Nerve fibers in the brain are a lot like wires: they need insulation so signals travel fast and don't get crossed. That insulation is made by cells called oligodendrocytes. In Down syndrome, when a person has an extra copy of chromosome 21, these cells take the wrong path from the very beginning: instead of becoming 'insulators,' they turn into support cells known as astrocytes. Insulation runs short, and the brain works like a cable with frayed shielding.
To see exactly when this happens, scientists grew tiny brain-like structures from stem cells, called organoids. Some carried the extra chromosome, others didn't. By day 21, the 'syndrome' organoids had fewer future insulators, and by day 60 they were producing noticeably less of the insulating substance myelin. Meanwhile, support cells became more numerous. It's as if the cells got the wrong instructions and were sent to the wrong department.
People used to think myelin suffered simply because of a general developmental delay. Now it's clear: the problem lies in the cells' career choice from the very start. This finding helps in the search for drugs that can put the cells back on the right track, so insulation is restored and signals in the brain travel more clearly.
Trisomy 21 organoids spontaneously 'talk' more often than healthy ones, but when you try to make them respond to a stimulus, they fall apart, like an orchestra that makes noise without a conductor but cannot play from the score.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.