The brain liquidates billions of mutant neurons around the time we're born

A study of more than 2,300 individual neurons suggests that the developing human brain removes many cells carrying major chromosome abnormalities around the time of birth. Researchers examined cerebral-cortex neurons from people ranging from 14-week-old fetuses to adults in their nineties, looking for copy number variations - chromosomal segments that have been gained or lost. More than a quarter of sampled fetal neurons showed these changes, compared with roughly 6% of neurons from babies after birth. The team repeated its analysis with a different single-cell sequencing method after spending years checking whether the unexpectedly high fetal results were genuine; the findings remained similar.

The sharp decline suggests that abnormal neurons are selectively eliminated during development, adding a possible quality-control system to the known process by which the brain regulates its cell numbers. Cells with more extensive chromosome changes also tended to have unusual activity in genes involved in building synapses, hinting at a feature that might mark them for removal, though how the brain identifies them remains unknown. The researchers argue that understanding this culling could shed light on childhood conditions such as autism and epilepsy, which may be influenced by disrupted brain development. The findings show a relationship between fetal genetic abnormalities and their later scarcity, but do not establish exactly how the cells are removed or whether their loss directly causes any particular condition.