A new study indicates that the brains of mice and primates develop along the same timeline, even though one is much longer-lived than the other.
Finding is ‘a shock for neuroscientists,’ challenges assumptions about disease and aging
Mice typically live two years and monkeys live 25 years, but the brains of both appear to develop their synapses at the same time. This finding, published in a recent study led by neuroscientist Bobby Kasthuri of Argonne National Laboratory and his colleagues at the University of Chicago, is a shock for neuroscientists.
Until now, brain development was understood as happening faster in mice than in other, longer-living mammals such as primates and humans. Those studying the brain of a 2-month-old mouse, for example, assumed the brain was already finished developing because it had a shorter overall lifespan in which to develop. In contrast, the brain of a 2-month-old primate was still considered going through developmental changes. Accordingly, the 2-month-old mouse brain was not considered a good comparison model to that of a 2-month-old primate.
That assumption appears to be completely wrong, which the authors think will call into question many results using young mouse brain data as the basis for research into various human conditions, including autism and other neurodevelopmental disorders.
“A fundamental question in neuroscience, especially in mammalian brains, is how do brains grow up?” said Kasthuri. “It turns out that mammalian brains mature at the same rate, at every absolute stage. We are going to have to rethink aging and development now that we find it’s the same clock.”
‘That is shocking’
Gregg Wildenberg is a staff scientist at the University of Chicago and the lead author of the study. He looked closely at the neurons and synapses firing in the brains of very young mice. He marveled that the baby mouse crawled, ate, and behaved just as one would expect despite having next to no measurable connections in its brain circuitry.
“I think I found one synapse along an entire neuron, and that is shocking,” said Wildenberg. “This living baby animal was existing outside of the womb six days after birth, behaving and experiencing the world without any of its brain’s neurons actually connected to each other. We have to be careful about overinterpreting our results, but it’s fascinating.”
Brain neurons are different than every other organ’s cells’ neurons because brain cells are post-mitotic, meaning they never divide. All other cells in the body — liver, stomach, heart, skin, and so on — divide, get replaced, and deteriorate over the course of a lifetime. This process begins at development and ultimately transitions into aging. The brain, however, is the only mammalian organ that has essentially the same cells on the first and the last day of life.
Complicating matters, early embryonic cells of every species appear identical. If fish, mouse, primate, and human embryos were all together in a petri dish, it would be virtually impossible to figure out which embryo would develop into what species.
At some mysterious point, a developmental programming change happens within an embryo and only one specific species emerges. Scientists would like to understand the role of brain cells in brain development as well as in the physical development within species.