In this video, we're going to talk about microglia, which
are glial cells of the central nervous system.
Their name comes from Greek words
meaning "small glue," which doesn't actually
make a lot of sense.
But when all the glial cells were having their names handed
out, the microglia were noticeably smaller
than all the other glial cells.
So the cells ended up being named microglia.
And all the other glial cells will sometimes
be referred to collectively as macroglia
because they all tend to be larger than the microglia.
Most or all microglia probably derived
from circulating monocytes from the bone marrow, that
then enter the central nervous system.
So that they are therefore from the part of the embryo
called the mesoderm, as opposed to the ectoderm
that all the other neural cells are derived from.
Microglia come in a few different shapes.
Resting microglia have a small soma and lots
of long, highly branched processes
heading out in every direction.
And when they're in this shape, they're
called resting microglia.
Active microglia are larger and just kind of blob shaped.
Some people say they're shaped like an amoeba.
So when they're shaped like this,
they're called active microglia.
When microglia are resting, they're
basically just sitting there sampling
the interstitial fluid.
They have all these long, highly branched
processes heading out, just keeping
an eye on the interstitial fluid.
And they're basically looking for trouble.
And when resting microglia do detect
that there's trouble going on, they
convert into active microglia.
They retract all these long, branch
processes and turn into these blob-like active microglia.
And the main type of trouble they're looking for
is inflammation.
And whether that's inflammation from injury
to the tissue of the central nervous system
or an infection of some sort, like a bacteria or a virus
has entered the central nervous system.
Now active microglia look and act like macrophages
anywhere else in the body, which is a type of cell that
moves around through tissues looking
for dead or damaged cells or foreign cells.
So active microglia will move or migrate over
to areas of inflammation and they're looking for things
like bacteria or other foreign cells
that have invaded and are causing
an infection or dead or damaged cells
of the central nervous system.
Now, if an active microglia finds a foreign cell
like a bacteria, it can secrete substances
that are called cytotoxic, that can kill these foreign cells.
For example, reactive oxygen species--
that I'll just shorten to "ROS"--
that could kill a cell like a bacteria.
And secretion of cytotoxic factors
is one of the main functions of microglia.
Now after the microglia killed the bacteria,
the bacteria would turn into just debris,
just broken up pieces of bacteria.
And there is nothing that microglia enjoy more than
chowing down on debris.
And it doesn't have to be foreign cell
debris, like a bacteria.
But any cell of the central nervous system, if it's dead
or its damaged and there's debris, basically
anything that's not a normal healthy cell
of the central nervous system, the microglia will eat it.
It'll take it inside its own cell
and break it down into smaller pieces.
So this is another main function of microglia,
which is called phagocytosis, which
means eating cells or eating debris.
Just like other types of cells do
all over the body, when they're like macrophages,
they look just like active microglia.
Now, these functions don't have to occur in this order.
If a microglia bumps into something
abnormal and phagocytoses it, that often
gets the microglia fired up and it may then
start at creating cytotoxic factors,
or other substances that interact with immune cells
to further fire up information.
Now after the microglia eats the debris,
it performs its next major function,
which is to take those broken up pieces of the debris
that it ate and stick little pieces out on its surface
for other cells to see and specifically
other cells of the immune system, like lymphocytes.
Lymphocytes will come in here and they will recognize
or they'll look at these pieces of the debris
that the microglia ate and is now
presenting on its surface for the lymphocyte.
Any molecule that a cell of the immune system can recognize,
we call an antigen.
So this function is called antigen presentation.
With the effect that these immune cells further
increase inflammation and try to make the inflammation more
specific to whatever the antigen was, particularly
if it's from a foreign cell.
So that the microglia is activated by inflammation
and the microglia also contributes to inflammation.
Therefore, microglia play a major role
in inflammation in the central nervous system.
In addition to these functions, microglia
appear to influence neurons, other glia, and other cells
of the immune system and vice versa,
through exchange of a variety of substances.