Author: Abaitel

  The mature human brain has many remarkable capacities: higher cognition, learning, creativity, imagination, and an appreciation of beauty. It may be hard to imagine that a brain capable of all this develops from a single fertilized egg. Drawing on what I learned about the brain over a semester, both in and outside class, this article offers a broad view of human brain development.

Early embryonic development and the fetal brain

  In a long and closely contested race, one competitor pulls ahead of a crowd of tail-propelled rivals. It reaches the destination by the narrowest margin: a warm, spherical cell. This cell is the ovum, the target pursued by the sperm. Each sperm carries different genetic information. Only the first to enter can combine its information with that of the ovum and help shape a new individual.

Figure 1. The race that begins a new life.

  As the successful sperm enters the ovum, chemical changes at the cell surface prevent other sperm from entering. When the genetic material of the sperm and ovum fuse, a zygote is formed. If development proceeds normally, its genetic information will eventually be present throughout the body of a new human being.

  Thirty hours later, the zygote completes its first division and becomes two cells. Further divisions follow. After three days, the cells form a cluster resembling a mulberry, with each “seed” corresponding to one cell. The cluster is therefore called a morula.

  Five days after fertilization, the morula changes markedly. Its cells separate into two groups rather than remaining a solid cluster. One group forms an outer wall around the remaining cells, the inner cell mass, producing a sphere that resembles a hollow plastic ball with a smaller ball attached to its inner surface. The morula has now become a blastocyst. The outer cell layer supports and nourishes the embryo, while the embryo itself develops from the inner cell mass.

Figure 2. A blastocyst under the microscope.

  The rounded blastocyst then implants in the uterine lining. After implantation, the inner cell mass separates from the outer layer and forms an embryonic disc two cell layers thick. About 12 days after implantation, some cells from the upper layer move between the two existing layers to form a third. Chemical signals from this layer cause cells in the top layer to differentiate into neurons, and that top layer becomes the neural plate. The neural plate will develop into the nervous system, including the brain.

  Between 18 and 20 days after implantation, the neural plate begins to change shape. By the third week, its edges rise to form the neural groove, then continue to curl inward until they meet above the groove and form the neural tube. The earliest outline of the brain is now present. The cavity of this tube later develops into the brain’s ventricular system, through which cerebrospinal fluid circulates and carries nutrients and waste products.

Figure 3. Formation of the neural tube.

  After the neural tube forms, neural cells continue to divide and the early brain continues to grow. Two months after implantation, differences in growth rate make distinct brain regions recognizable. The paired lateral bulges correspond to the cerebral hemispheres, while a later projection at the rear develops into the cerebellum.

  The differentiation of brain structures depends on an increase in the number of neural cells. During division, neural progenitor cells repeatedly move between the outer edge and the center of the neural tube. A progenitor preparing to divide moves from the periphery toward the center and divides there. The new cells then return to the periphery, where they grow until the next division. As division proceeds, these cells must be specialized and sent to different functional regions. Mature cells move from the outer area of the neural tube toward the inner region, then travel in different directions and differentiate into several cell types. Besides neurons, they may become macrophages that clear debris, myelinating cells that provide insulation, or astrocytes that help maintain the local environment. Neurons have the leading role in the brain, but these glial cells are also essential.

Figure 4. Glial cells and their functions.

  Glial cells form tracks along which neurons migrate from the center to other parts of the brain. Particular neurons match particular tracks, travel outward from a central point, and leave the track when they reach their destinations. We still cannot say exactly how neurons detach from these tracks or assemble into functional groups. What is clear is that the brain grows as newly divided neurons migrate outward and settle in place. Unlike the cortex of many other animals, the human cerebral cortex forms its innermost layer first, then adds successive layers outward until all six are present. Part of the skull remains flexible while the brain grows, allowing its volume to increase. A flat sheet containing all the neuronal cell bodies of the cortex would require a large area, so the cortex folds into gyri and sulci, fitting a greater surface area into a relatively small volume. Researchers have reproduced this folding process with a simple physical model.

Figure 5. Researchers reproduce the formation of cortical folds with a physical model.

  By the ninth month of pregnancy, the fetal brain contains nearly as many neurons as an adult brain. At this stage, it is about the size of a chimpanzee brain, roughly 350 cubic centimeters. A larger brain could not pass smoothly through the mother’s birth canal, so the fetus must be born. Because neuron numbers are already relatively stable, neuronal division cannot readily explain why the brain later grows to four times this volume. What drives that growth, and what gives us capacities that a newborn cannot possess? The next section considers possible answers.

Brain development in infancy and early childhood

  Birth frees the infant brain from the size constraint imposed by the mother’s birth canal. It also places the child in a world far richer in sensory input than the womb. Brain function depends on responding to external stimuli in increasingly effective and purposeful ways, so this surge in stimulation allows the brain to begin training its main functions. Birth is a watershed: before it, development is driven largely by increasing cell numbers; afterward, how those cells are organized and function becomes more important.

  The first obvious change is myelination. As noted earlier, some neural stem cells differentiate into glia, and some glial cells form myelin around neuronal processes, increasing the insulation of axons. Myelin increases rapidly in the cerebral cortex during the first month after birth. It allows neural signals to travel farther with less loss, making fine motor control possible. Fine movements require the cerebral cortex to send distinct electrical signals through neural pathways to distant effectors, which is possible only when signals travel efficiently along those pathways.

Figure 6. How myelin works.

  By about one year of age, infants can often be seen reaching for objects spontaneously. At first, they can move only all their fingers together, but with age they learn to move individual fingers. This may reflect further specialization of functional regions in the brain. Increasingly complex and precise neural activity is closely related to the growth of neuronal processes and the formation of synaptic connections. Much of the striking postnatal increase in brain volume comes from the growth of these intricate connections, which increase the total volume occupied by each neuron. Neural fibers sometimes extend between widely separated neurons. Some researchers propose that chemicals released by distant neurons attract the fibers; others suggest that pioneer fibers formed early in development establish paths for later fibers.

Figure 7. Schematic of neural connections.

  Neural connections are not simple, predetermined one-to-one pairings. They respond to the environment. One Italian boy, for example, was blind in one eye even though an examination found the eye itself to be normal. As an infant, he had experienced a mild infection in that eye, which was then covered with a bandage for two weeks. Deprived of input from that eye, the brain treated it as unusable, and connections from the other eye took over cortical territory that would normally serve both eyes. By the time the bandage was removed, this adaptation prevented the affected eye from forming effective connections with its target region, so the cortex could no longer receive its neural signals.

  During infancy and early childhood, widespread myelination and the formation of neural connections bring clear signs of consciousness and increasingly powerful problem-solving abilities. Does brain development stop at that point? It does not.

Later brain development

  The brain continues to develop throughout life. Neural connections formed earlier may later disappear, leaving those best suited to the person’s characteristics and environment. An individual is therefore shaped jointly by genes and experience. As the environment changes, the brain continues to change in subtle ways.

  The brain is generally considered to reach maturity after about 11 more years of development beyond age five. During that period, its size increases by another 5 percent. This small increase may help determine the fields in which we excel and the work we pursue. Later environmental input continues to fine-tune the brain, although the changes become smaller and less frequent. The amount of stimulation strongly affects how many neural connections are added during development, while the type of stimulation selectively activates and strengthens different neural circuits. Adults should therefore seek constructive experiences and practice that provide beneficial stimulation, helping shape both neural connections and personality.

  As we enter middle and old age, the brain inevitably declines. By age 70, a healthy older adult’s brain may have lost 5 percent of its earlier weight; by age 90, it may have lost 20 percent. Neurodegenerative diseases pose a greater danger, stripping patients of abilities, memories, their sense of self, and eventually almost everything else. Responding to neurological disease in an aging world has become a major challenge.

Figure 8. Beta-amyloid attached to neurons in a patient with Alzheimer’s disease.
Figure 9. Artistic depiction of neurodegenerative disease.

  This is the life of the brain as I understand it. It is also the life of every one of us. Understanding the brain and brain science is therefore an important part of understanding our own lives.