Development of the diencephalon (2nd cerebral vesicle) and hypophysis

Introduction

The diencephalon, together with the telencephalon, develops out of the prosencephalon. In contrast to the brainstem, here there is a massive spreading of the alar plates at the expense of the basal plates.

From the 5^th week (stage 14) two lateral cavities, the cerebral vesicles (future hemispheres), develop in the region of the prosencephalon. The diencephalon, on the other hand, arises out of the caudal portion of the prosencephalon. The diencephalon remains unpaired and represents the transition region between the telencephalon and the mesencephalon. The cavities of the telencephalon are connected with those of the diencephalon via the interventricular foramina (foramina Monroi). Through their massive increase in size, up to the end of the embryonic period, the hemisphere vesicles substantially surround the diencephalon. In this stage, there is a adhesion of the hemisphere vesicles with the diencephalon so that they form an anatomical unit.

media/module22/v9b_schemdevtelen.gif — **Fig. 75 -** Schematic diagram showing how the hemisphere vesicles grow around the diencephalon

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Fig. 75

In the upper image, the dashed green line indicates the region which will later be grown over by the hemisphere vesicles. The green shaded area in the lower picture corresponds to the adhesion locations between the telencephalon and the diencephalon.

Rostrally, the diencephalon ends at the level of a transverse plane through the optic chiasm and the interventricular foramina (lamina terminalis) while, caudally, it extends past the epiphysis and the mammilla bodies.

media/module22/v9d_dienceph.gif — **Fig. 76 -** Schematic diagram of the diencephalon boundaries at around the 8th week

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Fig. 76

Median section through the prosencephalon of a human embryo at around the 8^th week. The yellow shaded region shows schematically the spreading of the diencephalon. The commissural fibers of the telencephalon and diencephalon are shown in black (for their designations, see fig. 80).

The side walls of the diencephalon experience a very special transformation in which the roof, aral and floor plates are involved. As was already said, the basal plates are largely unaffected by this since, at the same time, they end with the sulcus limitans at the transition to the III^rd ventricle (according to some authors these structures should nevertheless extend further rostrally). From the junction of the mesencephalic aqueduct to the interventricular foramen, though, a hypothalamic sulcus appears on both sides in the side walls of the diencephalon.

media/module22/v3g_lamalaire.gif — **Fig. 77 -** Median section through the prosencephalon at around the 8th week

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Fig. 77

Note the rostral end of the basal plate at the transition to the diencephalon. In contrast, the alar plate extends out to the interventricular foramen.