Text settings Story text Size Small Standard Large Width * Standard Wide Links Standard Orange * Subscribers only Learn more Minimize to nav Late last week, we started seeing headlines about how new research was suggesting we might have two brains instead of the one that seems to show up on MRI scans. As is so often the case, these rumors can be traced back to a single press release put out by one of the universities where the work was done. You do not have two brains. It’s a silly way to look at the results of the new paper.
But there are far better ways to look at the results, ways that can help us understand how the brain gets put together and illustrate some of the general ideas behind developmental biology. So let’s take a look at what the research actually shows and place it in the context of brain development. Where are you? You may think body segments are something that show up in things like crustaceans and insects.
But our nervous system also develops in segments. Vertebrates have four major ones: the forebrain, midbrain, hindbrain, and spinal cord. (Some of these later develop segments within them, but we’ll skip over those.) The new paper is largely focused on providing part of the answer to an obvious question: how do those segments get there? That sort of question is the bread and butter of developmental biology, a field that focuses on how organisms start off as a single cell and, through a series of carefully timed and choreographed processes, produce all the tissues found in adults. To think about the question, we need a bit of context.
By the time the first cells that are committed to develop into neurons show up, the vertebrate embryo already knows its head from its tail, and its back from its belly. (We won’t go into how the embryo learns that, but we know a lot about that, too.) The embryo consists of three tissues at the time: the endoderm, which will line our guts, the mesoderm, which will form muscles and bone, and the ectoderm, which will go on to form the skin. Neural cells form as a thickening of the ectoderm that runs down the center of the embryo from head to tail. The center of these thickly packed cells drops down, while the sides fold up, eventually forming an oval-shaped tube that pinches off from the rest of the ectoderm. All the signals that run up your spine, every sound and shape you process, every thought you will ever have—all of these and more depend on the descendants of these cells.
A section across a vertebrate embryo as development proceeds. At first, there’s nothing on top but ectoderm. At the center of the left-right axis of the embryo, the ectoderm thickens and adopts a neural fate (upper right). The neural precursors then start shifting in a way that causes them to adopt a deep V shape (lower right).
This process eventually brings the two ends of the developing neural tissue together, and they merge, forming a tube (lower left). Credit: JOHN TIMMER A section across a vertebrate embryo as development proceeds. At first, there’s nothing on top but ectoderm. At the center of the left-right axis of the embryo, the ectoderm thickens and adopts a neural fate (upper right).
The neural precursors then start shifting in a way that causes them to adopt a deep V shape (lower right). This process eventually brings the two ends of the developing neural tissue together, and they merge, forming a tube (lower left). Credit: JOHN TIMMER (While this is being presented as a sort of “how vertebrates develop,” there are key differences among them. In mice, the formation of neural tissue happens nearly simultaneously along the entire head-to-tail dimension.
In chickens, the process starts in the head and moves slowly to the tail, such that there are already brain structures forming at a time when some of what will be the spinal cord doesn’t even know it will be a nerve cell yet. And tadpoles form a fairly simple spinal cord that gets expanded and reorganized as they change into frogs. So, while the general process is similar in all vertebrates, many species have adapted it to different styles of development.) By all appearances, all these newly formed neural cells look more or less the same. So we end up back at the original question: how do the segments of the nervous system form?
Remember that, by the time these cells start forming, the embryo already knows its head from its tail. That raises two very simple possibilities. One is that the ectoderm cells that the nerve cells form from already know where they are, and so the nerve cells inherit positional information from them. The alternative is that after the neural cells form, their non-neural neighbors can send signals to them to tell them where they are.
So if cells in the head make a different collection of signaling molecules from those in the tail, this can transfer positional information to the developing nervous system, telling it where to form the brain and where to form the spinal cord. Dividing up the brain The new work builds on decades of studies that have identified many key regulators of early processes. One of those earlier findings was that the ectoderm of the early embryo activates two genes, one in the front half of the embryo, one in the back. So nerve cells can inherit at least some crude positional information from the ectoderm they form from.
The key to this work is that the researchers modified a copy of these genes so that it activated fluorescent proteins wherever the gene was translated into a protein. So, they engineered mice where half of the early ectoderm glowed red, and the other half glowed cyan. These colors were maintained as the embryo formed nerve cells and the cells started to develop into the brain. They found that while the hindbrain glowed red, the rest of the brain glowed blue.
The inherited positional information set up one of the key boundaries in the brain. In other words, as soon as cells know they’re going to eventually develop into neurons, they know whether they can potentially form part of the hindbrain and not the mid- or forebrain. (This is, roughly, where the idea of “two brains” in the press release comes from.) Other experiments expanded on this. Similar genetic tools let them activate a fluorescent protein in individual cells in the early ectoderm. They found that, in 96 percent of the cases, the descendants of these single cells were all in the hindbrain, or all in the midbrain and forebrain.
They also worked with human stem cells and showed similar things were happening there: Exposure to the right signals would tell the stem cells where they were, and the nerve cells inherited that information as they formed. If you wanted to form neurons that are only found in the hindbrain, you have to first send the stem cells down the hindbrain path. Early cell labeling shows that the hindbrain and spinal cord (red) remain a separate fate from the midbrain and forebrain (cyan). Credit: Jokhai et. al.
Early cell labeling shows that the hindbrain and spinal cord (red) remain a separate fate from the midbrain and forebrain (cyan). Credit: Jokhai et. al. There obviously may still be some flexibility here—the 96 percent exclusive behavior they saw is not 100 percent. It’s not clear whether that’s a limitation of their experimental system or just a product of the fact that cells at the border between the front and back of the ectoderm can change fates if they wander a bit in the right direction.
Things happen very quickly in early development, and it’s also not clear how the timing of what we can see compares to the timing of what’s going on with the genes that were used to mark different cells. It’s also less certain what’s going on at the other key borders. Differences between the mid- and forebrain seem to arise later, and largely due to the action of signaling molecules that diffuse through the developing brain and provide positional information. In contrast, very little is kn
