Brain development consists of a series of complex and intricately coordinated stages that need to happen at specific times, and in a particular sequence at specific locations. Brain cells are formed during the last few months of pregnancy at a rate of about 200 cells per second. Not only that, but the cells have to find their way to their final positions in the brain cortex and elsewhere, some of them moving up to 1,000 times their own size. Each cell forms an extension, a so-called axon, through which it sends electrical signals. The total length of axons, if placed end-to-end, would reach four times around the globe. The nerve cells communicate via synapse connections, which function like electrical switches, and up to 1,000 synapses are generated each second during the first months after a child is born.
All of these processes are extremely complex and also vulnerable. Because the brain must undergo all of these processes during limited time windows, we have only one chance to develop a brain. Damage during early development is likely to be irreversible, and the consequences of derailing the time schedule or halting the developmental processes are therefore long-term.
Even though we get only one chance to develop a brain, yet no textbook discusses brain development from the point of view of toxic risks. In fact, we only understand major aspects of brain development, and much still needs to be learned. Although a more detailed map of the biochemistry is highly desirable, we also need to explore how best to protect the brain functions. We can no longer count on the blood-brain barrier (which envelopes the brain) to prevent toxic chemicals from entering the brain. Any interference with the intricate and complex biochemical processes that generate this exceptional organ is likely to cause more serious damage than in other organs.
So, exactly because the brain is so complex, and because optimal function depends on the integrity of the brain as a whole, brain development is highly vulnerable to toxic chemicals.