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).
in the five vesicle stage
at around the 38th day
of the myelencephalon
at around the 39th day
- Gracile nucleus
- Cuneate nucleus
- Aral plate
- Basal plate
- Pyramidal tract
(corticospinal tract) - Central canal
Cross-section through the caudal part of the myelencephalon. Note the structural similarities to the spinal cord that are present in this brain section.
Fig. 50Development 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.
in the five vesicle stage
at around the 38th day
of the myelencephalon
at around the 6th week
- IVth ventricle
- Roof plate
- Aral plate
- Floor plate
The side walls begin to pull apart. This makes the roof (roof plate) wide and thin. Similar to the spinal cord, the aral plates and basal plates with the sulcus limitans between them can be distinguished in the lateral wall.
Fig. 52in the five vesicle stage
at around the 44th day
of the myelencephalon
at around the 7th week
- IVth ventricle
- Roof plate
- Aral plate with somato- and viscero afferent portions
- Floor plate with somato- and viscero efferent portions
- Sulcus limitans
- Olivary nucleus
The roof plate is pulled still further apart and now canopies the rhombic groove of the IVth ventricle. By means of this deformation process the aral plate and the floor plate come to lie practically in a horizontal plane. Nevertheless, the sulcus limitans still runs between them. Moreover, via emigration of neurons from the aral plate, the olivary nucleus is formed ventrally in the bulbus spinalis.
Fig. 54Summary:
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.
at the five vesicle stage
at around the 8th week
of the myelencephalon
at round the 8th week
- IVth ventricle
- Roof plate
- Choroid plexus
- Olivary nuclei
- SSA (somatosensory zone)
- ASA (somatosensory zone)
- SVA (viscerosensory zone)
- AVA (viscerosensory zone)
- AVE (visceromotor zone)
- SVE (visceromotor zone)
- ASE (somatomotor zone)
Through the stretching and thickness reduction of the roof plate the medullary velum arises at which time a choroid plexus develops. In this stage the floor plates and the aral plates organize themselves on all sides into seven rows of nuclei zones (motor: medial; visceral: on both sides of the sulcus limitans; sensory: lateral). The olivary nuclei stem from the aral plates and are now clearly demarcated.
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- 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
If you move the mouse cursor over the ABCD areas in the left half of the upper illustration (cross section through the thoracic marrow) the corresponding structures of the myelencephalon are highlighted in the lower one.
Note: This is an idealized, schematic diagram.
- A
- Somatosensory zone
- SSA
- Special somatic afferent fibers
- ASA
- General somatic afferent fibers
- B
- Viscerosensory zone
- SVA
- Special visceral afferent fibers
- AVA
- General visceral afferent fibers
- C
- Visceromotor zone
- AVE
- General visceral efferent fibers (branchial arches)
- SVE
- Special visceral efferent fibers (X and IX)
- D
- Somatomotor zone
- ESG
- General somatic efferent fibers
The cranial nerves mediate the four general qualities – general because they are present throughout the body – as well as three further modalities that are only present in the cranial region (smell and taste, vision and hearing) and, for this reason, are designated as being "special".
The gray matter that is associated arranges itself in the neural tube into columns.
The motor nuclei zone of the cranial nerves arise from the floor plates in the cranial region:
- The columns of the general somatic efferent fibers (ASE) represent the continuation of the anterior horn of the spinal cord and, in the region of the myelencephalon, they deliver the somatomotor nuclei zone of the XIIth cranial nerves (hypoglossal nerve) that innervate the 4 occipital myotomes and the tongue. From these columns in the metencephalon and mesencephalon the nucleus zones of the cranial nerves III (oculomotor nerve), IV (trochlear nerve) and VI (abducens nerve) arise that subserve the control of the outer eye muscles (as derivatives of the prechordal mesenchyma).
- The columns of the special visceral efferent fibers (SVE) comprise the motor neurons of the cranial nerves IX, X and XI in the region of the myelencephalon. They subserve the innervation of the striated branchial arch muscles of the pharynx and larynx.
- The columns of the general visceral efferent fibers (AVE) contain the perikarya of the preganglionary parasympathic neurons. In the myelencephalon they form the dorsal nucleus of the vagus nerve that serves the control of the cardiac muscles as well as the smooth muscles of the intestines. Further parasympathic nucleus zones are the inferior salivatory nucleus of the glossopharyngeal nerve, whose target organ is the parotid gland, the superior salivatory nucleus of the facial nerve, which supplies the rest of the cranial glands, as well as the accessory oculomotor nucleus (Edinger-Westphal nucleus), which is responsible for the control of the inner eye muscles (smooth muscles of the iris sphincter muscle and the ciliary muscle).
From the aral plates arise the sensory nucleus zones of the cranial nerves:
- The columns of the general visceral afferent fibers (AVA) in the form of the caudal part of the nucleus of the solitary tract (vagus nerve), the neurons of which receive the enteroceptive afferent fibers from the digestive tract and the heart.
- In the rostral part of the nucleus of the solitary tract the columns of the special visceral afferent fibers (SVA) receive information from the taste buds of the mouth cavity via the cranial nerve fibers VII, IX and X.
- The columns of the general somatic afferent fibers (ASA) process touch stimuli, temperature and pain perceptions from the skin in the cranial region in the sensory nucleus of the trigeminal nerve. The nuclei zone extends over the whole lengths of the mesencephalon and rhombencephalon.
- The columns of the special somatic afferent fibers (SSA) receive sensory input from hearing and balance organs via the VIIIth 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)