The roof plate and the IIIrd ventricle
Similar to the rhombencephalon the side walls of the diencephalon spread apart, causing the roof of the IIIrd ventricle to be reduced to a single-layered epithelial lamina. In analogy to the IVth ventricle the choroid plexus of the IIIrd ventricle arises from this epithelial lamina (a derivative of the ependymal lining of the cavity system) and the richly vascularized tela choroidea (that derives from the pia mater).
At the location where the wall of the neural tube thins to become the ependymal epithelial lamina (roof of the diencephalon), a connection with the leptomeningeal tela choroidea (pia mater) occurs. This engenders the choroid plexus of the IIIrd ventricle that invades the ventricle lumen and forms cerebrospinal fluid.
In addition, the circumventricular organs arise in the region of the IIIrd ventricle, the region of the brain that has no blood-brain barrier. These are richly vascularized by fenestrated capillaries. Just as in endocrine glands, the high porosity of the vessels relieves the information exchange between blood and the cerebrospinal fluid. This is important from the point of view of neuroendocrine regulation (e.g., thirst, homeostasis of electrolytes and hormones).
The subfornical organ, the organum vasculosum of the lamina terminalis (or supraoptic crest) and the subcommissural organ are numbered with the circumventricular organs. Certain authors also count the epiphysis cerebri and the neurohypophysis as belonging among them. In addition, at the level of the IVth ventricle, the area postrema joins them.
The caudal region of the roof from the IIIrd ventricle is not involved in forming the choroid plexus. From this segment arises primarily the epiphysis cerebri (pineal gland) that makes its initial appearance as an epithelial thickening and after the 7th week bulges to become a solid organ with neuronal and glandular features. It is indeed a neuroendocrine gland. It consists of pinealocytes, which secrete melatonin and serotonin, and out of glial cells. Until puberty melatonin inhibits the formation of gonadotropic hormones or their control factors. Moreover, it plays an important role in the control of the circadian rhythm.
In adults there is a gradual involution of the epiphysis due to an accumulation of calcifications (acervulus) that can serve neuroradiologists as an orientation aid in x-ray images.