Muscle fibre-resolved spatial transcriptomics of regeneration, necrosis and border zones in human paraspinal muscle

Inside the human body, skeletal muscle is not a uniform block of tissue but a complex mosaic of individual fibers, each with its own identity and function. Some fibers are built for endurance, firing slowly and steadily, while others are designed for quick, powerful bursts. When these muscles are injured, the damage rarely affects every fiber at once. Instead, a single injury site often contains a chaotic mix: some fibers are dead, some are actively trying to repair themselves, and others appear perfectly healthy right next to the wreckage. For decades, doctors and scientists have relied on looking at these tissues under a microscope to understand what is happening. They can see which fibers are broken and which are mending, but the microscope cannot reveal the invisible chemical conversations happening inside those cells. It cannot tell us if a fiber that looks healthy on the outside is actually struggling on the inside, or if the boundary between a dead cell and a living one is as sharp as it appears.

A team of researchers at the University of Ljubljana set out to map these invisible boundaries in a specific type of muscle injury caused by cocaine use. This condition, known as toxic myopathy, creates focal lesions where muscle tissue dies and tries to regenerate. To solve the mystery of what is happening at the molecular level, the scientists turned to a powerful new technology called spatial transcriptomics. Think of this technology as a way to read the genetic instructions inside a cell while keeping a precise record of exactly where that cell is located in the tissue. By combining this genetic reading with high-resolution imaging, the researchers could examine individual muscle fibers one by one, distinguishing between those that were far away from the injury, those that were dead, those that were regenerating, and those that were sitting right on the edge of the damage.

The study focused on a small sample of muscle taken from the lower back of a patient who had suffered an acute injury linked to cocaine use. The researchers sliced this tissue into extremely thin sections and used a specialized system to detect the activity of nearly four hundred specific genes within each fiber. They then carefully traced the outline of every single fiber they could see, grouping them into categories based on what they looked like and where they were located. They compared the genetic activity of normal fibers far from the injury to the fibers right next to the dead and regenerating ones. They also checked the fibers to see if they were the slow-twitch type or the fast-twitch type, ensuring they understood the baseline identity of each cell before looking at the damage.

What they found challenged a long-held assumption in muscle pathology. For a long time, scientists have treated fibers that look normal under a microscope as a safe, neutral baseline for comparison. If a fiber looked healthy, it was assumed to be functioning normally. However, the genetic data told a different story. The researchers discovered that fibers sitting immediately next to a dead, necrotic fiber were not actually normal. Even though these border fibers looked structurally intact and healthy, their internal genetic activity had already shifted dramatically. They had begun to express a specific set of genes that closely matched the profile of the dying fibers next to them. It was as if the distress signal from the dead tissue had already crossed the invisible line into the living neighbor, changing its behavior before any visible damage appeared.

In contrast, the fibers that were actively regenerating showed a completely different pattern. These cells were busy turning on genes related to rebuilding the muscle's internal machinery and handling calcium, which is essential for muscle contraction. They were clearly in a state of repair, distinct from both the dead tissue and the normal tissue. The researchers also noted that the fibers far away from the injury site showed only very small differences between the slow and fast types, confirming that the massive changes they saw near the injury were driven by the damage itself, not by the natural differences between fiber types.

One of the most striking findings was the intensity of the "spillover" effect from the dead fibers. The genetic signature of the dead tissue, which was rich in signals associated with immune cells cleaning up the debris, seemed to bleed into the neighboring healthy fibers. This suggests that the environment around a dead muscle fiber is toxic and disruptive, altering the neighbors almost immediately. On the other hand, the influence of the regenerating fibers on their neighbors was much weaker. The healthy fibers next to a repairing cell did not show the same dramatic shift in their genetic activity as those next to a dead cell. This indicates that the process of repair is more contained and less disruptive to the surrounding tissue than the process of death.

The study also looked at whether the presence of a specific immune marker, called the membrane attack complex, could define different types of dead tissue. They found that this marker did not create a separate category of injury; the dead fibers were essentially the same whether or not they had this specific marker. This simplified the picture, suggesting that the core molecular state of a necrotic fiber is consistent, regardless of this particular detail.

By mapping these molecular states with such precision, the researchers have changed how we should view muscle injury. They demonstrated that in a focal lesion, there is no such thing as a truly "normal" fiber sitting right next to the damage. The border zone is a distinct biological state, a transition area where the healthy tissue is already reacting to the injury. This means that in future studies of muscle disease, scientists cannot simply use the fibers next to the injury as a control group, because they are already part of the problem. The findings provide a clearer, more detailed map of how muscle injury spreads and how the body attempts to heal, revealing that the boundary between life and death in muscle tissue is far more fluid and interconnected than a simple microscope image can show.