1.2. Brain Development
Brain development starts at the embryonic period, more specifically at the fifth week of pregnancy. Figure 1.1 illustrates the timeline of pregnancy by weeks and months of GA (16). In the first months of pregnancy and before birth, the brain experiences the most development and changes in shape, size and structure (17, 18). Most significantly, changes occur in the size of the brain and in the cortical folding. Growth continues rapidly until the brain is two to three years old, when the process slows and stabilizes and the developing brain becomes mature. There are three main tissue classes in the brain: gray matter (GM) or cerebral cortex, white matter (WM) or subcortex, and cerebrospinal fluid (CSF). Using MR Images …show more content…
Myelin promotes efficient neural signal transmission along the nerve cells. The appearance of the developing brain in structural MRI differs significantly from the appearance of the mature adult brain. The myelinated WM in T1-weighted MRI increases in intensity from hypo intense to hyper intense relative to GM. In T2-weighted MRI WM decreases from hyper intense to hypo intense relative to GM (17, 27, 28). Thus, the developing brain MR images are characterized by an inverted contrast of WM and GM as opposed to the developed brain as seen in figure 1.2. The inverted contrast is due to WM axonal growth where myelin sheath forms around the axon tracts (29, 30) (see figure 1.3) and change occurs in the cell water content as a result of decreasing both T1 and T2 times in order to avoid fetus motion (30, 31). By the completion of the myelination process, the brain tissue contrast appears in MR images similar to the adult brain tissue contrast while the brain structure, shape, and size are different …show more content…
1.4: Two-dimensional representation of the pallial and subpallial origins and the GABA-ergic and glutamatergic radial and tangential migratory paths in human fetus brain at 19 post-conceptual weeks. “Glutamatergic neurons originate in the pallial ventricular zone/subventricular zone (blue), with some tangential migration within the SVZ/VZ, but a dominant pattern of radial migration along radial glial fascicles in a plane perpendicular to the orientation of the cortical plate (radial pathways: blue arrows). GABA-ergic neurons in the human brain originate from both pallial ventricular zone/subventricular zone (blue) and subpallial ganglionic eminence (green). GABA-ergic neurons from ganglionic eminence display a pattern of migration initially tangential to the orientation of the cortical surface, before assuming a radial trajectory in intermediate zone towards the cortical plate (tangential pathway: dark green arrows); a pattern reported in more diverse mammalian systems. A pattern of GABAergic neuronal migration originating in SVZ/VZ and displaying a radial trajectory to cortical plate has been reported more recently in humans and non-human primates (light green arrows). GABAergic neurons originating from ganglionic eminence also migrate via subcortical paths to thalamus. Migratory pathways are displayed on a schematic slice of frontal cortex from a human fetus at 19 post-conceptual weeks. GE: ganglionic eminence; VZ: ventricular zone; SVZ: subventricular zone; IZ:
Brain development starts at the embryonic period, more specifically at the fifth week of pregnancy. Figure 1.1 illustrates the timeline of pregnancy by weeks and months of GA (16). In the first months of pregnancy and before birth, the brain experiences the most development and changes in shape, size and structure (17, 18). Most significantly, changes occur in the size of the brain and in the cortical folding. Growth continues rapidly until the brain is two to three years old, when the process slows and stabilizes and the developing brain becomes mature. There are three main tissue classes in the brain: gray matter (GM) or cerebral cortex, white matter (WM) or subcortex, and cerebrospinal fluid (CSF). Using MR Images …show more content…
Myelin promotes efficient neural signal transmission along the nerve cells. The appearance of the developing brain in structural MRI differs significantly from the appearance of the mature adult brain. The myelinated WM in T1-weighted MRI increases in intensity from hypo intense to hyper intense relative to GM. In T2-weighted MRI WM decreases from hyper intense to hypo intense relative to GM (17, 27, 28). Thus, the developing brain MR images are characterized by an inverted contrast of WM and GM as opposed to the developed brain as seen in figure 1.2. The inverted contrast is due to WM axonal growth where myelin sheath forms around the axon tracts (29, 30) (see figure 1.3) and change occurs in the cell water content as a result of decreasing both T1 and T2 times in order to avoid fetus motion (30, 31). By the completion of the myelination process, the brain tissue contrast appears in MR images similar to the adult brain tissue contrast while the brain structure, shape, and size are different …show more content…
1.4: Two-dimensional representation of the pallial and subpallial origins and the GABA-ergic and glutamatergic radial and tangential migratory paths in human fetus brain at 19 post-conceptual weeks. “Glutamatergic neurons originate in the pallial ventricular zone/subventricular zone (blue), with some tangential migration within the SVZ/VZ, but a dominant pattern of radial migration along radial glial fascicles in a plane perpendicular to the orientation of the cortical plate (radial pathways: blue arrows). GABA-ergic neurons in the human brain originate from both pallial ventricular zone/subventricular zone (blue) and subpallial ganglionic eminence (green). GABA-ergic neurons from ganglionic eminence display a pattern of migration initially tangential to the orientation of the cortical surface, before assuming a radial trajectory in intermediate zone towards the cortical plate (tangential pathway: dark green arrows); a pattern reported in more diverse mammalian systems. A pattern of GABAergic neuronal migration originating in SVZ/VZ and displaying a radial trajectory to cortical plate has been reported more recently in humans and non-human primates (light green arrows). GABAergic neurons originating from ganglionic eminence also migrate via subcortical paths to thalamus. Migratory pathways are displayed on a schematic slice of frontal cortex from a human fetus at 19 post-conceptual weeks. GE: ganglionic eminence; VZ: ventricular zone; SVZ: subventricular zone; IZ: