The Development of the Adult Brain: Nature on the brain's white matter tract development over the human lifespan.

The human body in general reaches adulthood shortly after sexual maturation: on the average, unless you have a pituitary disorder, long bone skeletal growth ceases by around age 19 and vertebral column growth by age 21. Perhaps a few bones in pelvis and skull continue very slow growth through middle adulthood.

The human brain, like the skeleton, reaches adult volume by age 21, but, unlike the rest of the body, certain portions of the brain, such as the white matter tracts within the brain which serve as communication pathways, are still growing and developing even at age 30, reaching a peak in the early to mid thirties and remaining at a relative plateau until about age 42, according to a new study published in Nature this month. After the early fourties of age, brain pathways slowly lose substance, inverting the upward climb of childhood through early adulthood, as aging occurs.

One brain parameter the authors studied and called "R1" is the inverse of the MRI T1 density. The T1 density of the brain's white matter decreases as more myelin is added to the tracts, since myelin's waxy or fatty component is dark on T1 views of the brain MRI. So the R1, the inverse of T1, increases as the brain grows more and denser communication pathways. The other parameter, diffusion MRI, measures the degree to which brain water is tightly bound within the brain pathways as they run in various directions, and so is a similar measure of brain pathway maturation, but one which seems to deteriorate more slowly with brain aging than MRI measures of myelin lipid density.

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ABSTRACT

Lifespan maturation and degeneration of human brain white matter

Jason D. Yeatman, Brian A. Wandell & Aviv A. Mezer

Nature Communications 5, Article number: 4932 doi:10.1038/ncomms5932

Received 06 June 2014 Accepted 08 August 2014 Published 17 September 2014

Properties of human brain tissue change across the lifespan. Here we model these changes in the living human brain by combining quantitative magnetic resonance imaging (MRI) measurements of R1 (1/T1) with diffusion MRI and tractography (N=102, ages 7–85). The amount of R1 change during development differs between white-matter fascicles, but in each fascicle the rate of development and decline are mirror-symmetric; the rate of R1 development as the brain approaches maturity predicts the rate of R1 degeneration in aging. Quantitative measurements of macromolecule tissue volume (MTV) confirm that R1 is an accurate index of the growth of new brain tissue. In contrast to R1, diffusion development follows an asymmetric time-course with rapid childhood changes but a slow rate of decline in old age. Together, the time-courses of R1 and diffusion changes demonstrate that multiple biological processes drive changes in white-matter tissue properties over the lifespan.