Contrary to some reports, you don’t have two brains

与某些报道相反，你并没有两个大脑

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.

上周晚些时候，我们开始看到一些新闻标题，称有新的研究表明我们可能拥有两个大脑，而不是核磁共振扫描（MRI）中显示的那一个。正如通常情况一样，这些传言可以追溯到进行该项研究的大学所发布的一份新闻稿。你并没有两个大脑。这是一种看待新论文结果的荒谬方式。但其实有更好的方式来解读这些结果，这些方式可以帮助我们理解大脑是如何构建的，并阐明发育生物学背后的一些通用理念。因此，让我们来看看这项研究到底展示了什么，并将其置于大脑发育的背景下进行探讨。

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.

你在哪里？你可能认为身体分节现象只会出现在甲壳类动物和昆虫身上。但我们的神经系统也是分节发育的。脊椎动物有四个主要部分：前脑、中脑、后脑和脊髓。（其中一些后来还会进一步分节，但我们暂且略过。）这篇新论文主要致力于回答一个显而易见的问题：这些分节是如何形成的？这类问题是发育生物学的核心，该领域专注于研究生物体如何从单个细胞开始，通过一系列精心安排和协调的过程，最终产生出成年个体所需的所有组织。

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.

要思考这个问题，我们需要一些背景知识。当第一批注定要发育成神经元的细胞出现时，脊椎动物胚胎已经能够区分头尾和背腹。（我们不会深入探讨胚胎是如何获知这些信息的，但我们对此也了解颇多。）此时的胚胎由三种组织组成：内胚层（将形成肠道内壁）、中胚层（将形成肌肉和骨骼）以及外胚层（将形成皮肤）。神经细胞是由外胚层增厚形成的，这条增厚带从胚胎的头部延伸至尾部。这些密集细胞的中心向下凹陷，两侧向上折叠，最终形成一个椭圆形的管状结构，并从其余的外胚层中分离出来。所有沿脊柱向上传导的信号、你处理的每一个声音和形状、你产生的每一个念头——所有这些以及更多功能，都依赖于这些细胞的后代。

(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.)

（虽然这被呈现为一种“脊椎动物如何发育”的范例，但它们之间存在关键差异。在小鼠中，神经组织的形成几乎在整个头尾维度上同步发生。在鸡胚中，这一过程从头部开始并缓慢向尾部移动，以至于当大脑结构已经开始形成时，部分未来将成为脊髓的组织甚至还不知道自己将成为神经细胞。而蝌蚪形成的脊髓相当简单，在它们变态发育为青蛙的过程中，脊髓会进一步扩张和重组。因此，尽管所有脊椎动物的一般过程相似，但许多物种已将其调整为不同的发育模式。）

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.

从各方面来看，所有这些新形成的神经细胞看起来都大同小异。因此，我们又回到了最初的问题：神经系统的分节是如何形成的？请记住，当这些细胞开始形成时，胚胎已经能够区分头尾。这提出了两种非常简单的可能性。一种是，形成神经细胞的外胚层细胞已经知道它们所处的位置，因此神经细胞从它们那里继承了位置信息。另一种可能性是，在神经细胞形成后，它们非神经细胞的邻居可以向它们发送信号，告知它们所处的位置。因此，如果头部的细胞产生与尾部细胞不同的信号分子组合，这就可以将位置信息传递给发育中的神经系统，告诉它在哪里形成大脑，在哪里形成脊髓。

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…

划分大脑：这项新工作建立在数十项研究的基础上，这些研究已经确定了早期发育过程中的许多关键调节因子。其中一项早期发现是，早期胚胎的外胚层会激活两个基因，一个在胚胎的前半部分，一个在后半部分。因此，神经细胞至少可以从它们形成的外胚层那里继承一些粗略的位置信息。这项工作的关键在于，研究人员修改了这些基因的一个拷贝，使其在基因翻译成蛋白质的任何地方都能激活荧光蛋白。因此，他们培育出的小鼠，其早期外胚层的一半发出红光，另一半发出青色光。当胚胎形成神经细胞并开始发育成大脑时，这些颜色被保留了下来。他们发现，后脑发出红光，而大脑的其他部分发出蓝光。这种继承的位置信息在大脑中建立了一个关键的边界。换句话说，一旦细胞知道它们最终将发育成神经元，它们就知道自己是否……