Finally, a big concept, and then I think we will finish for this particular topic, is transcytosis.
We're gonna move big things all the way across cells.
So, what's shown here are two endothelial cells,
and they're held together at their tight junction.
And that forms a pretty good barrier.
There are lots of blue balls outside that we might wanna get across
into the other side of the endothelium. How do we do that?
We could try to do it by a kind of, piecemeal endocytosis
where we have a little bit of cell drinking, and then it internalizes,
and then it moves across, and it dumps its contents
into the other side of the endothelial cell.
Conversely, what we can do is fuse endosomes and bake one big, gigantic pore.
And it turns out, transcytosis, moving across a cell like this, happens both ways,
but the most efficient way to get things across very quickly
in large amounts is fusing the endosomes to make these pores.
It plays a role in placental transport. So, babies?
Very metabolically active. You wanna get as much across as you possibly can.
Mom's circulation allows a great delivery of nutrition,
but we have to get it across the baby's endothelial cells
to get it into their bloodstream, and that happens by transcytosis.
The blood-brain barrier transport.
Normally, we have a very tight barrier, but sometimes,
we wanna move things at higher volumes,
and we will use this progressive endocytosis
and/or fused endosomes to accomplish that.
Thyroid epithelium will transport its thyroxin content out in exactly the same way.
Certain infections will take advantage of this transcytosis pathway to get across barriers.
And vascular permeability in tumors.
Tumors are incredibly edematous.
They're very, very leaky, and it's because those tumors drive this process quite efficiently.
So, with that, we've actually kind of looked at getting things across membranes.
Membranes are great barriers,
but now we have to move things in and out across them. Enjoy.