So let's look at contractile cells now, and contractile cells just aren't confined to muscle cells.
Lots of cells in the body can contract and in fact move.
This is a section through a sweat gland. It shows you profiles through a very coiled tube.
Imagine, if you cook some spaghetti or pasta and a long string of spaghetti, and you hold it up
near your kitchen bench, and you bring it down in contact with your kitchen bench,
and keep going down, you form a little coil of the spaghetti on your kitchen bench.
If you cut through that coil of spaghetti, you essentially see what you see in this slide.
You see profiles through various tubes.
In the case of spaghetti, they're solid tubes.
But in this case, these ducts of a sweat gland have epithelial cells that secrete the sweat into the lumens.
You don't see the lumens in great detail here because they're closed down,
but those epithelial cells are surrounded by myoepithelial cells.
They wrap around these tubes and they squeeze, they contract to bring about movement
of the secretory product or the sweat up through the lumen, up through the duct
to the surface of our bodies.
If you look very carefully down the bottom of the slide in the middle, you see a half of a profile of one of
these tubular structures. Look carefully on the border of that profile, and you can just make out some very fine
dark pink-stained little arms of these myoepithelial cells wrapping around this tubular part of the sweat gland.
They're myoepithelial cells.
You're used to contractile cells being in skeletal muscle or smooth muscle as you see here depicted
in the wall of the stomach and also cardiac muscle that I've just described earlier.
Smooth muscle is called smooth muscle because it's non-striated
and it forms the wall of a lot of tubes in our body—the gut, blood vessels, etc. and by slowly contracting,
those smooth muscle cells can move material through the lumens of the tubes in which they line.
It could be food, in the example of the gut. It could be blood flow regulation in blood vessels.
Here is skeletal muscle fibers. They're long fibers. They're multinucleated,
and the high content of pink you see staining there represents their high content of all the contractile
factory they require, and you can just make out some striations in those skeletal muscle fibers that
represent the repetitive contractile units.
These skeletal muscle fibers can be very, very long.
Sartorius muscle, for instance, passes from your hip region across your knee,
so it's a very long muscle and therefore, it makes sense to call the skeletal muscle cells
making up that muscle a fiber since it's a very long structure.
Smooth muscle cells don't have striations in them
because they tend to contract in a different way and in different directions.
You see them here with their nuclei forming the wall of perhaps the gut,
and these smooth muscle cells are very different to skeletal muscle cells because they can contract
and they can hold that contraction for some time. It's called muscle tonus.
and then they can relax and open up the lumen and in that way, they can force food, etc. along the gut tube.
Sometimes, they can contract and hold that contraction if, for instance, they're part of the wall of a blood vessel
and therefore, they can restrict blood flow through that blood vessel.
If you sit down and relax for a while, reading a book in the library or listening to a histology lecture,
you're not going to want to send a lot of blood to your lower limbs etc., so you tend to close off
blood flow to those areas to conserve both energy and also the flow of blood throughout the body.
I want to now move on and talk to you briefly about an epithelium that's responsible for
electrolyte and water transport and in this case, we're looking at an epithelial surface in the gallbladder.
You can see a group of cells, they're going from the bottom left to the top right of this picture,
and those cells have nuclei that are all lined up in a row, dark-stained structures.
They're slightly elongated because the cells themselves are tall, columnar cells.
They're going to sit on a basement membrane which is what I described earlier.
You can't see that very clearly here, and then that basement membrane is going to bind this epithelium
to underlying connective tissue which is that lightest area you see in the image.
Have a look at the epithelial cells in particular and you can see two major points.
One is you can see these little white streaks between the cells
and secondly, those white streaks don't go to the surface of the cells.
The cell surface is quite sealed. It's quite pink all the way along, and that's because this epithelial surface
is bound together by those tight junctions I mentioned the very first part of this lecture when describing epithelia.
Those tight junctions won't allow products to pass from the lumen into the underlying connective tissue,
so what the gallbladder does, the gallbladder is an organ that concentrates our bile,
so there's a massive amount of water absorption from the bile that's produced in our liver.
To absorb all that water and concentrate our bile, what the cell does is it doesn't allow water
to simply leak through the gaps between the cells.
It wants control over how much water absorption there is, so what these cells do is they pump sodium
into the basolateral zones between the cells. They pump sodium into the gaps between the cells,
and if you ever want to move water in the body, you move sodium first and water follows by simple osmosis,
and that's what's happening here.
Those little white streaks you see is where the water has passed in between the cells following the sodium
and that water will then flow down through the base of the cell or the base of the epithelium
into underlying connective tissue and into the bloodstream.
This is another example of a very specialized group of cells.