Chapter 153. The Diencephalon (second Vesicle) - Review of Medical Embryology Book

The diencephalon develops from the median portion of the prosencephalon and consists of a roof plate, 2 alar plates, and the third ventricle. There is a question of whether it has a floor or basal plate. It is bounded posteriorly by a plane passing behind the pineal gland and mamillary bodies and anteriorly by a plane passing just rostral to the optic chiasma and encircling the foramen of Monro. The lamina terminalis is considered to be a part of the telecephalon

THE ROOF PLATE consists of a single layer of ependymal cells covered by vascular mesenchyme (meninges). The 2 layers later combine to form the choroid plexus of the third ventricle, which closes it from above

The most caudal part of the roof plate does not take part in formation of the choroid plexus but develops into the pineal body or epiphysis. The latter initially appears as an epithelial thickening in the midline, begins to evaginate by week 7, and eventually forms a solid organ. Its structure and function are both neural and glandular
Occasionally, the roof plate forms another evagination near the foramina of Monro, called the paraphysis, which sometimes persists into postnatal life and may give rise to small cysts. It is usually seen in lower vertebrates
The roof plate is also thought to give rise to the epithalamus, a group of nuclei located on each side of the midline close to the pineal glan It may, however, arise from the alar plates

The epithalamic region is originally large, but it regresses to a small area where the habenular nuclei are seen. The latter form a link in the olfactory conduction path and are connected to each other, across the midline, by a group of nerve fibers collectively called the habenular commissure
Just posterior to the pineal body, a small commissure, the posterior commissure, connects the paramedian epithalamic nuclei

A distinct longitudinal groove, the hypothalamic sulcus, divides the alar plate into a dorsal and ventral region, the thalamus and hypothalamus, respectively

This sulcus is of a different nature than the sulcus limitans since it does not form a dividing line between motor and sensory areas

The thalamus is important in evolution. It begins as a simple relay station in the opticomesencephalic pathways, but gradually becomes a polysensorial connection, interposed between the sensory receptors and the cerebral cortex. Its major role is in humans

After proliferation, the thalami gradually bulge into the diencephalic lumen, and the 2 may fuse in the midline to form the massa intermedia or interthalamic connexus
The thalamic nuclear areas eventually form 2 distinct nuclear groups

A dorsal nuclear group: important for the reception and transmission of visual and auditory impulses
A ventral nuclear group: a passage and relay station for higher centers

The hypothalamus is the coordinating and effector receptor center of autonomic function in all vertebrates. It differentiates into a number of nuclear groups that serve as regulation centers of visceral functions, , sleep, digestion, body temperature, emotional behavior, et

One of the groups, the mamillary bodies, forms rounded elevations on the ventral surface of the hypothalamus on either side of the midline and connects the hypothalamus to higher centers of the rhinencephalon and lower brainstem centers
The lateral wall of the diencephalon also provides the pallidum which is the only striated intercerebral structure having a diencephalic origin

The floor of the diencephalon gives rise to the neural primordia of the eyes and between the 2, the funnel-shaped infundibulum and future neural lobe of the hypophysis