Damage to the spinal cord is often permanent because the nerve fibres responsible for carrying signals between the brain and body have little ability to regrow after injury. A new study from the University of Cambridge suggests that this repair failure may be partly programmed into human neurons themselves and identifies a decades-old hormone drug, lynestrenol, as a potential candidate for reversing some of that limitation in laboratory conditions.
Every year, an estimated 20,000 people in India sustain a spinal cord injury (SCI), with road traffic accidents accounting for around 45% of cases. Globally, the figure reaches . 700,000 to 1.2 million new cases annually. In most cases, the loss of movement and sensation is permanent. This is because the nerve fibres that carry signals between the brain and the spinal cord have a very limited capacity to repair themselves after injury in adulthood.
Certain animals, such as salamanders, retain the ability to regenerate spinal cord tissue throughout their lives. In contrast, the human central nervous system (CNS) loses this capacity during development.
Why this happens, and whether it can be meaningfully addressed, has been a long-standing question in neuroscience. The study published in Cell Reports by researchers at the University of Cambridge now offers new evidence on both fronts, using a human-specific laboratory model to investigate the mechanisms behind this repair failure.
Each neuron has a long, slender projection called an axon, the fibre responsible for transmitting signals from the brain to the muscles via the spinal cord. After injury, these axons need to regrow for any functional recovery to occur. Evidence from rodent studies has long suggested that immature neurons retain some capacity to regrow their axons, but that this ability is progressively switched off as neurons mature, a process that appears to be encoded in gene activity rather than simply a structural limitation.
The researchers report what they describe as the first direct evidence that a similar developmental shutdown occurs in human cortical neurons. The researchers found that neurons derived from more mature organoids (miniature tissue structures grown from human stem cells), showed a 34.5% reduction in axon regrowth capacity compared to those from younger organoids. This finding suggests that the decline in regenerative potential is, at least in part, built into human neurons as they mature.
STUDY FINDING Cortical neurons derived from mature human organoids (150–290 days in culture) showed a 34.5% reduction in axon regrowth after injury compared to neurons from younger organoids (75–100 days). Researchers report this is the first direct evidence of this developmental shutdown in human CNS neurons.
This sets up a key question the research then attempts to address: if the repair block is genetically encoded, could targeting the relevant gene network restore some growth capacity?
To study this in human tissue, the team engineered a corticospinal connectoid, a miniature, laboratory-grown circuit representing the human pathway from the brain to the spinal cord. The two regions are kept physically separate but linked by a gel bridge, allowing signals to pass between them.
The model has three core components:
A cortical organoid: a cluster of human brain tissue growing cortical neurons with outgrowing axons
A hydrogel bridge: a gel structure across which the axons travel, mimicking the pathway from brain to spinal cord
A spinal organoid with attached muscle clusters: which contracts in response to electrical signals, demonstrating that the model produces functional motor output
This design addresses a recognised limitation of previous models in which brain and spinal tissue are grown fused together, making it difficult to study the behaviour of cortical neurons in isolation.
Using single-cell transcriptomics, which measures gene activity in individual cells, the team identified a network of genes that appears to function as a developmental brake on axon growth. A central node in this network is PTEN, a gene already known from rodent studies to limit axon elongation. The researchers confirmed that inhibiting PTEN in the connectoid model led to measurable increases in axon growth cone activity at an injury site, validating the network as a meaningful target.
Building on this, the team performed a computational screen of 323 FDA-approved medications, searching for compounds whose known mechanisms of action align with reversing the transcriptional changes associated with the repair shutdown. From this screen, six drugs were shortlisted and tested in human cortical neuron cultures. Lynestrenol is a synthetic progestogen used since the 1960s for menstrual disorders and as an oral contraceptive. Lynestrenol produced the largest observed effect, with results indicating a 2.06-fold increase in axon length in injured cortical neuron cultures following treatment. The finding was observed in a cell culture assay, not in the connectoid model or in human patients.
Lynestrenol belongs to the progestogen family of hormones. Its parent compound, progesterone, has been investigated separately for neuroprotective effects in traumatic brain injury. While early Phase 2 clinical trials in 2006 showed some promising signals, subsequent Phase 3 trials did not confirm benefit in a 2014 study for traumatic brain injury. Lynestrenol’s potential role in axon repair appears to operate through a distinct mechanism and would require independent clinical investigation.
The authors also note that lynestrenol’s effect on axon repair may differ between males and females, and that this sex-specific variation has not yet been studied.
Lynestrenol itself may not be the answer to spinal cord repair, but it shows us that, in principle, it should be possible to directly target human neurons and regenerate their axons.Dr. András Lakatos, Senior Clinical Fellow and Senior Author, Department of Clinical Neurosciences, University of Cambridge. Source: University of Cambridge press release, May 2026
These results are preclinical and should be interpreted with appropriate caution. The study used a single human embryonic stem cell line, which limits conclusions about individual variability or disease-specific differences. The organoid model does not incorporate immune cells, blood vessels, or scar-forming tissue, all of which play significant roles in the biology of real spinal cord injuries and in restricting repair.
Axon regrowth in this study was observed only over short distances in laboratory conditions, and no functional recovery was demonstrated. However, the authors point out that even short-distance regrowth may carry clinical relevance: in cervical spinal cord injuries, axons may only need to bridge a small number of spinal segments to re-establish connections involved in hand function. From a translational standpoint, repurposing an existing approved drug carries a potential regulatory advantage over developing a new compound from scratch, since safety and tolerability data already exist.
Researchers emphasise that further studies are needed across multiple cell lines, animal models, and eventually clinical trials before any clinical application could be considered.
SAFETY NOTE: Lynestrenol is not approved or indicated for spinal cord injury or any neurological condition. Do not self-medicate. Individuals who are pregnant, those on existing hormonal therapy, or those with a history of hormone-sensitive conditions, blood clotting disorders, or cardiovascular disease should consult their treating specialist before making any changes to their medications.
The Cambridge study contributes new, human-specific evidence that the failure of nerve fibre repair after spinal cord injury is not simply a structural problem, it appears to be, at least in part, a programmable one, encoded in the gene activity of maturing neurons. The identification of lynestrenol as a compound that may influence this program in laboratory conditions is a preliminary but notable finding, adding a potential new direction to an area where treatment options remain extremely limited.
Whether this early laboratory signal translates into clinically meaningful nerve repair in humans remains to be established through substantially more research. For now, findings from this study suggest the repair capacity of human neurons may not be entirely lost, and that it may, under the right conditions, be partially restored.
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