Astrocytes

In this video, we're going to talk about astrocytes. And in their name comes from the Greek words for "star cell." Astrocytes are glial cells of the central nervous system, which are derived from neural stem cells. Astrocytes have a soma of variable number and branches of processes. But they often have a lot. They often have quite a few processes that are highly branched, which is how they got their name of star cells. Because some people thought they look like stars when they looked at them under the microscope. And at the end of their processes, they have special structures called end-feet. Let me draw in a few of these end-feet that are at terminus of the astrocyte processes. So that's end-feet. And that's all of these structures at the end the astrocyte processes. And astrocytes are work horses. They have arguably more functions than any other cell type of the nervous system. So first, we can talk about how the astrocytes really form the scaffold for the entire central nervous system. These cells really occupy a huge amount of the space of the central nervous system and form the majority of the structure that actually makes up the brain and the spinal cord. So they provide the structural support and the place for all the other cells to be, like the neurons and all the other glia. The second function we can talk about is called the glial scar. And that refers to what astrocytes do if there's injury somewhere in the central nervous system. So if there is some type of injury somewhere in the brain or the spinal cord, what we see is that astrocytes proliferate. They divide and form more astrocytes. And they migrate over to the area of injury. They then surround that area of injury. And their processes hypertrophy. They grow these much larger, thicker, longer processes that actually serve to wall off that area of injury. And between all the processes, they form a thick tissue, kind of like scar tissue that we see elsewhere in the body. But that scar tissue elsewhere in the body is formed by different cell types because the astrocytes are only present in the central nervous system. So this whole process of astrocytes reacting to injury has multiple names. It's called gliosis, or it's called astrogliosis, or it's called astrocytosis, or it's called reactive astrocytosis. And the actual scar tissue that's produced is called the glial scar. And the grill scar probably performs kind of a similar structural role to the astrocyte's role as the general scaffolding for the central nervous system. Because probably what they're trying to do is wall off an injured area. And particularly if there's a cavity, if a hole has formed, and they're trying to shore that area of structural support from this wall. Another function of astrocytes is homeostasis. And homeostasis just means trying to keep everything in optimal conditions in homeostasis of the interstitial fluid, that is the fluid between all the cells of the central nervous system. And this is very important because the neurons require a very fine-tuned, a very even keel environment for them to function properly. And if certain things like the concentrations of certain ions, and particularly potassium ions, if those concentrations are abnormal, the neurons can't function properly. So one thing the astrocytes do is that they're constantly monitoring the interstitial fluid. And they're either taking in ions or they're releasing ions to try to keep those ion concentrations exactly the same all the time, in homeostasis. Another thing they do for neuron sometimes is they release lactate into the interstitial fluid. And the reason they do this is because neurons have very little internal energy stores in their cells. Neurons are completely dependent on a continuous supply of oxygen and glucose to have all the adenosine triphosphate they need to perform their functions. Now, astrocytes do have some internal energy stores in the form of glycogen. And they can convert some of that to lactate and secrete that, so that the neurons can use lactate in a pinch if they have lost access to continuous oxygen and glucose. Another function of astrocytes is contributing to something we call the blood-brain barrier. And this is a barrier that prevents large molecules in the bloodstream-- so I'll just draw a little blood vessel passing through the central nervous system. And this prevents large molecules from leaving the blood to enter the central nervous system unless the cells actually want that large molecule to enter. Components of the blood vessels themselves play a role in this blood-brain barrier. But astrocyte processes also play a role. And, in particular, it's the end-feet. These end-feet, on the end of their processes, are plastered all over the blood vessels that are passing through the central nervous system. And they play a role in preventing certain large molecules from leaving the bloodstream and entering the brain. So that between the astrocyte end-feet and components of the blood vessels of the central nervous system, there's a quite effective barrier function between the blood and the central nervous system. And this includes a spinal cord. But we traditionally call this the blood-brain barrier. And one more very important function that the astrocytes perform is that they help to clear out synapses between neurons. And so synapses are the connections between neurons and their target cells. So if we drew an axon of one neuron coming down here. And this will be the terminal of the axon. And then this axon is forming a synapse with say the dendrite of another neuron here. Just like the astrocytes are using their end-feet to surround the blood vessels that pass through the central nervous system, the astrocytes are also extending their processes and placing their end-feet all over synapses so that the end-feet are plastered all over the synapses and they're actually helping to clear out those synapses. And we'll cover this more when we cover synapses. But basically they're clearing out the molecules that communicate between neurons and their target cell, called neurotransmitters. And this is very important to reset the synapse so that it can be used again for communication between the neuron and its target cell. Because if neurotransmitters just lingered in the synapse, then that synapse wouldn't be functional any more. It would just constantly be turned on. When instead, the neurons need to be able to rapidly turn on and off the synapses to be able to communicate information effectively. In addition to these functions, astrocytes appear to influence neurons and other glia, and vice versa, through exchange of a variety of other substances. So as you can see, astrocytes are very hard working cells in the central nervous system. No other cell of the nervous system appears to do such a huge variety of functions like the astrocytes do.