The mesencephalon represents the most anterior part of the brainstem and, morphologically, is the cerebral vesicle that has the simplest construction. It forms rostrally the basic tube-shaped continuation of the rhombencephalon. Its walls consist of centrally lying gray matter (aral plate and basal plate) as well as of peripherally adjoining white matter (marginal zone). The central cavity is originally broad, but during its development closes down into a narrow canal, the mesencephalic duct (aqueduct of sylvius). This connects the IIIrd ventricle of the diencephalon with the IVth ventricle of the rhombencephalon. In contrast with the rhombencephalon there is no broadening and divergence of the side walls. In addition, the mesencephalon represents the only cerebral vesicle that experiences no further subdivision.
Development of the floor: basal plate and peduncle
From the stage 13, the neuroblasts of the basal plate form two groups of somatoefferent (motor) nuclei zones for the cerebral nerves III (n. oculomotorius) and IV (n. trochlearis). These are involved in the innervation of the outer eye muscles. Note that the IVth cerebral nerve is the only one that leaves the brainstem dorsally.
The parasympathetic innervation of the inner eye muscles takes place from the Edinger-Westphal nucleus zone (nucl. accessorius n. III) (m. constrictor pupillae and m. ciliaris).
Still controversial is the origin of the neuroblasts that form the red nucleus as well as the substantia nigra. Being discussed are emigrations from the aral plates or from the basal plates.
With regard to the red nucleus and substantia nigra, subcortical centers of the extrapyramidal motor functions are involved.
The marginal zone finally closes ventrally through the formation of the peduncle (crura cerebri). This consists of nerve fibers that descend to deeper lying centers from the cerebral cortex.
Development of the roof: aral plate and quadrigeminal plate (tectum)
The four hills of the tectum arise through the emigration of neuroblasts from the aral plates. These nerve cells settle in the median section of the roof and then form there two longitudinal ridges, separated by a median sulcus. A transverse groove, which arises later, further divides the two strands, creating the four hills of the tectum, the rostral and caudal colliculi. At the quadrigeminal plate, the lateral geniculate nuclei of the diencephalon are joined, the medial and lateral geniculate bodies
The nucleus zone of the rostral colliculi contains information from the eye retina. The anterior hill then is also in the service of steering optical reflexes. In contrast, the caudal colliculi represent interfaces of the auditory pathways that are involved in controlling auditory reflexes. In the lower vertebrates the quadrigeminal plate has an additional task in the extensive processing of optical and acoustic information. In higher mammals with more pronounced visual and auditory cortexes the tectum is however only responsible for steering the corresponding unconscious reflex processes.
Synopsis of mesencephalon development
The mesencephalon represents the anterior part of the brainstem and the morphologically most inconspicuous cerebral vesicle. It is the only one that will not be further subdivided into secondary vesicles. This originally tube-shaped brain section with wide lumens experiences a thickening of its walls at the expense of the clearing that will thereby gradually close down into a narrow canal, the mesencephalic aqueduct.
- Out of the dorsal part, the roof, develops the quadrigeminal plate with the rostral and caudal colliculi. These go over laterally into the lateral and medial geniculate nuclei of the diencephalon. In higher mammals the tectum is in the service of reflex responses to optical and acoustic stimuli.
- The ventral part, floor, comprises the somatomotor nucleus region of the IIIrd and IVth brain nerves as well as the visceralmotor (parasympathic) nucleus region of the IIIrd brain nerves. Whether the neuroblasts that for the red nucleus and the substantia nigra are engendered via the basal or the aral plates, is still contested.
- Finally, the marginal zone thickens to become the peduncle (cerebral crus). Here, an accumulation of fibers is involved that extend from the cerebral cortex at the mesencephalon and further to the subordinate centers.