Basics of brain development

How the Human Brain Develops – From Before Birth to Young Adulthood

1. When Does Brain Development Begin?
Brain cell (neuron) production in humans begins just 42 days after conception (called Embryonic Day 42 or E42). By the middle of pregnancy, most neurons have already been made. Once they are formed, these neurons travel to different parts of the brain and start forming connections with each other—this is how basic brain networks are built. By the end of pregnancy, important pathways like the thalamocortical pathway (which connects the thalamus to the cortex) are already in place.

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2. How the Brain Grows After Birth
Brain development doesn’t stop at birth. In fact, it continues for many years. During the preschool years, the brain grows quickly—becoming four times bigger and reaching about 90% of adult size by age 6.

However, the brain continues to change in structure and function throughout childhood and adolescence. These changes help children and teenagers develop more advanced thinking, learning, and behavior.

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3. Early Brain Connections and Pruning
At birth and during early childhood, the brain has many more connections between neurons than an adult brain does. Over time, some of these connections are removed (pruned) through a natural process. This pruning is shaped by a child’s experiences and environment. It’s what makes the young brain highly adaptable and able to learn quickly.

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4. What Happens in the Teenage Brain?
While the total size of the brain doesn’t change much during the teenage years, there are big changes inside:

• Gray matter (the part of the brain that contains neuron cell bodies, dendrites, and synapses) decreases, especially in areas involved in thinking and emotions. This happens because the brain removes unused connections to make it more efficient.

• White matter (made up of myelinated axons that help different parts of the brain communicate) increases, thanks to a process called myelination. Myelin acts like insulation around wires, helping signals travel faster and more smoothly.

These changes help improve thinking speed, problem-solving, and decision-making. Brain development starts in the back of the brain and moves forward to the front (prefrontal cortex), which controls planning, judgment, and impulse control. This is why full brain maturity isn’t reached until the mid-to-late 20s.

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5. The Balance of Brain Systems
During adolescence, the brain’s reward system (which drives motivation and emotions) develops earlier than the prefrontal cortex (which controls thinking and self-regulation). This imbalance is why teens may take more risks or make emotional decisions before their reasoning skills are fully developed.

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The First Steps in Brain Development: The Birth of Brain Cells

In the second week after conception, the embryo is a simple, flat, two-layered structure. By week three, through a process called gastrulation, it transforms into three layers. This is a major turning point in development.

One of these layers gives rise to neural stem cells—also called neural progenitor cells. These are the special cells that will eventually become all the different cell types of the brain and spinal cord.

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Key Stages in Brain Development

Here are the main steps in how the brain forms and changes over time:

1. Differentiation – When stem cells become specific types of brain cells.

2. Migration – When new neurons move to their proper places in the brain.

3. Proliferation – When cells multiply rapidly to build brain tissue.

4. Myelination – When axons are covered with myelin to speed up signals.

5. Cell Death in Glial Populations – Some support cells (glia) are removed naturally.

6. Synaptic Exuberance and Pruning – Extra connections are made, then pruned back based on experience.

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To Be Continued...
Source: National Library of Medicine – PMC Article