So a few things.

The brain tissue is formed in layers.

Each layer has specific types of neurons and specific types of connections. Which connect specific regions.

when your brain develops as an embryo, there are two primary ways that neuron connections are formed to make sure the right type of neuron with the right pathway is made.

1. Scaffolding. Othe types of neurons and cells are grown in a way so that neurons can "grow up them". Think of like lattice fences for vine plants. Chemical signals are also sent that tell these neurons where to grow and where to connect. Kind of like a "hey buddy, over here".

2. The other big one is folding of brain sections.

This part is kinda insane but as the brain is developing it starts sectioning out pretty early and then these sections do this folding thing. Where they sort of turn themselves inward. This folding also helps form the layers of the brain within the sub organs of the brain. Like the brain stem, hippocampus thalamus (other parts of limbic system) etc. There are a lot of sub organs in the brain.

And I guess the 3rd thing I should mention even though you probably figured it out is that these two things cannot occur in adults.

The scaffolding thing isn't quite as simple as I've said. But trust me. It can't occur in adults either.

So now you know two fundamentals of neuro development that even a fair number of people in my field seem to be ignorant of.

Now what does this mean. ?

It means you can't just "repair" or "replace" missing tissue.

This is why there aren't any mammals that regrow brain tissue. None regrow cortical tissue.

The only neurogenesis (neuron growth) observed is a small tiny area in the hippocampus (part of limbic system ) and that's a whole other complicated thing. But I'm willing to give you the basics if you want. Just ask.

Okay so. Back to fixing the brain.

Same problem as fixing severed nerves in the spine but actually worse. You can't get a neuron to make a connection if there isn't scaffolding. So you can strengthen something if there is something there that new connections can follow. This is why some people can restore some nerve loss. If there are some nerves still connected then with physical and occupational therapy, new connections can be formed and strengthened. (I'm talking about limbs, not brain. Such as with spinal injury. In strokes it works differently; by reprogramming existing architecture).

But if there is nothing then that is not possible.

You also can't easily tell a stem cell to grow into a very specific type of neuron and connect in the right way.

A large portion of neurons are inhibitory. And they must be placed properly.

I give this example to explain why stem cells can't fix a brain

Let's say you have a circuit board. And some circuits are burnt out. So you cut up a bunch of pieces of solder and thin wires and scatter it on top of the circuit board and try to power it up.

There is pretty much no way that adding stem cells to the brain won't result in bad things happening.

There was a study that did this.. It gave people brain tumors.

There are still on going studies to potentially use stem cells but honestly there is a very good reason why mammals can't regrow cortical tissue.

I did a quick search and was surprised to see people still trying to do this line of research. As I said with the circuit board example, you can't control the errors that would be created.

Brain neuron connections (aka the architecture) is the mind. You can't alter these without people loosing something.

A few years ago (7 ish years?) when the study I'm thinking of came out, there was a lot of talk and a lot of studies were cancelled. I was working in pharm Alzheimer's research at the time at a clinic. I remember people thought it was going to be some miracle fix. But the brain is much more complicated than , like regrowing a damaged kidney or something.
Which also isn't currently possible with modern medicine.

https://en.wikipedia.org/wiki/Development_of_the_cerebral_cortex

Here this "neural tube" is talking about that folding I was referring to. During embryo development.

https://en.wikipedia.org/wiki/Neural_tube

There is also cortical folding (brain wrinkles) that form during later development. That's a different thing but also ads to the difficulty of repairing the brain.

You should really look up human brain organoids, how they are created, and what we are doing with them. You can rent one and make it..do..think things. Sometimes they grow eyes.

the problem would be getting a specific type of stem cell to do that, likely a pluripotent rather than a totipotent(which is usually a blastocyst after fertiliation) to differentiate into a nerve cell and not continue growing or dividing. because cancer behaves pretty much like a stem cells, if not some are stem cells themselves.

Just to point out. This article theorizes neurogenesis in the hippocampus MAY be possible in humans because it's been supported to occur in rats.

This is exactly what I said. It's theorized to occur in humans in a very specific area of the hippocampus (not in cortical regions or any other area of the brain,). It's not supported with evidence to occur. It's only theorized to occur.

This article is more specifically talking about the research on exercise and hippocampus neurogenesis in rats. Which showed a positive correlation.