Formation of the primitive cerebral vesicles
Before the 25th day that is before the closure of the anterior neuroporus, three cerebral vesicles arise at the anterior end of the neural tube
- the prosencephalon
- the mesencephalon
- the rhombencephalon
In the region of these cerebral vesicles two kinks of the neural tube ventrally occur, one of which is in the region of the mesencephalon known as the cranial flexure (flexura mesencephalica), the other in the transition region between the rhombencephalon and the spinal cord known as the neck bend (Flexura cervicalis).
The anlage of the spinal cord and the three named cerebral vesicles together form the basis of the CNS. In addition, the cerebral vesicles contain the higher sensory organs: smell, vision and hearing.
Indeed, in the case of the optic vesicles protrusions of the prosencephalon wall are involved. They grow out laterally and, in the surface ectoderm, they induce the formation of the lens placodes
The olfactory placodes are induced by the adjoining mesenchym as well as by two protrusions of the neuroepithelum in the region of the anterior prosencephalon
The formation of the otic or auditory vesicles begin with the formation of the ectodermal auditory placodes which are induced by the neuroepithelium of the myelencephalon at the level of the die rhombencephalon.
Besides the sequence that has been mentioned from the 4th to the 5th week three primitive cerebral vesicles exhibit an additional, segmental articulation. Here, narrow, temporary thickenings are involved that are known as neuromeres (or rhombomeres in the region of the rhombencephalon). In the fifth week (stage 14) can be distinguished in a human embryo:
- 1 telencephal neuromere (A)
- 4 diencephal neuromeres (B)
- 2 mesencephal neuromeres (C)
- 8 rhombomeres (at the rhombencephalon) (D)
These segmented structures are connected to homeotic genes. The relationships with the segmental organization of the paraxial mesoderm and the branchial arch will be discussed in the context of face development