For generations, scientists imagined the brain assembling itself like a building — floor by floor, one neuron type at a time. New research from Irene Varela-Martínez at the Institute of Science and Technology Austria overturns that orderly picture, revealing that neural stem cells branch into distinct developmental lineages from the very beginning, each following its own rhythm and scale. The cerebral cortex, it now appears, is not the product of a single sequential program but of parallel stories unfolding simultaneously — a finding that quietly reframes our understanding of how minds are mad
Neural stem cells branch early into distinct neuron lineages, reshaping brain development understanding
The brain builds itself through parallel branches, not a sequence
Mark
So the old model said neural stem cells were like a factory with a production schedule—make ET-PNs first, then switch to IT-PNs. What made you suspect that wasn't right?
Mimi
The data showed overlap. When we looked at when each type was being produced, they weren't cleanly separated in time. That was the first hint that something more complex was happening.
Mark
And the MADM technique let you see the actual family trees of these cells?
Mimi
Exactly. You can watch a single stem cell divide and follow all its descendants. You see which neurons it produces, when, and in what quantities. That's how we discovered the branching—it's not a sequence, it's a fork in the road that happens right at the beginning.
Mark
Why does it matter that ET-PNs come in small clusters while IT-PNs come in larger groups?
Mimi
Because it explains the timing. Small clusters get used up fast, so ET-PNs dominate early. But IT-PN lineages keep producing neurons in bigger batches for longer, so they take over later. The brain's layering makes sense now—it's not about switching programs, it's about different production scales.
Mark
You're now looking at how brains got bigger across evolution. Are you asking whether these same branching mechanisms changed?
Mimi
That's part of it. If you want more neurons, you need stem cells that can generate more neurons. The question is: did the branches themselves change, or did the stem cells get better at proliferating? Understanding the lineage level is key to answering that.
Der Puls
- A foundational assumption in neuroscience — that the brain builds itself in strict sequence, one neuron type before the next — has been shown to be wrong.
- Two major classes of cortical neurons, long thought to arise one after the other, actually emerge from separate developmental branches operating in parallel from the start.
- The apparent dominance of one neuron type early in development turns out to be a matter of scale: small concentrated bursts versus large sustained waves, not a matter of order.
- Using a precision lineage-tracking technique called MADM, researchers could follow individual cell divisions and reconstruct the brain's developmental family tree with unprecedented clarity.
- The discovery reorients the field toward a new question: how have these parallel developmental programs shifted across species to produce the ever-larger, more complex brains of humans?
For generations, scientists imagined the brain assembling itself like a building — floor by floor, one neuron type at a time. New research from Irene Varela-Martínez at the Institute of Science and Technology Austria overturns that orderly picture, revealing that neural stem cells branch into distinct developmental lineages from the very beginning, each following its own rhythm and scale. The cerebral cortex, it now appears, is not the product of a single sequential program but of parallel stories unfolding simultaneously — a finding that quietly reframes our understanding of how minds are made, and perhaps how they grew larger across the arc of evolution.
The cerebral cortex — the brain's outermost layer, responsible for thought, perception, and movement — is built from many distinct types of nerve cells. How those cell types emerge during development has long been a mystery, and the prevailing answer has now been overturned.
Irene Varela-Martínez, a postdoctoral researcher at the Institute of Science and Technology Austria, has spent years reconstructing the family trees of neurons. Her findings, published in Science Advances, reveal that neural stem cells do not produce different neuron types in sequence — they branch into separate lineages from the very beginning, each following its own path.
The cortex relies on two major classes of projection neurons: IT-PNs, which connect regions within the brain, and ET-PNs, which send signals outward toward the spinal cord. Scientists long assumed ET-PNs came first, filling deeper cortical layers, with IT-PNs following later into the upper layers. The timing seemed to confirm it.
But Varela-Martínez, working first in Madrid and then in Austria, found that production of both types overlaps in time — and that each follows a distinct developmental rhythm. Using a technique called MADM to track individual cell divisions, she mapped the branches of radial glial cells, the stem cells from which cortical neurons arise. What she found was early divergence: at least two separate branches operating from the start, not in sequence. One branch produces only IT-PNs; the other generates both types. ET-PNs emerge in small, concentrated clusters that exhaust quickly, while IT-PN lineages are larger and distributed across all six cortical layers — explaining why ET-PNs appear to dominate early before IT-PNs take over in sustained waves.
The cortex, it turns out, is assembled not by a single master program but by parallel developmental branches running simultaneously. Varela-Martínez is now asking how these programs have changed across species — and whether the mechanisms she's uncovering in mice might illuminate the evolutionary expansion of the human brain itself.
The human brain's outer layer, the cerebral cortex, is a densely woven tapestry of different nerve cells working in concert to let us see, think, and move. Yet the origin story of how these distinct cell types emerge during brain development has remained largely mysterious—until now.
Irene Varela-Martínez, a postdoctoral researcher at the Institute of Science and Technology Austria, has spent years tracing the family tree of neurons with the precision of a genealogist. Her latest findings, published in Science Advances, overturn a long-held assumption about how the brain builds itself. Rather than neural stem cells following a strict developmental sequence—producing one type of neuron first, then switching to another—they actually branch into separate lineages from the very beginning, each following its own path.
The cerebral cortex contains two major classes of projection neurons, the long-distance messengers of the brain. Intra-telencephalic projection neurons, or IT-PNs, wire different regions of the cortex together, even connecting the left and right hemispheres. Extra-telencephalic projection neurons, or ET-PNs, send their signals outward, toward the spinal cord and beyond. For decades, neuroscientists assumed these two types arose in sequence: ET-PNs first, populating the deeper cortical layers, followed later by IT-PNs settling into the upper layers. This made intuitive sense given the cortex's layered architecture and the way it fills in during development, from inside out.
But Varela-Martínez's work revealed a more intricate reality. Working initially at the Centro Nacional de Biotecnología in Madrid under Marta Nieto, she discovered that the production of these two neuron types actually overlaps in time. More intriguingly, each type follows its own distinct developmental rhythm. To understand why, she moved to Austria and employed a specialized technique called MADM—a method that allows researchers to track individual cell divisions and reconstruct entire lineages with remarkable precision. By following radial glial cells, the neural stem cells from which cortical projection neurons arise, she could map their developmental branches.
What emerged was a picture of early divergence. The radial glial cells split into at least two separate developmental branches right from the start, not sequentially as previously imagined. One branch produces only IT-PNs. The other generates both ET-PNs and IT-PNs. Crucially, these branches operate on different scales. ET-PNs emerge in small, tightly clustered groups that exhaust themselves quickly. IT-PN lineages, by contrast, consist of much larger populations of cells distributed throughout all six cortical layers. This difference in scale and distribution explains the apparent paradox: early in development, ET-PNs dominate because they're produced in concentrated bursts, but as development continues, IT-PNs take over because they're generated in sustained, larger waves.
The implications ripple outward. The cerebral cortex, it turns out, is not assembled through a single master program but through parallel developmental branches that operate from the beginning. This finding reshapes how neuroscientists understand the basic architecture of the brain. Now, Varela-Martínez has turned her attention to an even larger question: how did brains evolve to become bigger and more complex? She's investigating how the developmental programs of neural stem cells have changed across species to generate ever-larger numbers of neurons. The mechanisms she's uncovering in mice may hold clues to understanding the evolutionary expansion of human intelligence itself.
Bemerkenswerte Zitate
The cerebral cortex is built through an early branching process, in which different neuronal lineages arise in parallel from the very beginning— Irene Varela-Martínez, postdoctoral researcher at ISTA
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