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.