This forms – like all plexus choroideï – cerebrospinal fluid (liquor cerebro-spinalis) as an ultrafiltrate of the blood. In addition, the plexus mediates the transport of nutrients and electrolytes and it eliminates toxic metabolism products.
The extreme reduction of the wall thickness in the dorsal part of the neural tube leads to a meeting of the ependymal epithelium (lamina epithelialis) with the leptomeninx (pia mater and arachnoidea), whereby a choroid plexus arises. This extends into the ventriclar space and forms the cerebrospinal fluid.
- In contrast to the roof, in the regions of the side walls and the floor, a thickening of the neural tube occurs. There the floor plate (medial) and the aral plate (lateral) lie beside each other, separated by the sulcus limitans. Parallel to this transformation process the columns of gray matter that cohere to each other in the spinal cord fall apart into individual nuclei zones (of which some can exhibit considerable longitudinal stretching). From the 28th day the motor nuclei zones of cranial nerves V to XII arise from the floor plates of the rhombencephalon, and somewhat later, at around the 5th week, the corresponding sensory nuclei zones arise from the aral plates . All of these nuclei zones lie along 7 longitudinal trajectories of which the vegetative are beside the sulcus limitans while the somatosensory are situated laterally and the somatomotor medially. In the chapter concerning the disposition of the cranial nerves the arrangement of these nuclei zones will be extensively discussed. Here it is only noted that the ventral displacement of the white matter due to the transformation processes mentioned above contributes to the fragmentation of the floor plates and the aral plates into individual nuclei zones.
- From the side walls of the myelencephalon the lower portions of the cerebellum (inferior cerebellar peduncule) emerge. They guide the spino-cerebellar, bulbo-cerebellar and vestibulo-cerebellar fibers.
|As a reminder|
- The olivary nuclei consist of neurons that emigrate from the aral plate and settle in the bulbus spinalis. This is the first supra-spinal structure that arises. The olivary nuclei represent a special nuclei zone of the reticular formation and form an interface for involuntary motor functions.
Disposition of the cranial nerves
Overview of myelencephalon development
- Formation of the pons and the simultaneous diverging of the side walls of the neural tube with the arrangement of the aral plate and floor plate in a slanted plane
- Extension of the cavity system while the IVth ventricle is being formed
- Reduction of the wall thickness of the roof of the IVth ventricle to form the medullary velum
- Displacement of the gray matter into the floor of the IVth ventricle and, as the nucleus zone of the cranial nerves, arranging to trajectories that correspond to an extension of the posterior horn (somatosensory nucleus zones), lateral horn (viscerosensory and visceromotory nucleus zones) and the anterior horn (motor nucleus zone)