In a Cambridge laboratory, tiny versions of the human brain and spinal cord have begun to reveal one of medicine's most stubborn mysteries: why nerve damage that should be permanent might not be. Dr. András Lakatos and his team at the University of Cambridge have grown miniature circuits from stem cells that mimic how the brain and spinal cord connect to drive human movement—and in doing so, they've uncovered evidence that the supposed irreversibility of nerve damage may be far less fixed than we thought.
The discovery hinges on understanding axons, the cable-like nerve fibers that transmit information between neurons to activate muscle contractions. As humans develop from embryo to fetus to infant, neurons form these critical connections. But at some point during development, the central nervous system essentially "forgets" how to regrow these axons after damage. This loss of capacity explains why spinal cord injuries can lead to permanent disability—the inability to walk or grasp—and why it presents such a challenge in neurodegenerative diseases like motor neuron disease and multiple sclerosis.
By growing brain and spinal cord organoids—pea-sized 3D models that resemble parts of the human cerebral cortex—Lakatos's team built a working circuit in a dish. They kept the brain and spinal cord tissues apart, just as they exist in the human body, and watched as nerve fibers from the brain organoid grew across the gap to connect to the spinal cord tissue. The circuit was functional enough to trigger muscle contractions, proving the model captured the essential architecture of human neural wiring.
Then came the crucial finding. By maintaining this system for more than a year, the researchers identified a critical turning point: around day 150 of development—corresponding to the mid-trimester of pregnancy—axons retained the ability to regrow after damage. But after that milestone, their regrowth capacity plummeted dramatically. "Neurons taken from less mature organoids regrew long fibers after injury, but those from more mature organoids showed a sharp drop in their ability to regrow," explained George Gibbons, the study's first author from the Department of Clinical Neurosciences.
The team then identified the culprit: a network of genes that acts like a molecular switch, progressively restricting axon growth as neurons mature. Here is where hope emerges. By blocking key regulators of this network, the researchers switched the regrowth ability back on. When they searched a database of existing drugs for compounds that could act on this gene network, one candidate stood out: lynestrenol, a hormone medication already licensed for menstrual disorders and contraception. Testing it on damaged neurons revealed something remarkable—it significantly boosted axon regrowth in mature neurons that had lost this capacity.
The findings, published in Cell Reports, don't erase the complications of nerve repair. Scar tissue and inflammation also restrict axon regeneration at injury sites. But by isolating and understanding the neuron-specific mechanisms that block regrowth, the researchers have illuminated a path forward. The fact that less mature neurons can extend axons through hostile environments suggests that restoring their intrinsic growth capacity could bypass some of these obstacles. With a known drug already showing promise, the journey from laboratory discovery to therapeutic possibility has suddenly become clearer.