Module
22
Icon module 22

Formation of the secondary cerebral vesicles and the cerebral flexures

Illustration

Over the course of the fifth week, a subdivision of the prosencephalons occurs into the telencephalon and the diencephalon. Parallel to this from the rhombencephalon the metencephalon and the myelencephalon arise.
Since the mesencephalon experiences no further articulation, the brain now consists of 5 secondary cerebral vesicles

The kinks in the region of the cranial flexure (flexura mesencephalica) and cervical or cranial flexure (flexura cervicalis) increase and between the metencephalon and the myelencephalon a bending of the neural tube in the other direction occurs, the pontine flexure (flexura pontina).

From the sixth week a distinct growth of the cerebral vesicles of the telencephalons on both sides occurs and the appearance of further bending leads to a stronger spreading of the derivates of the roof plate (dorsal collection in the visceral and somatosensory area) as opposed to the derivates of the floor plate (ventral collection in the visceral and somatomotoric area). At the same time, massive cell movements take place that lead to a collecting of gray substances in the periphery so that, centrally, white substances dominate at the end. This represents a distinct difference between the brain and spinal cord as will be shown in the context of their respective development. The tremendous development of the hemisphere vesicles of the telencephalon from the 6th and 7th weeks distinguishes humans from the other higher mammals

One notes that at around the eighth week (stage 23) the floor plate does not extend past the third cerebral vesicle. Thus motoric core regions are absent rostral of the mesencephalon. In contrast, the roof plate reaches the diencephalon.

Fig. 18 - Median view of the prosencephalon (telencephalon and diencephalon) at about the eighth week
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  1. Sulcus diencephalicus dorsalis
  2. Dorsal part of the roof plate
  3. Sulcus limitans
  4. Floor plate
  5. Sulcus hypothalamicus

Legend
Fig. 18

The floor plate ends at the transition to the diencephalon while the roof plate extends to the interventricular foramen.

At around the 38th day (stage 16) a ventrally directed rolling up of the neural tube around the cranial end of the notochord takes place. In this the third vesicle (metencephalon) gradually comes to lie on top while the advancing bending in the region of the cranial bend has the result that the most anterior two cerebral vesicles are put in front of the notochord.

Navigation
Fig. 18a - Side external view in the stage of the 5 cerebral vesicles
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P
Prosencephalon (telencephalon + diencephalon)
M
Mesencephalon
R
Rhombocephalon (metencephalon + myencephalon)
1
Cranial flexure (flexura mesencephalica)
2
Cervical flexure (flexura cervicalis)
3
Pontine flexure (flexura pontina)

Legend
Fig. 18a
Fig. 18b - Side external view in the stage of the 5 cerebral vesicles
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P
Prosencephalon (telencephalon + diencephalon)
M
Mesencephalon
R
Rhombocephalon (metencephalon + myencephalon)
1
Cranial flexure (flexura mesencephalica)
2
Cervical flexure (flexura cervicalis)
3
Pontine flexure (flexura pontina)

Video

Animation in plasticine of the development of the CNS (stages 10-23).