Where Nature's neurons start in the brain is not necessarily where they end

The development of the fetal brain involves the generation and migration of billions of neurons during pregnancy

By retracing genetic mutations in the brains of deceased adults, researchers have tracked cell migration in the developing fetal brain for the first time

The development of the human brain is still a very mysterious process, starting from the embryonic neural tube and ending with the more than 100 billion interconnected neurons in the neonatal brain

These nerve cells are often generated far from where they ultimately reside and function in the new brain, and this migration has been studied extensively in animal models with chemical or biological tracers, but never directly in humans research has been carried out

In a new paper published online April 20, 2022 in the journal Nature, scientists at the University of California, San Diego School of Medicine and the Rady Children's Institute for Genomic Medicine describe a new method developed by studying healthy adults who recently died of natural causes.

"Every time a cell divides into two daughter cells, by chance one or more new mutations emerge that leave clues that can be read by modern DNA sequencers

"By studying ways to interpret these mutations in the brain, we are able to uncover key insights into how the human brain is formed compared to other species

The structure of the human neocortex underlies species-specific traits and reflects complex developmental programs

While the human body has 3 billion DNA bases and more than 30 trillion cells, Gleason and his colleagues found that only a few hundred DNA mutations may appear in a fertilized embryo after the first few cell divisions or in early brain development

To understand the cell types displaying these mosaic mutations, they developed methods to isolate each major cell type in the brain

However, they also found that the mutations found on the left and right sides of the brain differed from each other, suggesting that - at least in humans - the two brain hemispheres separated during development much earlier than previously suspected

"This study," the authors say, "solves the mystery of why these focuses are almost always confined to one hemisphere of the brain

Reference: “Somatic mosaicism reveals clonal distributions of neocortical development” by Martin W.