The hypothalamus

The alar plates (side walls and floor of the diencephalon)

From the 7^th week (stages 16-19) a thickening of the diencephalon walls occurs as well as the formation of three copious swellings that protrude into the III^rd ventricle. Dorsally to ventrally, these are the epithalamus, the thalamus and the hypothalamus. In 70% of individuals the thalami adhere to one another in the median plane (interthalmic adhesion), whereby the III^rd ventricle receives a ring-shaped form.

media/module22/v9a_develtelenceph.gif — **Fig. 80 -** Schematic median view of the anterior brain at around the 8th week

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Fig. 80

Besides the most important derivatives of the alar plates (shaded violet part), this median section through the anterior brain shows the lamina terminalis and die commissure plate (19, light blue). Moreover, the commissures (dark red), the furrows in the diencephalon as well as the end segment of the basal plate (orange) can be recognized. This spreads together with the sulcus limitans.

The epithalamus

The epithalamus (stage 16) develops out of the roof and the upper portions of the alar plates. As was already mentioned, the epiphysis (stage 15) and the choroid plexus of the III^rd ventricle, which is composed of the lamina epithelialis and the tela choroidea, belong to it. The epithalamus is delimited form the thalamus by the dorsal diencephalic sulcus (stage 17). Sometimes, in humans, a diverticulum, the paraphysis, arises above the foramina of Monro that in lower vertebrates is always present. The habenular nuclei arise laterally in the epithalamus that are assigned to the limbic and olfactory systems and play an important role in the release of emotional reactions as well in steering behavior. There are also two small transversal connections, the habenular commissure and the posterior commissure (that already belong to the midbrain). The massive growth of the thalamus leads to a disappearance of the dorsal diencephalic sulcus and to a caudal relocation of the elements of the epithalamus

media/module22/v9c_develtelencepha_st20.gif — **Fig. diencephale01 -** Cross section through the prosencephalon at stage 20 (ca. 49 days)

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This schematic cross section through the interventricular foramina shows the two cerebral hemisphere as well as the choroid plexus of the lateral ventricle and the III^rd ventricle. The ventricular eminence and the diencephalon (hypothalamus and thalamus) can be clearly seen.

media/module22/v9c_develtelencepha_st23.gif — **Fig. diencephale02 -** Cross section through the prosencephalon at state 23 (ca. 56 days)

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In contrast with the previous stage, the considerable growth of the cerebral hemispheres is apparent. Thereby the telencephalon slowly surrounds the diencephalon (green dotted line). The structures of the diencephalon develop and extend laterally. This increases the connection between the telencephalon and the diencephalon.

media/module22/v9c_develtelencepha_w11.gif — **Fig. diencephale03 -** Cross section through the prosencephalon (fetal period)

Legend

Through the considerable growth of the hemispheres the telencephalon increasingly surrounds the diencephalon (green dotted line). Through the arch-shaped expansion of the hemispheres various parts near the ventricles are moved together into the temporal lobes. To these semicircular structures are numbered the caudate nucleus, the hippocampus and the choroid plexus of the lateral ventricle.

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The hypothalamic sulcus

The hypothalamic sulcus divides the alar plate into dorsal and ventral regions, from which the thalamus and the hypothalamus emerge. The hypothalamic sulcus thus differs fundamentally from the sulcus limitans that runs between the basal plate and the alar plate!

Illustration

The basal ganglia and their embryonic development

In contrast with the epithalamus, over the course of evolution the thalamus gained in importance. Originally, the thalamus represents an interface in the optic tract but as a result of phylogenetic development it increasingly became a polysensorial node (integration, modulation, coordination).
In this it comes to a progressive subdivision of the thalamus into specific areas that are connected with the striate bodies (see development of the telencephalon), the afferents (which, after a changeover, project to the primary cortical areas) and the cerebral cortices. The subthalamic nucleus regions (Luys' bodies) are to be found in the caudoventral region of the diencephalon and assigned to the switching nuclei of the basal ganglion systems.

Legend

Fig. 81

This median section through the forebrain, besides the most important derivatives of the aral plate (differing shades of violet), shows the lamina terminalis and the commissure plate (light blue). Moreover, the commissures (dark red), the furrows in the diencephalon as well as the end segment of the basal plate (orange) can be seen. These spread together with the sulcus limitans.

The hypothalamus develops out of the subthalamic alar plate and from the base of the diencephalon (at most, some parts of the atrophying basal plate are also involved). Out of this emerge the hypothalamic nucleus region including the corpora mammillaria and the neurohypophysis. This nucleus region is in the service of steering vegetative functions (emotional states, eating behavior, sleep, bodily temperature, fluid balance, etc.).

The median ganglionic eminence

From the lateral wall of the alar plates the neuroblasts migrate into the white matter of the telencephalon and form there the median ganglionic eminence (future globus pallidus) as the only nucleus region of the basal ganglia that derive from the diencephalon.

The optic vesicle

At around the 19^th day the ocular anlagen are already recognizable as two lateral bulges on the floor of the prospective prosencephalon. Since up to this point the anterior neuroporus is not yet closed, two corresponding optic sulci (foveolae opticae) appear on the inside of the neural groove. The development of the ocular anlagen is induced by the prechordal plate.
From the 25^th day (stage 9), that is after the neural tube closes, the ocular anlagen widen to become the optic vesicles (vesiculae ophthalmicae), which remain connected with their area of origin, the future boundary between floor and side wall of the diencephalon, via the ocular peduncle (pedunculus ophthalmicus).