Gliogenesis seems to foreshadow neurogenesis in some ways. The glia, for example, are formed from cells located in the same region that produces neurons. According to some research, some glial precursor cells can also form neurons under the right circumstances. (Lee, et all, 2000)
Almost all neurons are made in the second trimester. Neuronal genesis is a truly fascinating process. The creation of neurons is much like the creation of other cell in that they are produced through mitosis, but the fascinating thing about neurons is what happens next. When neurons divide in the fetal brain, they divide near central part of the neural tube, close to the ventricles. If all the dividing neurons remained where they were created the brain would simply be a ball of cells, but clearly it is not. Because the brain is differentiated into different sections with different functions and is often arranged in layers, the neurons must migrate according to the proscribed pattern. The daughter neurons move, or rather one daughter cell of each divided pair, climbs to a new location following glial cell fibers initially, and later following the pathway of previously moved (pioneer) neurons. The neurons appear to be following a chemical gradient along the glial cells, which act as a sort of scaffold. The neurons continue to be made and spread out throughout the brain, but rather than starting from the outside and filling in, the brain is built from the inside to the outside. It is almost as if the newer neurons are extruded out along the pathway made by the older neurons. The process is similar to what one sees when a volcano builds new land. Once in place, the neurons begin to send out axonal and dendritic projections along the chemical gradients which suggest the presence of other neurons. Most of the projections, however, are not sent out until after birth. In this way, the brain at birth is similar to a house that has its wiring roughed in, but the detail connections for plugs and light sockets have not yet been made.
Texturing the brain: major fissures
The brain hemispheres separated when the telencephalon formed, but the brain is still fairly smooth until the gyri form. The first set of major fissures form in the brain, creating the major lobes of the cerebrum. There are four major lobes on each side of the brain. While they may look symmetrical, each side generally serves a unique, although sometimes related function, in the adult brain. For example, when looking at a pattern, an adult will use the left side of their brain to look at the details of the pattern and the identical structure on the right side to look at the pattern at a global or big picture level. A child, however, will frequently use both sides of the brain to look at the pattern both locally and globally. In children with damage to one side of the brain, the brain frequently adapts by using the other side for both sorts of functions. Unfortunately, the same is not generally true in adult brains. (Nelson, et al, 2006, pg 103)
The major lobes of the cerebrum, four in each hemisphere are the frontal lobe, the temporal lobe, the parietal lobe, and the occipital lobe. The lobes are located beneath the skull bones bearing the same names. Not surprisingly, the frontal lobe is in the front part of the brain, above the eyes. It is the last part of the brain to mature and does not become fully myelinated until about thirty years old. This part of the brain is responsible for reasoning, conscious evaluation, the logical parts of language, emotion, movement and problem solving. The temporal lobe can be found medial to the ears. This part of the brain contains structures responsible for auditory processing, memory, and some of the auditory component of speech recognition. Interestingly, the pattern recognition aspects required for language comprehension, for both auditory language and for sign language, are processed in the temporal lobe regardless of whether or not there is an auditory component to the language. (Nelson, et al, 2006, pg 106) The occipital lobe is located about where a head hits the pillow when a person lies down on his or her back. It is responsible for processing visual input, including some aspects of pattern recognition. It is the first of the lobes to begin maturation, beginning even before birth. The parietal lobe is located about where a woman wears a high pony tail. It is responsible for processing input mainly from the epidermis, and evaluates sensations like, temperature, pressure, pain, and pleasure.
The lobes of the cerebrum form by differentiating from the original mass of the telencephalon. The temporal lobe is the first to begin differentiation in about the 4
month when the lateral cerebral fossa begins to form a groove that will become the lateral cerebral sulcus, separating the temporal lobe from the frontal lobe. The groove that will become the sulcus deepens as the frontal and temporal lobes grow larger. At about six months other groves begin to appear on the surface of the cerebrum which will grow into the other major sulci, including the central sulcus dividing the frontal and parietal lobes and the occipital sulcus, dividing the parietal and occipital lobes, by about eight months. Again, the sulci appear to deepen as the brain tissue expands around the groove without filling it in.