Development of a Brain

Even the most complex structure in the body begins its development with the union of egg and sperm cells: approximately 24 hours after fertilization, the maternal and paternal chromosomes have combined to form the genetic makeup of the new individual. The first cell division begins. After 96 hours, the fertilized egg cell has developed into a sphere of about 30 cells that looks a little like a ripe mulberry, hence its name: morula. The cells of the morula are tiny because the first cells divide repeatedly into two halves without increasing in size. In the morula, the cells begin to differentiate into outer and inner cells. Three to four days after fertilization, fluid has collected in the center of the cell sphere – the morula has become a blastocyst. At one point in this hollow sphere, the innermost cells form a small cluster called the embryoblast. Only these cells develop into the embryo, while the rest become auxiliary organs such as the fetal parts of the placenta and the amniotic membranes.

While the embryo travels down the fallopian tube and the mother is still completely unaware of what is happening in her body, the division of labor in the developing human being progresses. The cells of the embryoblast fold into the three germ layers: endoderm, mesoderm, and ectoderm. The endoderm later develops into the internal organs, while the mesoderm develops into bones, muscles, and connective tissue. The ectoderm develops into the skin and, in a process called neurulation, the central nervous system, including the brain.

On the 18th day of the embryo's life, around the time when the mother realizes she may be pregnant, the first indentation forms in the ectoderm, which shortly thereafter constricts: the neural tube, the precursor to the Spinal cord Three protrusions, called brain vesicles, form at its front end. The embryo has now completed its journey and found a place in the uterine lining. It is now about two millimeters in size.

In the following days, the upper part of the neural tube with the brain vesicles bends slightly, and the first signs of the cerebral hemispheres become visible. Massive cell migration causes this area to enlarge significantly and become increasingly distinct from the spinal cord. Four weeks after fertilization, the Eye spots form and the heart begins to beat. After six weeks, the foundations for brain structures such as the Pons and cerebellum, thalamus, basal ganglia, and cerebral Cortex begin to develop. In the ninth week, tiny fingers and toes are already visible, and the spinal cord begins to control the first movements.

After three months, the embryo, now called a fetus, is twelve centimeters long and has well-developed structures in the Midbrain and hindbrain, but its cerebral cortex is still smooth and undifferentiated. It is not until around the 24th week that the first furrows typical of the human brain begin to form. This process continues after birth – until around the child's first birthday.

The structure of the human brain is only genetically determined in broad terms. Its fine structure is the result of a complex organizational process in which environmental factors also play a role. These include the mother's diet and any illnesses or exposure to toxins. ▸ Like mother, like child

The young neurons develop from stem cells in a tissue layer of the neural tube. To get a rough idea of this process, you can divide the number of neurons in the brain by the months of pregnancy: this gives an average value of 250,000 new neurons per minute. From there, they migrate to their destinations in the brain and begin to specialize for their tasks during this migration: for example, into photoreceptor cells or olfactory cells. Their tasks depend on when they were created and on chemical factors in their environment. First, the inner layers of the Cerebrum develop, and the younger cells migrate past the older ones to form the outer layer. In doing so, they use the radial glial cells, whose long processes grow outward across the layers of the brain, as a kind of railing to hold onto (see info box).

Once a Neuron has arrived at its destination, it must connect to its target region. If a neuron is located in the Retina of the eye, for example, it must dock onto the visual center in the thalamus. To do this, it extends an "arm," a neurite, at the tip of which is a growth cone ▸ The neuron: form and function. It paves the way for the neurite, similar to an axon, through the dense tissue, sometimes even reaching the other half of the brain. Where this growth cone grows is determined, on the one hand, by attractive and repulsive substances on the surfaces of the surrounding cells. "This allows proper roads or channels to form, which the neurites use for orientation," explains Paul G. Layer, professor emeritus of developmental biology and neurogenetics at Darmstadt Technical University. On the other hand, growth factors influence where the neurite extends. These are small proteins that are emitted by the target regions of the neurites and can be detected by the growth cone with receptors on its numerous tentacles. "Unlike the substances on the cell surfaces, growth factors can act over certain distances," explains Layer. The neurite then grows to where the concentration of the growth factor is highest.

Once the growth cone has reached its destination, the Neuron is still far from finished. "First, the cell Nucleus must be informed that the neurite has arrived," says Layer: "To do this, the growth factor must be transported backwards through the neurite into the cell body. Once there, it triggers a signaling cascade that activates genes in the cell nucleus that are necessary for the neuron's survival. If this signal is not received because the neurite has not found its target, the neuron undergoes apoptosis, which means it commits suicide." The growth factor is therefore more of a survival factor.

In fact, the mass production of neurons in the embryo's brain leads to there being far too many neurons at times. They compete with each other: only those with the most stable connections remain. Up to 80 percent of neurons are broken down again.

Most of the successful axons are later enveloped by extensions of the Oligodendrocytes. This process, known as myelination, begins in the oldest structure of the brain, the Brain stem ▸ The Brainstem, and continues into the youngest ones. They isolate the axons from the processes in their environment and enable the electrical impulses with which the cells communicate to be transmitted up to a hundred times faster ▸ Nerve Cells in Conversation. Myelination continues into adulthood in the Frontal lobe of the Cerebrum. ▸ The frontal lobe

The brain is by no means fully developed at birth. Birth is rather the starting point for an enormous growth spurt: in the first year of life, the brain grows threefold and the head reaches three-quarters of its adult size. This growth spurt is partly due to the formation of new neurons and partly to the fact that the nerve fibers become thicker through myelination and the neurons establish numerous synapses with their neighbors. This formation of synapses initially occurs rapidly and indiscriminately, as if one were trying to get to know everyone at a party through "speed dating." At the age of three, children have twice as many synapses as adults. At the party, you eventually have to decide who you actually want to talk to. The neurons experience something similar: only the most active synapses remain, while the others are broken down. This process is called synaptic pruning and continues in vertebrates until puberty. During childhood, it can cost us billions of synapses every day. That sounds alarming, but it actually leads to more efficient cognitive processes.

This slow development of the brain, heavily influenced by the experiences a child has in its environment, makes humans so adaptable, so intelligent, but also so impressionable.

Further reading

• Dan H. Sanes, Thomas A. Reh, William A. Harris: Development oft he Nervous System. Elsevier Publisher 2012.
• Lise Eliot: What's Going on in There?: How the Brain and Mind Develop in the First Five Years of Life. 1999

Originally published on March 17, 2016
Last updated on February 20, 2026