White matter refers to the structural connectivity between neurons, the processing units of the brain. After neurons migrate to their destination they develop outgrowths, called axons. These outgrowths can be seen as the pipes within the brain that allows the processing units to talk to each other. In order to do so, neurons use electro-chemical signals to transfer information along the axons. Here we will introduce the concepts of synaptogenesis, myelination and pruning.
Around 24-26 weeks post-menstrual age (PMA) the first axons (the pipes) grow towards the cortical plate (see Grey matter
). Once they reach their target, they connect to the neurons through synapses in a process called synaptogenesis. These connections allow for the differentiation of the different parts of the cortex that govern for example movement and sensation. Axons continue to grow until shortly after birth, however, synapses are being formed until 2 years of age.
One important aspect of synaptogenesis is that the brain "over-connects" its regions in the beginning. This means that more synapses are formed than are necessary over a lifetime. Therefore, even though the amount of synapses increases rapidly after birth for a short period of time, a process called pruning starts to take over, which reduces their total numbers. During development different parts of the brain reach their maximum synaptic density at different time points, however, in general we lose approximately 40% over our lifetime.
Communication within the brain occurs through electro-chemical signals. One aspect of this kind of information transport is that it loses signal intensity the longer the information has to travel. The brain counters this loss by using myelin sheathes as insulation. Around 29 weeks PMA, myelin sheathes start to form around axons, which allow for much better signal transfer. Myelin is made of a fatty substance with a white colour, which is the reason why the system of axons or structural connectivity is referred to as white matter. The process of myelination continues even into late adolescence.
One way to investigate the structural connections is by using diffusion magnetic resonance imaging (dMRI)
. With dMRI researchers can estimate where bundles of axons are and which brain regions connect with each other.