Embryonic Development of the CNS
The brain is a large, complex organ. In order to understand it fully, an understanding of its embryonic development is necessary. You see, the brain could look like a big blob to you. But, after figuring out how it develops, you should be able to easily distinguish at least five levels in it, and even more regions.
The entire nervous system develops from an infolding of the outer embryonic layer. This outer layer also gives rise to skin. But a long strip of this tissue begins to differentiate into nervous tissue (step #2 in this figure), and then it infolds (step #3 in this figure)... as it infolds, it pinches off and forms the neural tube (as in step #4 in this figure). This tube is literally a tube of cells that will divide to produce the entire CNS, both the brain and the spinal cord. Because it forms from infolding of the long strip of developing nervous tissue, it ends up forming inside the embryo, below the surface.
It is easy to imagine that a tube of cells can grow into a spinal cord. But it is a little trickier to imagine that this tube will also grow into the brain! The brain is so large, how does it develop from a tube? Obviously, the tube is going to have to swell a whole lot.
At first, the tube has three swellings at what is going to be the rostral end of the embryo. These three swellings continue to swell, and then turn into five swellings.
These pictures are my attempt to show you how that happens. One tube (at the left) gets three swellings (in the middle) and then those swellings expand further to turn into five swellings (at the right). The correct term for a "swelling" is a brain vesicle.
There are many terms in these diagrams. These are the terms people use when referring to regions of the brain. I expect you to learn all the terms that are in regular, straight green print in these figures (don't worry about the terminology in italics in this figure... that would be excessive). Please note that the full name of the "medulla" is the "medulla oblongata," but since it is the only medulla we'll discuss in the brain, I'll typically refer to it as simply the medulla.
The brain, even in an adult, can be said to be composed of a forebrain, midbrain, and hindbrain. You should be able to see that these brain regions are the regions derived from the embryonic neural tissue that exists at the 3 vesicle stage of neural development. So, for example, all of the forebrain structures in an adult brain (including the large cerebral cortex and the areas for learning and memory) come from cell divisions of the cells in the forebrain vesicle that exists in the 3 vesicle stage.
When the 3 vesicles become 5 vesicles, they do so by the top and bottom vesicles each splitting, but the middle one staying as is. So, the forebrain splits into the telencephalon and diencephalon . The hindbrain splits into the pons & cerebellum vesicle and the medulla oblongata vesicle. The midbrain still remains as the midbrain.
Here's the drawing from your book of a brain, viewed from a midsagittal cut.
Do you see all the different regions? Well, the stuff in light and darker orange is all tissue derived from the forebrain; the light orange is telencephalic, and the dark orange is diencephalic. The pink areas (and the white cerebellum) are derived from the midbrain and hindbrain.
Can you see how the brain still has the telencephalic tissue on top of the diencephalic tissue, on top of the midbrain, on top of the pons and cerebellum, which is on top of the medulla? Just like it did in the embryo. Now it is time to see how those five vesicles expand to make up this brain.
Here's an animation of the development of the brain. This one took longer than most of the others, but I don't think it is wonderful. It's certainly not perfect. But I wanted to get the idea of growth of each of the brain vesicles across to you, and I think this accomplishes that. The developing brain in this animation is being viewed from the side... you are watching the left half of the CNS form, and anterior is to the left.
You may have noticed how large the tissues that were derived from the telencephalon are in the brain. In that figure above, the light orange area is huge! In order for the telencephalic vesicle to make so much nervous tissue it has to grow faster than the other vesicles. The adult brain, when viewed from the outside, is completely surrounded by the telencephalon (as you can see in this animation). One can only see the rest of the brain when one cuts into it like in the drawing from your book above. So, try to observe how the telencephalon has to grow more than the other brain vesicles.
The other interesting item to note is the development of the cerebellum. The cerebellum develops from the same vesicle that produces the pons (the metencephalon is the specific name for that vesicle). The cerebellum has to grow off the posterior of the pons, and it is a very large structure in adults. So, it also has to grow rather quickly and a lot.
Using development to understand the brain
The neural tube turns into the brain and spinal cord. As it swells to make vesicles, and those vesicles expand, the neural tube sets up the order of the regions of the brain. First of all, the region where no swelling occurs on the neural tube will become the spinal cord. Because it is part of the same tube that gives rise to the brain, the spinal cord and brain remain continuous in the adult.
From caudal to rostral, the spinal cord connects to the medulla, which connects to the pons, and then to the midbrain, which connects to the diencephalon. These regions are all in a line, like a stick, even in an adult brain. I like to think of the CNS as looking like a lollipop... the stick is all of the regions above (everything that is not telencephalic), and the candy is the telencephalic portion.
In order to totally understand this, I have to go over some terminology. First of all, the word "brainstem." The brainstem is called that because it is part of the stick, and it includes the medulla, pons, and midbrain. It does not include the cerebellum or the diencephalon. I haven't even included the cerebellum in the lollipop picture, because it is the only part of a brain that a lollipop doesn't have (unless a bit of wrapper remains stuck to the candy). The brainstem carries out very essential duties, like being in control of our breathing. It also has roles in smell, taste, hearing, and balance, and it controls our facial movements as well.
The cerebellum grows out of the same vesicle that the pons grows out of. But the cerebellum grows only posteriorly. And then it grows so much that it covers most of the brainstem on the posterior surface in an adult. Of course, there is only one place where the cerebellum is actually connected to and continuous with the rest of the brain, and that is the pons. Right? Does that make sense? Since it grows out of the pons, it remains connected at that point.
The diencephalon lies directly above the brainstem, but below the telencephalon. The diencephalon somewhat caps the top end of the brain stem, and is a bit wider. It has functions in processing sensation and in controlling involuntary functions... I'll discuss that briefly in the diencephalon page.
The telencephalon is bigger in humans than in other animals, especially the region of the telencephalon that is called the cerebrum. Other vertebrates have a the same general layout for their CNS, but their telencephalon doesn't grow as much as ours (of course, some have really neat specializations within their brains that we don't have-- you can't win them all!). Our telencephalon gives us our perception of senses, offers our movement commands, provides for thought, memory, learning, and personality. There are a few pages about the telencephalon in this unit.
© 2011 STCC Foundation Press