159. Commissures of The Telencephalon

  1. Introduction: the commissures of the telencephalon are the fiber systems that cross the midline and connect homologous portions of the right and left halves of the cortical hemispheres. The most important make use of the lamina terminalis which is the zone of junction of the 2 telencephalic vesicles, corresponding to the closure of the anterior neuropore, and extends from the roof plate of the diencephalon to the optic chiasma
    1. THE RHINENCEPHALON COMMISSURES appear before those of the neocortex, probably as a result of progressive maturation of various cortices as well as phylogenetic order
    2. FUNCTIONAL UNITY OF THE 2 HEMISPHERES, particularly of the 2 halves of the symmetric central nervous system, is assured by the systems of association, thus necessitating commissures in the subjacent layers and even in the spinal cord
  2. The anterior commissure is the first to appear and is seen at the end of month 2, connecting the 2 olfactory bulbs by its anterior processes and the 2 convolutions of the hippocampus (right and left temporal lobes) by its posterior processes. Its fibers pass in the inferior part of the lamina terminalis
  3. The hippocampal or fornix commissures connect the two hippocampi and the 2 paleocortices in the area of the trigonum. It appears during month 3
    1. THE COMMISSURAL FIBERS arise in the hippocampus, follow the posterior pillars of the fornix, and converge on the lamina terminalis near the roof plate of the diencephalon and in the middle of the lamina terminalis. It tends to regress somewhat after the corpus callosum appears
      1. The above fibers then continue to form an arching system, just outside the choroid fissure as the anterior pillars, which connect to the mamillary bodies
      2. The anterior and posterior pillars of the same side join the hippocampus and the homolateral mamillary bodies. Thus, in the true sense, they are not commissures but rather a system of intrahemispheric association fibers
  4. The corpus callosum appears by week 10 and connects nonolfactory areas of the 2 hemispheres (neocortex). It is not fully formed until about month 6
    1. INITIALLY, THE CORPUS CALLOSUM forms a small bundle in the lamina terminalis just rostral to the hippocampal commissure, but with expansion of the neopallium, it expands anteriorly and posteriorly, arching over the thin roof of the diencephalon
      1. Growth of the corpus callosum in an anterior direction pulls the area of the lamina terminalis away from the fornix commissure, thus out from the lamina terminalis locally, forming the septum pellucidum (may contain a small cavity)
        1. It has been suggested that the septum represents the apposed walls of the 2 hemispheres anterior to the lamina terminalis
  5. Appearance of several other commissures in the lamina terminalis
    1. TWO APPEAR JUST BELOW AND ROSTRAL to the stalk of the pineal gland: the posterior commissure and the habenular commissure
    2. THE THIRD, THE OPTIC CHIASMA, seen in the rostral wall of the diencephalon contains fibers from the medial halves of the retinae which cross on their way to the lateral geniculate bodies and the anterior colliculi
  6. Pathology related to the commissures: the most frequently affected development is that of the corpus callosum and may involve total or partial agenesis or hypogenesis, with an abnormally thin corpus. Possible causes for the derangement are
    2. A DEFICIENCY OF THE PATHWAY, the lamina terminalis
    3. AGENESIS OF THE CORPUS CALLOSUM, seen in 2% of cases with cephalic pathology, usually results in debility and epilepsy, however, some types have no clinical symptoms because the callosal fibers reach the neocortex via the anterior commissure

commissures of the telencephalon: image #1