Humans not only have bigger brains but also ones that are more wrinkled. This is superefficient: more brain in the same mug. But how do you grow a wrinkly brain? How do these gorges and plateaus paint the picture of your brain? It is part of the story that makes us human.
An origin story
The secret heroes in this story are a special type of cells called the basal radial glial cells. But before we get to them, we need to tell an origin story. Not of THE origin of life but the origin of your life and mine. Each of us started as a very small clump of cells in the womb of our mother. As that clump begins to grow and organize itself, the predecessor of the brain arises: the neural plate. In the end, the neural plate consists of six layers. Three of those are important to our story, with sci-fi type names: the subventricular zone at the bottom, the intermediate zone, unsurprisingly in the middle, and the cortical plate on top.
Image credit: https://www.cell.com/developmental-cell/pdf/S1534-5807(23)00580-4.pdf
It’s a hilly affair…
The weird thing is this. The bottom of the neural plate isn’t flat in humans, it meanders up and down as if it has hills and valleys. So, this is where our secret heroes enter, the basal radial glial cells. There are much more of them in the hills than in the valleys. And on this undulating surface they grow a fiber network on top of the neural plate, like the top half of a spider web. Neurons, which grow from our heroes, can climb up, from the bottom where they came from to the top of to the cortical plate, increasing the mass on top of the hill. And so the hills grow bigger and the valleys become deeper, leaving us with a folded instead of a flat brain.
But there’s more. Neurons in animals with wrinkly brains, such as humans and chimpanzees, also climb up that fiber network much more like a mountain road, going left and right and taking all sorts of turns. On the other hand, neurons in animals with smooth brains, like mice, go straight up. This means that the hills and valleys that were already there get more wrinkled because of neurons messing about.
Image credit: https://www.cell.com/developmental-cell/pdf/S1534-5807(23)00580-4.pdf
Scientific mimicry
For a long time, scientists weren’t sure whether the outside pressure of the skull caused the brain to wrinkle or if it was instead due to pressure from the inside. Interestingly, scientists can mimic the folding pattern quite well with simple gels and by adding pressure from the inside to these gels, the same wrinkles appeared as in a human skull. So, the growth from the inside is basically like a pressure cooker.
Video: https://youtu.be/f-JraqXhinY?feature=shared&t=20
Article: https://www.cell.com/developmental-cell/pdf/S1534-5807(23)00580-4.pdf
