In the embryo, as in the adult, the roof of the diencephalon is very thin
comprised only of ependyma plus adjacent pia called the
tela choroidea. When
blood vessels invade the tela choroidea the
choroid plexus of the
3rd ventricle, fourth ventricle, and
lateral ventricles
develop. The thin medial wall of the ventricle is the
choroid fissure; its most rostral point is at the
interventricular foramen.
In the adult the choroid fissure is “C”-shaped and continuous in the medial
walls of the parietal and temporal lobes.
Early in development, the diencephalon develops 2 pairs of prominent
swellings in the walls of the 3rd ventricle. The swellings represent only
the alar plate; there is no basal plate. The largest mass is the
thalamus dorsally,
separated by the
hypothalamus
and
hypothalamic sulcus.
Where the roof plate thickens along the medial wall of the thalamus are the
smaller swellings of the
epithalamus comprised
of the a) midline pineal gland, b) paired habenular nuclei, and c) paired
stria medullaries.
The anterior wall of the diencephalon’s slit-like 3rd ventricle is the
lamina terminalis. At this anterior location in the 3rd ventricle, the
interventricular foramina (of Monro) connects the 3rd ventricle with the
lateral ventricles of the developing hemispheres.
