158. Histogenesis of The Cerebral Cortex

  1. By the end of month 1, the telencephalic pallium (cortex), in the human fetus consists of a germinal layer and an ependymal layer, which together form the stratified cellular wall of the neural tube
  2. By the end of month 2, medullary differentiation is well advanced, and the cells migrate toward the surface to form the mantle layer. Thus, the wall of the hemisphere has the general primitive structure of the neural axis with the germinal or mantle layer on the interior and the marginal or future molecular layer on its exterior
  3. During month 3, the cortical layer is formed by cells from the mantle layer migrating toward the surface. This cortical layer is thin in the archi- and paleopallial regions and thick in the neopallial region. Various cortical regions differentiate from the cortical layers
    1. THE PRIMORDIAL OLFACTORY CORTEX begins first between 2 and 3 months, and we see the hippocampus (archeocortex) and paleocortex
      1. A 3-layer cortex forms in these areas
    2. DIFFERENTIATION OF THE NEOCORTEX extends from the beginning of month 3 to the end of month 6. It is characterized by extensive cellular migrations, resulting in the formation of 6 cellular layers
      1. In the adult, the cerebral cortex forms a layer of gray matter approximately 3 to 5 mm thick
      2. The cellular migration and layering of cells are first seen at the level of the insula and parietal cortex. Thus, the somesthetic system, which ends in the ascending parietal convolutions of the brain, is functional in the fetus very early, before the special senses like sight and hearing
      3. Cellular migrations then appear at the level of the frontal and occipital cortices
      4. In month 6, the neurons form their processes and in month 7, the various types of cortical structures are established, , motor, sensory, associative, and intermediary, according to the proportion of specialized cells they contain
    3. IN CONTRAST TO OTHER REGIONS of the nervous system, the germinal layer is active for only several months after birth
  4. Cortical organization: 5 or 6 major types have been described, as well as their distribution in various areas of the cerebral hemispheres
    1. THIS HAS PROVED VALID in functional terms even though variations do occur
  5. Cellular migrations are increasingly evident as they relate to the phylogenetically more recent structures
    1. THEY ARE DISCRETE in the spinal cord, notable in the brainstem, important in the cerebellum and in the archeocortex, and at their maximum in the neocortex
      1. As a result of their degree of development in the brainstem, cerebellum and archeocortex, and neocortex, the gray matter in these areas is peripheral and the white matter central, which is just opposite to that in the spinal cord, where the white matter is peripheral and the gray matter is central
    2. THE MATURE INFANT is born with most of its cortical neurons, approximately about 9 to 14 billion, besides nerve fibers, neuroglia, and blood vessels
      1. Only the neuroglial cells continue to multiply actively and separate the neurons
      2. The connections of each neuron increase progressively and may reach a huge number, in the order of 10,000 synapses per cell
      3. The total surface of the adult cerebral cortex has been estimated at about 285,000 square millimeters, with a volume of about 300 cubic centimeters
      4. The cerebral cortex varies in thickness from 55 to 5 mm

histogenesis of the cerebral cortex: image #1