CHAMPAIGN, Ill. -- A simple study of rat brains has added more substance to the idea that the adult brain is still a work in progress, even well after puberty, say University of Illinois researchers. While overall size may not change, the composition of nerve fibers in a key area does.
The scientists looked at the cellular makeup in the splenium, the thick, round, back portion of the rat corpus callosum, a mass that connects the two hemispheres of the brain, at 60, 120 and 180 days of age. Well after puberty's onset at about 40 days, axons (nerve fibers) continued to be wrapped by myelin -- white matter that both insulates axons and speeds the transmission of impulses between neurons. The number of unmyelinated axons decreased at the same time presumably because they are turned into myelinated axons.
The study -- reported in the March 16 issue of Developmental Brain Research -- was the first demonstration "that the area occupied by myelinated axons increases until 180 days of age" and likely beyond in the rat. There had been indication in the literature that myelination continues in adult cats.
"People have thought that myelination stopped during development, probably at about age 12," said Janice M. Juraska, a neuroscientist and professor of psychology at the UI. "This study is part of a rethinking on the growth of the brain. It is becoming clear that the corpus callosum changes in size at later ages than previously believed."
By 120 days of age (about in the mid-20s in humans), the area analyzed appeared to have stabilized in the amount of blood vessels and glial cells, but there was a significant decrease in the area occupied by unmyelinated axons and glial bodies. "An increase specifically in splenial myelination in the adult rat may lead to increases in the amount of visual information transferred between the hemispheres," the researchers wrote.
Axons that have not become myelinated can exchange communication signals with each other, but at a slower pace than myelin-covered axons. If axons later lose their myelin sheaths, communication is disrupted. A breakdown of myelin is associated with demyelinating diseases such as multiple sclerosis as well as many other metabolic and inflammatory disorders.
"The majority of work in humans is done with MRI [magnetic resonance imaging]," said Joseph L. Nunez, a doctoral student in the neuroscience program. "We, however, were able to look at the splenium and fibers within it, which cannot be done in humans. Human work has suggested that structural changes later in life within the brain involve language acquisition, but we are seeing changes in a primary sensory region. How this relates to the rest of the corpus callosum, we don't know."
The National Science Foundation, UI Research Board and the National Institute of General Medical Sciences funded the study through grants to Juraska and Nunez.
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