24th November 2023

White and Grey Matter Trajectories

We have already discussed that DTI (by FA) is an established MRI technique for imaging myelination events from as early as the second trimester. DTI may also be used then, to study white matter tracts (bundles of myelinated axons connecting different brain regions). This is commonly through DTI tractography – 3D reconstruction of DTI data to visualise specific fibres and tracts. Prior to the development of DTI tractography, T1 and T2 sMRI sequences were commonly manipulated to provide white matter insights.

Studies taking each approach show that major white matter tracts, including many fibres projecting from subcortical to cortical structures, first develop during the fourth or fifth month of gestation. Thalamocortical and association tracts develop after, during the seventh gestational month or later. All major white matter tracts are established by gestational term. However, total brain white matter volume gradually increases until 30–40 years of age.

Whilst DTI tractography is useful to map structural connectivity within the brain, functional MRI (fMRI) can tell us about functional connectivity. In both blood oxygen level dependent (BOLD) and arterial spin labelling (ASL) fMRI, changes in blood flow are understood to represent alterations in regional neural activation. In resting state fMRI (rs-fMRI), changes in blood flow are not provoked by activity such as mental or physical tasks. rs-fMRI studies show that canonical sensory, motor and association networks – such as the visual, auditory, somatomotor, default mode and dorsal attention networks – are evident as early as 26 weeks gestation. These networks mature during childhood and adolescence, during which time longer-range connections emerge (e.g. between anterior and posterior regions of the cortex) and shorter-range connections weaken. The network most studied developmentally is the default mode network (DMN), yet the exact age at which the DMN emerges is disputed.

Grey matter (regions of neuronal bodies, dendrites and unmyelinated axons) is best imaged by sMRI. Studies show that grey matter volume, thickness and surface area increase early on in childhood but shrink thereafter (from 8–10 years of age). This trajectory is determined by synaptic dynamics – the periods of rapid synaptogenesis followed by synaptic pruning discussed earlier – combined with progressive myelination (decreasing the volume of grey matter relative to white matter).