155. The Brain: The Telencephalon (first Vesicle)

  1. Introduction: the telencephalon is the most rostral part of the brain vesicle and consists of 2 lateral outpocketings, the cerebral hemispheres, and a median part, the lamina terminalis. The hemispheres grow simultaneously in lateral, longitudinal, and parietal directions. The growth is dominated by the great development of the neocortex, which, in humans, occupies about 90% of the cerebral surface. The pallium or vault of each hemisphere forms the cortex. The floor gives rise to the striated bodies, and between the cortex and the bodies, the lateral ventricles are formed
  2. Development during the first 2 months of embryonic life
    1. 30- TO 32-DAY EMBRYO: each hemisphere contains an evagination consisting of a vault (pallium or future cortex) and a lateroventral region (floor or future striatum)
      1. The 2 lateral cavities of the telencephalon communicate with the cavity of the diencephalon via the 2 foramina of Monro
    2. 45-TO 50-DAY EMBRYO: the vault and floor begin to differentiate. The pallium thickens slightly, and one can recognize the archeo-, paleo-, and neopallial regions which will give rise to the corresponding areas of the cortex
      1. The floor becomes greatly thickened as a result of marked activity of its germinating zone and gives rise to the primordia of the striated nuclei, the lateral striated body on the outside, and the median striated body on the inside
      2. The interhemispheric or choroid fissure is forme The junction between the 2 vaults is very thin, and along the fissure the choroid plexuses invaginate
      3. The ventricular cavities narrow and differentiate as a result of parietal growth. One sees the lateral ventricles, the foramina of Monro, and the third ventricle
    3. 50-DAY TO 2-MONTH EMBRYO: the neopallium extends and engulfs the paleopallium on the ventral side and the archeopallium on the dorsal side so that the telencephalon progressively surrounds the diencephalon
      1. In a parallel manner, the various cells of the striatum develop and contribute to the thickening of the area of junction of the telencephalon and diencephalon. This permits the 2 structures to become continuous, and a wide area is formed that helps join the hemispheres to the rest of the neural axis
        1. Efferent and afferent fibers to and from the cortex arrange themselves in this junctional zone to form the internal capsule
  3. Development after the second month
    1. PALLIAL DEVELOPMENT: the neopallium grows and compresses the archeopallium on the inside and the palleopallium below. The striated bodies become paramedian. The pallial zones give rise to distinct cellular layers which are the primordia of the cortex
      1. The parietal growth produces some volume reduction of the ventricular cavities
      2. The mass of white matter between the cortex, the ventricles, and the central gray nuclei also increases, and myelinated fibers come to and leave the cortex
    2. FLOOR DEVELOPMENT is characterized by the appearance of the striated nuclei. The latter are traversed by fibers of the internal and external capsules
      1. The lateral nucleus striatum gives rise to the neostriatum which forms the caudate nucleus and putamen. The claustrum also may be derived from it
      2. The median nucleus striatum gives rise to the paleostriatum or globus pallidus. This nucleus is responsible for most of the strioencephalic connections. It fuses laterally with the putamen to form the lenticular nucleus
      3. The amygdaloid nucleus or archeostriatum differentiates from the most ventral region of the floor, below the lenticular nucleus
      4. The septal formations are seen in humans but have lost their great importanc They are derived from the median telencephalic parts that blend ventrally with the diencephalon. They form an important junction where the hippocampal, olfactory, and neocortical pathways connect. They send efferent fibers to the hypothalamus and the reflex centers of the brainstem

the brain: the telencephalon  (first vesicle): image #1