Summary: A milestone neuro-regenerative study has shattered a long-standing medical dogma by proving that “irreversible” damage to the connections between the human brain and spinal cord can be reversed. The research utilizes sophisticated, patient-derived 3D stem cell organoid circuits grown in the lab for over a year to replicate the central nervous system’s development.
By analyzing these human “mini-brains” and spinal pathways, scientists identified a precise genetic network that acts as a maturity switch, systematically shutting down axon regrowth after the mid-trimester of pregnancy. Remarkably, the investigators demonstrated that deploying a licensed hormone drug, lynestrenol, blocks this genetic restriction, successfully switching axon regeneration back on and offering a new therapeutic track for paralysis, motor neuron disease, and multiple sclerosis.
Key Facts
- The Paralysis Block Identified: During embryonic and fetal development, neurons form complex highways of axons—the nerve fiber “cables” that carry movement signals from the brain to the spinal cord to trigger muscle contractions. Historically, medicine accepted that central nervous system neurons permanently lose this growth capacity over time, rendering paralysis from traumatic spinal injuries or neurological diseases permanent.
- The Living Muscle Circuit: Building on their 2021 cortical organoid research, Cambridge scientists grew distinct brain and spinal cord organoids apart in a dish, mimicking human biology. Over time, nerve fibers spontaneously bridged the physical gap, forming a highly sophisticated, functioning 3D circuit that successfully triggered contractions in attached, miniature muscle clusters.
- The Day 150 Maturity Shift: By tracking this living human model for more than a year, the team located the exact developmental window where regeneration halts. Up until day 150, corresponding directly to the mid-trimester of human pregnancy, damaged axons regrew long fibers effortlessly. After day 150, as the central nervous system neurons matured to form synapses, a sharp, permanent drop in regeneration capacity occurred.
- Toggling the Genetic Switch: Genetic expression analysis revealed an integrated network of genes that operates as an absolute developmental switch to restrict axon growth. By biochemically blocking the key regulators of this network, the team successfully tricked mature human neurons into reverting to an embryonic state, turning the axon regrowth mechanism back on.
- Chemical Intervention via Lynestrenol: The researchers screened an extensive database of molecular drug compounds to find candidates capable of acting on this specific genetic pathway. They identified lynestrenol, a contraceptive hormone drug traditionally licensed for managing menstrual disorders. When applied to the damaged mature models, lynestrenol significantly boosted axon regrowth.
- Bypassing the Animal Research Blindspot: Much of global nerve regeneration data has relied on rodent models, whose neurons behave differently from human cells. These human-derived organoids bridge this clinical knowledge gap, providing a pristine, highly accurate proxy for patient biology while actively advancing efforts to reduce animal testing in global science.
Source: University of Cambridge
Cambridge scientists have grown miniature circuits in the lab that mimic how the brain and spinal cord connect up, which underlies our movements. They used this model to show how damage to these connections previously considered ‘irreversible’ could, in fact, be reversible.
As we develop and grow from embryo to fetus to infant, our nerve cells (neurons) form connections, allowing information to be transmitted between the brain and the spinal cord. A key component of each neuron is the axon – the nerve fibre ‘cable’ that transmits information to other neurons to activate muscle contractions.
At some point, we lose the ability to grow axons in the central nervous system, or this ability is at least greatly impaired or slowed down. This means that damage to the brain and spinal cord becomes permanent, leading to devastating disabilities, such as the inability to grasp or walk. This is often the case for traumatic spinal cord injury and can be a feature of many neurological diseases, including motor neurone disease or multiple sclerosis.
In 2021, Dr András Lakatos and colleagues at the University of Cambridge developed ‘mini brains’ using human patient-derived stem cells – special cells that have the potential to develop into most human cell types – which they guided to grow into pea-sized brain ‘organoids’. These organoids were 3D models that resemble parts of the human cerebral cortex. The team used these to demonstrate molecular problems in motor neurone disease and potential ways to prevent them.
Now, in research published in Cell Reports, Dr Lakatos’s team has taken its research a step further, building a mini version of the connected human brain and spinal cord system in the lab by recreating these tissues using organoids.
In the human body, the brain and spinal cord tissues are distinct but connected by axons, so the researchers kept the brain and spinal cord organoids apart. They saw that nerve fibres from the brain tissue grew across the gap to connect to the spinal cord, forming a working circuit that could even cause tiny muscle clusters to contract.
By growing this human system in the dish for more than a year, they found that up until around day 150 – which corresponds to the mid-trimester of pregnancy – the axons were able to regrow after damage, but after this time, their growth was greatly impaired.
George Gibbons from the Department of Clinical Neurosciences at the University of Cambridge, the study’s first author, said: “Neurons taken from less mature organoids regrew long fibres after injury, but those from more mature organoids showed a sharp drop in their ability to regrow. In other words, poor regeneration is built into human neurons as they mature in the central nervous system.”
By analysing the gene expression – a sign of how active the genes are – in neurons that connect the brain and the spinal cord, they were able to identify a network of genes that acts as a ‘switch’ restricting the axon growth ability while the neurons mature to form connections (synapses). Amazingly, blocking key regulators of this network switched back on the ability of axons to grow.
The team then scanned a database of drug compounds to search for those that act on the genes in this network and identified as a candidate lynestrenol, a hormone drug licensed for managing certain menstrual disorders and as a contraceptive. When they tried this drug on damaged neurons, they found that it significantly boosted axon regrowth.
While scar tissue and inflammation may also restrict axon repair, exploring and tackling neuron-specific causes – the subject of this study – is very important. This is supported by evidence that axons of less mature neurons can grow through non-permissive environments that characterise injury sites.
Senior author Dr András Lakatos, who led the project at the Department of Clinical Neurosciences, said: “When the brain and spinal cord are damaged, the nerve fibres that carry movement signals from the brain to the spinal cord rarely grow back. That’s why paralysis is usually permanent. But we didn’t know exactly when the ability of axons to regenerate becomes limited. Our model provides a good indication that this block happens during development, and it can still be reversed after this point.
“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. Although we still need to show that this strategy will also help to re-establish appropriate connections between the brain and spinal cord cells, this gives us hope that one day we may be able to treat conditions previously thought untreatable.”
Organoid models are an important way of understanding human biology. While animal models – for example, mice and rats – are useful for studying our biology as they share some similarities with humans, their differences ultimately limit what we can learn. Organoids grown from human stem cells can more closely mimic human biology.
Dr Lakatos added: “Much of what we know about nerve regeneration comes from rodents, whose neurons behave differently from human neurons. Our sophisticated organoid models help bridge the knowledge gap from animal models to what we see in patients. They are also an important contribution to efforts to reduce the use of animals in research.”
Organoids, often referred to as ‘mini organs’, are being used increasingly to model human biology and disease. At the University of Cambridge alone, researchers use them to repair damaged livers, understand Crohn’s disease in children, and model the early stages of pregnancy, among many other applications.
Funding: The research was funded by the UK Research and Innovation Medical Research Council and Spinal Research.
Key Questions Answered:
A: Because of a natural shutdown built straight into our central nervous system. When humans grow from embryos to infants, our nerve fibers, or axons, easily stretch out to connect the brain and spinal cord. However, the University of Cambridge discovered that around day 150 of pregnancy, a genetic “switch” clicks on inside our maturing neurons, permanently crippling their ability to grow or repair themselves. When an adult suffers a spinal injury, the brain’s movement signals are permanently cut off because the adult cells are genetically blocked from regrowing.
A: By directly overriding the cell’s internal maturity clock. After identifying the precise gene network that blocks axon growth, Cambridge researchers scanned medical databases for drugs that could manipulate those exact genes. They discovered lynestrenol, a hormone drug normally used for birth control and menstrual disorders. When applied directly to damaged, mature human neurons, lynestrenol successfully bypassed the developmental block, toggling the genetic switch back to an embryonic state and significantly boosting axon regrowth.
A: While this is an extraordinary leap forward, it is an early, conceptual scientific victory. Lynestrenol itself may not be the final drug used in human spinal surgeries, but it serves as an ironclad proof-of-concept that human axons can be regenerated after the maturity block has locked in. The next major hurdle for scientists is proving that these newly regrown nerve fibers can successfully wire themselves back into the correct, highly precise anatomical connections across the human body.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this neurology and spinal cord injury research news
Author: Fred Lewsey
Source: University of Cambridge
Contact: Fred Lewsey – University of Cambridge
Image: The image is credited to Dr András Lakatos
Original Research: Open access.
“A human corticospinal organoid-slice connectoid model informs enhancer strategies for post-injury axon regrowth” by George M. Gibbons, Tanja Fuchsberger, Mai Abdelgawad, Stefano L. Giandomenico, Kornélia Szebényi, Veselina Petrova, Lea M. D. Wenger, Daniel N. Olschewski, Jeremi Chabros, Leila Muresan, Rachael C. Feord, Muhammad Asif, James W. Fawcett, Susanna B. Mierau, Ole Paulsen, Madeline A. Lancaster, and András Lakatos. Cell Reports
DOI:10.1016/j.celrep.2026.117399
Abstract
A human corticospinal organoid-slice connectoid model informs enhancer strategies for post-injury axon regrowth
Axon elongation in the mammalian central nervous system (CNS) declines during development, limiting regenerative capacity after birth. Intrinsic regulators of this process are promising repair targets, as immature axons can regrow in tissues otherwise not conducive to regeneration. Yet the precise timing and mechanisms underlying the cessation of axon growth in the human CNS remain unresolved.
Here, we developed a three-dimensional human corticospinal motor organoid-slice connectoid platform mimicking the developmental axon elongation program and its subsequent restriction through maturation.
Cortical and spinal slices establish functional connections while remaining spatially segregated, enabling cortical cell-type-specific observations without direct confounding effects by spinal cells. Using single-cell transcriptomics, computational analyses, axon regrowth assays, and live imaging, we identified transcriptional alterations contributing to decreased axon growth in maturing human cortical projection neurons.
We further demonstrate that this decline can be reversed using compounds and repurposable drugs targeting a maturation-associated transcriptional shift, promoting post-injury axon repair.
