Tanycyte

Transcription

In this video, I want to talk about ependymal cells. But to do so, first let me draw the brain and the spinal cord. So I'll just draw a big circle for the brain and I'll draw a long structure like this for the spinal cord. Because there are spaces inside the brain and inside the spinal cord that are full of fluid. And these spaces are connected together. And they are connected to this little skinny canal that goes down the spinal cord. And don't worry about the anatomy of this. We'll go into this in more detail in other videos. But these spaces are full of a kind of fluid called cerebral spinal fluid for the brain and the spinal cord. And the lining of these spaces is called the ependyma, which is made up of ependymal cells. The ependyma is named for a Greek word for covering. And the ependymal cells that make up the ependyma are glial cells of the central nervous system derived from neural stem cells. If we zoom in here and look at some of these ependymal cells, we'll see that they form a simple, cuboidal epithelium. Simple, meaning that they're just one layer of cells; cuboidal, meaning that they're shaped like little cubes; and epithelium, meaning they're a covering, in this case the lining of a cavity. So let's say that this is the side facing the cerebral spinal fluid, which I'll just write as "CSF" for short, for Cerebral Spinal Fluid. And that this side faces the interstitial fluid of the central nervous system, all the fluid between the cells of the brain and the spinal cord. And I'll just write "IF" as short for Interstitial Fluid. On the side of the ependymal cells facing the cerebral spinal fluid are a large number of little tiny processes called microvili, that increase the surface area of the ependymal cells on that side. They also have some processes that are a little longer, called cilia, that are these mobile, whip-like structures that kind of whip around and help move the cerebral spinal fluid around. One of the main functions of ependymal cells is to form a barrier between the cerebral spinal fluid and the interstitial fluid. So to some extent, they limit the movement of cells and large molecules between these fluid-filled spaces and the interstitial fluid of the tissue itself. Now as barriers go, the ependymal cells form a fairly leaky barrier, particularly if we were to compare it to the blood-brain barrier created by the capillaries in the central nervous system and the astrocyte end-feet. And the fact that this is a relatively leaky barrier is actually useful for medical purposes because there are areas where we can sample the cerebral spinal fluid and send it to the laboratory. And when we analyze the cerebral spinal fluid, we can often get a lot of information about what's happening in the tissue of the brain and the spinal cord because it's a relatively leaky barrier. The second major function of ependymal cells is to participate in secretion of the cerebral spinal fluid. So secreting CSF, cerebral spinal fluid. Specialized ependymal cells and capillaries form little tufts in some of these spaces in the brain. And this is actually where the cerebral spinal fluid is secreted into these spaces, so that there will be capillaries very close to the ependymal cells. And in these little tufts, fluid will be secreted across the ependymal cells to create the cerebral spinal fluid.