Read about all five ages of the brain in our special feature
Setting the stage
By the time we take our first breath, the brain is already more than eight months old. It starts to develop within four weeks of conception, when one of three layers of cells in the embryo rolls up to form the neural tube. A week later the top of this tube bends over, creating the basic structure of fore, mid and hindbrain.
From this point, brain growth and differentiation is controlled mainly by the genes. Even so, the key to getting the best out of your brain at this stage is to have the best prenatal environment possible. In the early weeks of development, that means having a mother who is stress-free, eats well and stays away from cigarettes, alcohol and other toxins. Towards the end of the brain-building process, when the fetus becomes able to hear and remember (see 鈥淭he five ages of the brain: 2 Childhood鈥�), sounds and sensations also begin to shape the brain.
Advertisement
In the first two trimesters of pregnancy, though, development is all about putting the basic building blocks in place: growing neurons and connections and making sure each section of the brain grows properly and in the right area. This takes energy, and a variety of nutrients in the right quantity at the right time. In fact, if you consider the size of the construction job at hand 鈥� 100 billion brain cells and several million support cells in four major lobes and tens of distinct regions 鈥� it is a truly staggering feat of evolutionary engineering.
One nutrient we know the brain needs early on is folic acid, which is crucial for closing the neural tube. Deficiencies can lead to defects like spina bifida, where part of the spine grows outside the body, and anencephaly, a fatal condition in which much of the brain fails to develop. There鈥檚 some evidence that vitamin B12 deficiency has similar effects ().
The role of other nutrients is less well understood in these early weeks 鈥� partly because deficiencies often occur as a component of either general malnutrition or poverty, which brings in the confounding factors of poorer general health or health education, and partly because early deficiencies are difficult to link to problems that may not show up until months or years later.
We do know from animal studies, however, that malnutrition 鈥� particularly a lack of protein 鈥� stunts the growth of neurons and connections, and that iron and zinc are needed for neurons to migrate from where they form to their final location. Long-chain polyunsaturated fatty acids are required for synapse growth and membrane function.
Yet while a good diet and taking prenatal vitamins, if prescribed, is usually enough to take care of the fetus鈥檚 early brain needs, deficiencies caused by an inefficient placenta can hinder development. Factors that can affect the placenta include maternal high blood pressure, stress and smoking. Excesses of nutrients can also be bad news. Poorly controlled diabetes, for example, can cause a potentially toxic excess of glucose in the developing brain.
Luckily, the brain has a large safety net, producing double the neurons it will ultimately need. Even so, some damage may not be reversible. Fetal iron deficiency, for instance, affects brain function for up to three years, even if the child receives supplements from birth.
As for toxic substances, the good news is that fetuses are well protected from the outside world, and even from their own mother. The placenta is a highly selective barrier studded with protein pumps that help prevent unwanted substances in the mother鈥檚 bloodstream from reaching the fetus.
Toxic effects
It is not a perfect barrier, however, and toxics such as mercury, nicotine and alcohol can squeeze through. The effects depend on the dose and timing of exposure, but in animal studies nicotine has been shown to affect the functioning of neurotransmitters, while mercury has been linked to cell loss in the cerebellum and parts of the cortex. Alcohol is known to kill off neurons and alter the action of certain neurotransmitters, although how much is needed to cause the physical and cognitive defects of fetal alcohol syndrome or more subtle forms of damage is unclear.
Some toxics do not need to go through the placenta to have an effect. For example, cigarette smoke restricts blood flow to the fetus, depriving it of oxygen and nutrients. Exactly how this affects the brain in human fetuses is unclear, although there are some indications that the corpus callosum, which links the two hemispheres, and the , involved in social behaviour, may be smaller in adolescents exposed to cigarette smoke in the womb. A smaller OFC correlated with less 鈥渃aring鈥� behaviour in female teens, suggesting smoking may cause behavioural problems in offspring. However a recent study found between exposed and non-exposed adolescents (International Journal of Epidemiology, vol 38, p 158).
Stress, too, can be toxic. In animals, exposure to the mother鈥檚 stress hormones can lead to anxious behaviour and hyperactivity in the offspring. A recent longitudinal study of over 7000 mothers and babies at Imperial College London concluded that maternal stress may account for up to 15 per cent of diagnoses of attention-deficit hyperactivity disorder.
If you are reading this, however, chances are you got off pretty lightly in the first nine months of life, or that any problems were outweighed by good parenting or education (see 鈥淭he five ages of the brain: 2 Childhood鈥�). In the next age, the young brain starts to learn and remember, and that means we get our first chance to really use that amazing organ.
Read about all five ages of the brain in our special feature
