Chapter 158. Histogenesis of The Cerebral Cortex - Review of Medical Embryology Book

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
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
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

THE PRIMORDIAL OLFACTORY CORTEX begins first between 2 and 3 months, and we see the hippocampus (archeocortex) and paleocortex

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

In the adult, the cerebral cortex forms a layer of gray matter approximately 3 to 5 mm thick
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
Cellular migrations then appear at the level of the frontal and occipital cortices
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

IN CONTRAST TO OTHER REGIONS of the nervous system, the germinal layer is active for only several months after birth

Cortical organization: 5 or 6 major types have been described, as well as their distribution in various areas of the cerebral hemispheres

Cellular migrations are increasingly evident as they relate to the phylogenetically more recent structures

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

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

THE MATURE INFANT is born with most of its cortical neurons, approximately about 9 to 14 billion, besides nerve fibers, neuroglia, and blood vessels

Only the neuroglial cells continue to multiply actively and separate the neurons
The connections of each neuron increase progressively and may reach a huge number, in the order of 10,000 synapses per cell
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
The cerebral cortex varies in thickness from 55 to 5 mm