Histology is the study of tissue.
And we have four basic types: muscle, nervous, epithelial, and connective.
We are going to focus on those last two in this video.
Your cells are able to carry out all their functions thanks to compartmentalization.
Each organelle has a membrane around it which maintains specific concentrations of the particular
chemicals inside, and to make sure they don't diffuse into other areas of the cell.
Just imagine if the hydrolytic enzymes in the lysosome were not fenced off - they'd
wreak havoc all over the place.
Similarly as a multi-cellular organism with differentiated cells, your tissues and organ
systems are compartmentalized.
If they weren't, all the cells in your body would form, well, a pile of goo.
And, well, it wouldn't be very fun, since you'd lack the ability to move. But I guess
that wouldn't matter since your stomach acid would have destroyed your brain cells and
you wouldn't have a cardiovascular system to deliver oxygen to all of your cells.
I know what my preference is.
So the point is compartmentalization is important.
It's critical.
And it happens thanks to epithelial tissues.
These guys coat the internal and external walls of your organs, and various body cavities
and surfaces.
They have strong cytoskeletons, linked via junctional complexes, making epithelial tissues
quite robust.
It is the reason why you can pinch your skin without ripping part of it off.
Epithelial tissues also protect the tissues within from infection or injury.
Between slabs of epithelial tissue, you have connective tissue.
As epithelial tissue have no blood supply, they rely on the connective tissue underneath
which is well vascularized, for support.
However, that is not the only function of the connective tissue.
Your connective tissues insulate and protect, store fluid and energy, and allow you to move.
There are four kinds of connective tissues: proper, blood, cartilage, and bone.
All four have in common the fact that they are mostly made up of extracellular matrix.
The extracellular matrix bears mechanical pressures, such as tension and compression.
Now, lets examine a cross section featuring both epithelial and connective tissues.
While the cells in the connective tissue are all spaced out, the epithelial cells are all
connected to each other, forming an epithelial sheet.
They are also connected to the extracellular matrix of the connective tissues thanks to
the basal lamina, which is a specialized portion of the extracellular matrix
How are the epithelial cells held together?
This responsibility falls on the junctional complexes.
At the most apical position, we have the occluding junctions.
Then, moving toward the basal end: cell to cell anchoring junctions, channel forming
junctions, and finally cell to matrix anchoring junctions.
Each has a specific purpose.
Occluding junctions, also known as tight junctions, form a very tight seal, so substances can't
pass between the cells in the epithelial layer.
Cell to cell anchoring junctions tie the epithelial cells together.
These come in two flavours: adherens junctions and desmosomes.
While adherens junctions connect the actin filament bundles in each cell to those of
its' neighbours, desmosomes do the same with the intermediate filaments.
Actin is a more dynamic molecule, allowing for coordination of cell movement, while intermediate
filaments are stronger.
Next we come to the channel forming, or gap junctions, which let small, water soluble
molecules pass between cells.
Lastly, matrix anchoring junctions include hemidesmosomes, which link the intermediate
filaments in the cell to the extracellular matrix, and actin-linked cell-matrix junctions.
Which, you guessed it, link actin filaments within the cell to the extra cellular matrix.
Let's take a closer look at each of these junctions.
Tight junctions can be thought of as zip locks - they form thanks to the interactions of
the trans-membrane proteins claudin and occludin.
I always remember this as santa claudin occludes tight chimneys.
Claudin and occludin each have four transmembrane domains.
When two cells are in close proximity, the claudins on one cell interact with the claudins on the other cell,
and the occludins on one cell interact with the occludins on the other cell.
Both the adherens junctions and the desmosomes use cadherins to form the connections between
cells.
While tight junctions resemble zip locks, you can think of cell to cell anchoring junctions
as resembling velcro - the cadherins link together, and the sheer number make them
hard to tear appart.
Gap junctions are made up of six connexons.
They allow water soluble molecules to go back and forth between neighbouring cells, without
contacting the extra cellular fluid, creating a means of effective cellular communication.
Hemidesmosomes have integrins, rather than cadherins, as transmembrane anchors.
The actin links at the basolateral end of the cell also use integrins.
The tight, seamless connection between cells in epithelial tissues, allow for the maintenance
of polarity, meaning that the concentrations of substances on either side of the epithelial
barrier are different.
The cells of the epithelial sheet themselves all match in polarity, with an apical side
facing the lumen, and the basal side facing the basal lamina.
This polarity is the fundamental property of epithelial tissues that allows for compartmentalization.
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