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

Illustration

Tabular overview

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.

media/module22/v7a_planC_st16.gif — **Fig. 57 -** Side view of the CNS
in the five vesicle stage
at around the 38th day

media/module22/v7b_develpontC_st16.gif — **Fig. 58 -** Cross-section
through the metencephalon
at around the 6th week

Legend

Fig. 57, 58

In this developmental stage the lateral walls in the region of the metencephalon lie closer together than in the myelencephalon, but the topography remains very similar.
Note the emigration of the neuroblasts from the aral plate dorsally while the rhombic lips are forming (yellow)

media/module22/v7a_planC_st18.gif — **Fig. 59 -** Side view of the CNS
in the five vesicle stage
at around the 44th day

media/module22/v7b_develpontC_st18.gif — **Fig. 60 -** Cross-section
through the metencephalon
at around the 7th week

Legend

Fig. 59, 60

At this point in time, the dorsolateral portion of the aral plates form the rhombic lips that gradually grow over the roof of the IV^th ventricle. The pontine nuclei develop out of the ventral portions of the aral plates. The marginal zone is considerably thickened.

media/module22/v7a_planC_st23.gif — **Fig. 61 -** Side view of the CNS
in the five vesicle stage
at around the 8th week

media/module22/v7b_develpontC_st23.gif — **Fig. 62 -** Cross-section through
the metencephalon
at around the 8th week

Legend

Fig. 61, 62

The rhombic lips are now fully formed. They cap the roof and bend themselves into the lumen of the IV^th ventricle. Only at around the 12^th week do they extend beyond the edge of the roof plate. The ventral part of the aral plate contains four groups of sensory nucleus zones while the floor plate comprises three groups of motor nucleus zones. The thickness of marginal zone has again increased.

As a reminder

SSA: special somatic afferent fibers (somatosensory)
ASA: general somatic afferent fibers (somatosensory)

SVA: special visceral afferent fibers (viscerosensory)
AVA: general visceral afferent fibers (viscerosensory)

AVE: general visceral efferent fibers (visceromotor branchial arches)
SVE: special visceral efferent fibers (visceromotor X and IX)

ASE: general somatic efferent fibers (somatomotor)

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 6^th week the rhombic lips of the metencephalon arise. At the beginning the cerebellar anlage only bends towards the cavity of the IV^th ventricle and only after the 12^th 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 IV^th ventricle

As a reminder

Differentiation of the cerebellum (roof of the metencephalon)

media/module22/v7d_develcervel_st1623.gif — **Fig. 63 -** Schematic dorsolateral side view (stage 16)
and dorsal view (stage 23) of the brainstem

Legend

Fig. 63

Left:
Due to the increasing bending of the pontine flexure, the side walls are pushed apart (blue arrow) and the rhombic lips (yellow) move towards the myelencephalon (red arrow).

Right:
Dorsal view of the brainstem of an embryo in the 8^th week. The roof of the IV^th ventricle has been removed. On the floor of the IV^th ventricle the aral and floor plates can be distinguished. Note the rhombic lips that join up cranially in the median due to the formation of the cerebellar plate.

At around the 12^th 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).

media/module22/v7d_develcervel_12w.gif — **Fig. 64 -** Dorsal view of the rhombencephalon at around the 12th week

Legend

Fig. 64

Dorsal view of an embryo's brainstem in the 12^th week. The cerebellar plate arises out of two lateral thickenings and almost completely covers the floor of the IV^th ventricle. The fissura posterolateralis walls off the nodulus and flocculus. Note the delayed development of the median part (vermis).

At around the 14^th 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.

media/module22/v7d_develcervel_16w.gif — **Fig. 65 -** Dorsal view of the rhombencephalon at around the 16th week

Legend

Fig. 65

Dorsal view of an embryo's brainstem in the 16^th week. The growth of the cerebellar hemispheres outweighs that of the vermis. Transverse grooves separate the phylogenetically older portions (archicerebellum) from the hemispheres (neocerebellum), namely rostral to the lingula and caudal to the flocculonodular lobe.
Note the lateral openings (foramen of Luschka) as well as the telencephalon in the lateral recess of the IV^th ventricle, which already partially covers the mesencephalon.

As shown above, the cerebellar anlage only grows dorsally after the 12^th week.
After the 16^th 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 IV^th ventricle

media/module22/v7e_develcervel_11w.gif — **Fig. 66a -** Development of the cerebellum, sagittal section

media/module22/v7e_develcervel_12w.gif — **Fig. 66b -** Development of the cerebellum, sagittal section

Legend

Fig. 66a

Development of the cerebellar anlage at around the 11^th week. The blue arrows indicate the beginning, dorsally directed extraventricular growth and the pink arrows the spreading into the IV^th ventricle.

Fig. 66b

Development of the cerebellum at around the 12^th week.
Note the development of the choroid plexus in the roof of the IV^th ventricle.

media/module22/v7e_develcervel_16w.gif — **Fig. 67 -** Development of the cerebellum, sagittal section

media/module22/v7e_develcervel_20w.gif — **Fig. 68 -** Development of the cerebellum, sagittal section

Legend

Fig. 67

From the 14^th week the cerebellum is subdivided into an anterior lobe and a posterior lobe by the fissura prima (A arrow). The fissura posterolateralis (B arrow) separates the flocculonodular lobe from the vermis and the hemispheres. The roof of the IV^th ventricle is successively overgrown by the cerebellum.

Fig. 68

Advancing division of the cerebellums into lobes and folds by the appearance of newer fissures from the 16^th week.

media/module22/v7e_develcervel_20w463.gif — **Fig. 69 -** Development of the cerebellum, sagittal section

Legend

Fig. 69

The same situation as in Fig. 68 but the cerebellar architecture is shown more schematically (archicortex: yellow, paleocortex: blue, neocortex and dentate nucleus: pink).

NB: Indeed, in the depths of the white matter of the cerebellum, four cerebellar nuclei are to be found on each side: the nuclei fastigii (interface of the archicerebellum), the nuclei emboliformis and globosus (interfaces of the paleocerebellum) as well as the here representatively nuclei dentatus (interfaces of the neocerebellum).

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

Rappel

From a functional point of view one can in summary say that the cerebellum contains afferent fibers from the motor cortex, from skin receptors, from the musculoskeletal system, from the eyes and ears.

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.