Diencephalon

The diencephalon is a subdivision of the prosencephalon located between the telencephalon and the mesencephalon. It is composed of bilateral structures arranged on either side of the third ventricle: thalamus, hypothalamus, epithalamus, and subthalamus. These structures are involved in regulating numerous sensory, motor, endocrine, autonomic, and behavioral functions.

During embryonic development, the diencephalon forms through the division of the prosencephalon, which itself arises from the anterior neural tube and initially differentiates into three primary vesicles: prosencephalon, mesencephalon, and rhombencephalon. The prosencephalon subsequently subdivides into the telencephalon (which gives rise to the cerebral hemispheres) and the diencephalon.

The two halves of the diencephalon are connected by the interthalamic adhesion. The walls and roof of the third ventricle are formed by the lamina terminalis and the choroid tela. The third ventricle communicates with the lateral ventricles via the interventricular foramen and with the mesencephalon through the cerebral aqueduct. Several recesses are also present, including the optic recess, the neurohypophyseal (infundibular) recess, and the inframammillary recess.

Certain regions of the diencephalon correspond to circumventricular organs, where the blood-brain barrier is reduced, such as the subcommissural organ and the subfornical organ, which play sensory and secretory roles. The diencephalon is connected to the cerebral hemispheres via the internal capsule.