Hi Mike, Lindsay here (from the Cornell biomedical engineering lab).
How many different types of blood vessels are there in the brain?
Your understanding of the blood vessels in the brain is more than sufficient; we generally only talk broadly about arteries→ capillaries→ veins. I know you’re an academic so below is an NCBI article that goes into greater detail;
Are all of the blood vessels that we are seeing in the movies capillaries?
All of the outlined vessels are capillaries, but you do sometimes see the larger vessel types in the videos. These vessels are far larger than a single cell in diameter and so they don’t exhibit the same stalling phenomenon we are interested in. (Keep reading to make sense of how bigger vessels make it into the movies at all)
How are capillaries different from arteries? In some of the movies, we can see large bands of blood vessels that seem to be all tangled together. However, in most of the movies, the blood vessels are more or less distinct, and separated from one another. Do these two different groupings of blood vessels serve different purposes?
Note the illustration from Mayfield Clinic—it is good to start thinking about how capillaries are a kind of intermediary between veins and arteries. The middle capillary beds are where the exchange you mentioned occurs. There is certainly variation in the complexity of these capillary beds; some capillaries may stray farther from the main beds making them appear more distinct, while some remain tangled. As far as I know, the function of exchange is similar, the variation is just the vasculature’s attempt to service more volume.
Where are the neurons, and why don't we see them?
Not a silly question at all in fact I’m glad you asked, the answer is pretty cool. All of those brain cells you mentioned are there, we just aren’t seeing them because of how we image. The imaging modality used to create the movies is called “in vivo two photon excited fluorescence microscopy.” Breaking down this crazy name may answer your question:
“two photon excited fluorescence” – the microscope employs a 2-photon laser to selectively excite fluorescent dyes we have artificially added. In other words, the microscope only shows us what we want to see. In the case of blood flow imaging, we add a fluorescent dye that labels all of the plasma (liquid part of the blood). As the plasma is the only thing that carries a fluorescent dye excitable by the laser, it is the only thing that emits light! So within the movies you are watching, the other tissues are all there, we just don’t see them. The microscope starts at the top of the brain, above the dura (a protective covering) and images successively deeper areas, the big vessels that you see in the beginning are in the dura, and the occasional big ones you see deeper are servicing the capillaries.
“in vivo” of course means the mice are alive, blood pumping, breathing etc during the imaging session. So as the focus of the microscope moves deeper and deeper from the top of the brain, the images taken are moving in both time and space.
In sum- we don’t see the other stuff because we didn’t put in a dye for the laser to interact with, so they don’t really emit any light! It is a strange concept to get used to that light exists out of our visible spectrum and behaves differently/ illuminates different things… the first time I used fluorescence I thought of the magic markers I had when I was a kid that only became visible with a blacklight! Similar idea as far as the light thing goes. (When the laser is on in the lab during an imaging session we can’t even see it, we have to use a special telescope kinda thing to see where the beam is!)
Does the term morphology only refer to the shape of the blood vessel, or does morphology refer to something more than simply the shape of the blood vessel?
We mostly refer to the shape, sometimes the path. The vessels for the most part display the same shape and kinds of paths, but we occasionally stumble upon one that looks like it took a sharp turn or branched more than normal—variations that we need to be careful with as they sometimes confuse the computer’s total vessel count.
Please ask any clarifying questions and don’t hesitate to be honest if I just confused you more!