The formation of the cranial vesicles and flexures provides further development and segmentation of the brain and occurs in two phases (3, 10, 11, 12). The spinal cord is caudal to the myelencephalon and remains unsegmented.
The cranial neural tube during the fourth embryonic week divides into three morphologically and histochemically distinct primary vesicles:
- Prosencephalon (forebrain)
- Mesencephalon (midbrain)
- Rhombencephalon (hindbrain)
During the fifth embryonic week, five secondary vesicles develop from the primary vesicles:
Prosencephalon – two secondary vesicles
- Telencephalon: The telencephalon forms the cerebrum and basal ganglia.
- Diencephalon: The diencephalon develops into the hypothalamus, thalamus, epithalamus, neural retina, and pineal body.
Mesencephalon – one secondary vesicle
- Mesencephalon: The mesencephalon remains unchanged and gives rise to the tectum (superior and inferior colliculi), midbrain tegmentum, and cerebral peduncles.
Rhombencephalon – two secondary vesicles
- Metencephalon: The metencephalon forms the pons and cerebellum.
- Myelencephalon: The myelencephalon develops into the medulla.
Concave bending of the neuraxis (primary flexures) occurs during the fourth embryonic week.
- Mesencephalic flexure: The mesencephalic flexure occurs around the mesencephalon.
- Cervical flexure: The cervical flexure occurs at the junction of the metencephalon and spinal cord.
- Pontine flexure: A third flexure, the pontine flexure, occurs during the fifth embryonic week between the mesencephalic and cervical flexures and is concave ventrally. The roof of the mesencephalon becomes thinned as a result of the pontine flexure and produces the roof of the fourth ventricle.