Module
22
Icon module 22
Nervous system

Development during the embryonic phase (first 8 weeks)

Over the course of the embryonic phase bulging occurs on the two cerebral vesicles on the sides of the telencephalon out of which the hemispheres develop. Their arched roofs form the cerebral covering, the pallium, while the floor, the subpallium, develops to become the striatum. Between the pallium and the subpallium lies the lateral ventricle.

Fig. 90 - Cross section through the prosencephalon in stage 20 (ca. 48 days)
media/module22/v9c_develtelencephA_st20.gif

  1. Location where the diencephalon is covered by the expansion
    of the cerebral hemispheres
  2. IIIrd ventricle
  3. Hypothalamus
  4. Thalamus
  5. Medial part of the ganglionic eminence
  6. Lateral part of the ganglionic eminence
  7. Choroid plexus of the lateral ventricle
  8. Cortex of the right hemisphere
  9. Lateral ventricle
  10. Cortex of the hippocampus (archicortex)

Legend
Fig. 90

In this cross section through the prosencephalon at the level of the interventricular foramen (corresponding to A) the anlagen of the cerebral hemispheres as well as the choroid plexus in the lateral ventricles and the IIIrd ventricle (middle) can be clearly recognized. On the floor of the prosencephalons lie the medial (diencephalon) and the lateral (telencephalon) portions of the ganglionic eminence; the pallium is still thin in this phase. The green arrow indicates the kink location at the transition between the telencephalon and the diencephalon.

From stage 14 (ca. 33 days) the cerebral hemispheres are recognizable as two lateral bulges of the telencephalon.
The neuroblasts of the ventricular zone emigrate from two regions and populate two portions of the hemispheres. Thus, in the pallium, gray matter arises as the cortex; in the subpallium gray matter is in the form of the ganglionic eminences that emerge from the striate bodies. In contrast to the pallium, which in this phase thickens only minimally, the subpallium swells considerably so that the ventricle space becomes increasingly constricted. Later, in the lower parts of the interhemispheric fissure (fissura longitudinalis cerebri) the commissures of the two hemispheres become connected with each other. The corpus callosum represents the mightiest commissure (see below: commissures of the telencephalon).

Fig. 91 - Cross section through the prosencephalon in stage 23 (ca. 56 days)
media/module22/v9c_develtelencephA_st23.gif

  1. Location where the diencephalon is covered by the expansion
    of the cerebral hemispheres
  2. IIIrd ventricle
  3. Hypothalamus
  4. Thalamus
  5. Medial part of the ganglionic eminence
  6. Lateral part of the ganglionic eminence
  7. Choroid plexus in the interventricular foramen and lateral ventricle
  8. Pallium
  9. Lateral ventricle
  10. Cortex of the Hippocampus (archicortex)
  11. Insular cortex
  12. Amygdala

Legend
Fig. 91

Cross section through the prosencephalon at the level of the interventricular foramen (corresponding to A) somewhat later. The hemispheres gradually cover the diencephalon (dotted line). The ganglionic emimence is relocated to the side and the location of the connection to the diencephalon thickens. Note the insular cortex anlage.

Finally, the two hemispheres meet in the median plane, remaining dorsally, though, separated from each other by the interhemispheric fissure (with the falx cerebri).  The medially lying adhesion location of the hemisphere roofs at the diencephalon stay thin and epithelialial. In this a longitudinally running groove arises against the ventricular space, the choroid fissure, out of which the choroid plexus of the respective hemisphere arises in stage 19 – 23 (ca. 7th to 9th weeks). The neopallium greatly expands and pushes back ventrally the palaeopallium (olfactory bulb and tubercle) and dorsally the archipallium (hippocampus).
At the same time, through the animated mitosis activity in the ventricular zone, a thickening of the subpallium occurs.

Fig. 92 - Cross section through the prosencephalon in stage 23 (ca. 56 days)
media/module22/v9c_develtelencephB_st23.gif

1
Location where the diencephalon is covered by the expansion
of the cerebral hemispheres
2
IIIrd ventricle
3
Hypothalamus
4a
Epithalamus
4b
Thalamus
5
Medial part of the ganglionic eminence
6
Lateral part of the ganglionic eminence
7a
Choroid plexus in the lateral ventricle
7b
Choroid plexus in the IIIrd ventricle
8
Pallium
9
Lateral ventricle

Legend
Fig. 92

In this cross section caudal to the interventricular foramen (see section level B) the further rostral lying connection between the cerebral hemispheres is not affected.  Note and compare the kink locations at the transition between the telencephalon and the diencephalon (green arrows) with the later figures sectioned at B. Also note the atrophy of the epithalamus that is, however, still visible.