the blood brain barrier; are blood vessels different in the brain so medicine can’t get in when it could cross into other tissue from blood elsewhere in the body?
In: Biology
The blood brain barrier is a physical barrier that separates the brain from any blood. The barrier is very selective in what is allowed through, so medicines need to be carefully designed to cross it, if it even can.
Yes! Other blood vessels can have slight gaps between each of the cells in the vessel wall, but there are very tight junctions in the vessel walls of the blood brain barrier to limit the molecules that can enter.
This makes it more difficult to get some medicines into the brain. Some very small molecules can enter the brain by diffusion and some molecules can be picked up by transporter proteins on the blood brain barrier and carried across.
Attaching medicines to liposomes (fat-based carrier molecules) also lets medicines travel through the barrier to the brain; I think this is the world of nanomedicine which seems to be the future of drug delivery!
It’s not so much the blood vessels themselves that are different. The blood vessels in the brain do have special features, but the [blood-brain barrier](https://en.wikipedia.org/wiki/Blood%E2%80%93brain_barrier) is mainly related to a kind of cell called an [astrocyte](https://en.wikipedia.org/wiki/Astrocyte). Astrocytes have extensions called feet that stick to the blood vessel and wrap around its outer wall. Most blood vessels in the brain are covered by a dense layer of astrocyte feet that form a [physical barrier](https://en.wikipedia.org/wiki/Glia_limitans). Any substance that “wants” to get into a neuron or other brain cell must first cross through astrocytes. There are some exceptions though (for example, parts of the brain that are exposed to the bloodstream with no blood brain barrier include the pineal gland, some parts of the hypothalamus, and some parts of the brain stem).
Here’s a good [picture](https://s3.cad.rit.edu/cadgallery_production/images/uploads/faculty-f-projects/340/951.jpeg) showing how astrocyte feet help to form the blood brain barrier. You can see that astrocytes basically get to choose what enters the brain and what doesn’t. Some substances are able to enter passively (usually small molecules that are often fat soluble), but others (such as glucose) need to be actively transported across. Many drugs and toxins lack the ability to cross the blood brain barrier as a result of being too large or not very fat-soluble (non-lipophilic). In general, astrocytes usually allow nutrients through while blocking most other substances from reaching neurons and other brain cells.
edit: just as a technical note so that this doesn’t mislead you, the blood brain barrier is not only formed by astrocytes. Other cell types are also involved. The endothelial cells of the blood vessel itself are also a key part of the blood brain barrier as another answer says. Another kind of cell involved is the [pericyte](https://en.wikipedia.org/wiki/Pericyte). But astrocytes are the main cellular link between neurons and the bloodstream. Continuous capillaries are not unique to the brain.
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