Development of the metencephalon (4th brain vesicle – pons, cerebellum)
The metencephalon develops in the anterior region of the rhombencephalon from the pons to the mesencephalic isthmus. Two parts develop from the metencephalon
- The posterior or dorsal part or roof out of which the cerebellum develops, a coordination center that serves movement and balance
- The anterior or ventral part or floor from which the pons arises
Development of the Pons (floor)
The pons represents the rostral extension of the myelencephalon and is also structurally related to it. Although the lateral walls of the neural tube approach each other again, the fundamental morphologic features of the aral and floor plates remain preserved. They deliver the nucleus zones of cranial nerves V to VIII.
In the metencephalon the floor plates also deliver three columns of the motor nucleus zone: the column of the general somatic efferent fibers (ASE), the column of the special visceral efferent fibers (SVE) and the column of general visceral efferent fibers (AVE). In addition, there is a considerable thickening of the marginal zone ventral to the floor plates because, at this place, numerous fibers pass past it that connect the spinal cord with the cerebral cortex and the cerebellum (see below).
From the aral plates the columns of the general visceral afferent fibers (AVA), those of the special visceral afferent fibers (SVA) and the columns of the general somatic afferent fibers (ASA) emerge. Moreover, from the aral plates, neurons migrate ventrally so that the typical thickening, termed the pons, arises. The perikarya of these neurons accumulate at the pontine nuclei (pons nucleus) from which fibers pass below the intersection of the median plane (decussation) to the cerebellum. These ponto-cerebellar fibers delimit the floor of the rhomboid fossa laterally as the middle inferior cerebellar penducle. Finally, the pons nucleus becomes the integrating interfaces in the information transmission between the cerebral cortex and the cerebellar cortex as well as the spinal cord.
|As a reminder|
Development of the cerebellum (roof)
The cerebellum develops from the dorsolateral part of the aral plates. These thicken and incline themselves medially, whereby at around the 6th week the rhombic lips of the metencephalon arise. At the beginning the cerebellar anlage only bends towards the cavity of the IVth ventricle and only after the 12th week do they extend further dorsally. Caudally, the rhombic lips lie wide apart, but converge cranially. There they meet up and thus form a transversal thickening, the cerebellar plate, which slowly grows over the roof of the IVth ventricle
At around the 12th week this increase in circumference forms a central plate, the vermis as a purely axial structure as well as two lateral thickenings, the future cerebellum hemispheres (neo-cerebellum). On the backside of the cerebellum plate a transverse groove arises, the fissura posterolateralis that delimits the nodulus from the vermis as well as the flocculus from the cerebellar hemispheres (flocculonodular lobe).
At around the 14th week the cerebellum is divided by the fissura prima into an anterior lobe and a posterior lobe.
The anterior lobe thus comprises the cerebellar lobes rostral to the fissura prima, namely the lingula.
The posterior lobe consists of the cerebellar lobes caudal to the fissura prima with the exception of the flocculonodular lobe.
As shown above, the cerebellar anlage only grows dorsally after the 12th week.
After the 16th week further fissures arise that organize the cerebellum into lobes and folds (folia) resulting in, like with the cerebrum, a considerable surface area enlargement of the cerebellar cortex. Over the course of the development the cerebellum will completely cover the roof of the IVth ventricle
The structure of the cerebellum mirrors its phylogenetic development.
- The archicerebellum (archi = Greek for "old") consists of the flocculonodular lobe (vestibulocerebellum) and lingula (vestibulocerebellum and spinocerebellum). It represents the phylogenetically oldst part and takes part in spatial orientation as well as maintaining balance. The nucleus fastigii is assigned to the archicerebellum as its interface
- The paleocerebellum comprises the vermis as well as the lobus anterior (with the exception of the lingula). It is phylogenetically younger and processes the musculoskeletal information in order to steer the posture and position reflexes (spinocerebellar pathways). The associated cerebellar nuclei are the nuclei globosi and emboliformes
- Finally, the neocerebellum is made up of the lobus posterior (cerebellar hemispheres). This is phylogenetically the youngest part. The dentate nuclei function as the interfaces. The neocerebellum slides in between the two older portions, and in humans it is especially strongly developed. Its task consists in the focused monitoring of the supraspinal motor functions (corticopontocerebellar connections). In contrast to the paleocerebellum the development of the neocerebellum at birth is not yet complete
Overview of metencephalon development
The metencephalon corresponds to the rostral part of the rhombencephalon whereby, just as with the myelencephalon, a pushing apart of the side walls is involved so that the aral plates come to lie lateral to the floor plate.
With regard to the development of the ventral and dorsal portions, however, there exist the following differences:
- The ventral portion or floor develops into the pons. The thickening refers mainly to the fibers that connect the cerebral cortex, spinal cord and cerebellum with one another. The nuclear zones of the associated cranial nerves emerge from the aral and floor plates and the neurons that migrated out of the aral plate form the pontine nuclei in the depths.
- The dorsal part or roof, the future cerebellum, is a coordination center that serves movement and balance. The cerebellar anlage emerges from the fusion of the rhombic lips in the median. Through the emigration of neuroepithelial cells from the rhombic lips the surface cerebellar cortex arises as well as the cerebellar nuclei in the depths.