Neural Stem cells and intelligence
Can stem cells be injected into your brain to form functional neurons? Based on what current evidence says, yes. Two, would this enhanced nervous tissue proliferation make you smarter? That is more ambiguous.
Neural Stem Cells (NSCs) are essentially made by differentiating embryonic or human fibroblast-derived stem cells into neural cells using distinct growth factors, such as Fibroblast Growth Factor (FDF), Growth Differentiation Factor (GDF), and retinoic acid.
Neural stem cells have shown to integrate well into monkeys’ brains, countering neurodegeneration even after two years. It has also been successfully transplanted in mice; More importantly, Phase I Clinical Trials of Neural Stem Cell Transplantation for spinal cord injury began in mid 2018. In 2017 itself, experimental therapies involved injecting neural stem cells into the bloodstream or cerebrospinal fluid, and have improved neurological outlook for Traumatic Brain Injury (TBI) patients.
Given the successive, current and long term success of neural stem cell transplantations, neural stem cells injecting into the body (NSCs are not injected directly into the brain) will integrate with existing connections and enhance function. Although transplantation methods may differ according to the purpose and subject, it has shown to include intraspinal or intrathecal (injection into the cerebrospinal fluid or the CSF) injections.
However, will this increased connectivity make you smarter? Intuitively one could argue yes, because the greater the neurons and connections, the greater neural communication, i.e. higher capacity for thinking. Current research provides minimal answers - and rightly so - because injected NSCs are currently only use as experimental treatment for diseases such as Amylotrophic Lateral Sclerosis (ALS), Multiple Sclerosis (MS), and cerebral palsy.
Generally, results have been promising - showing greater neurotrophic factor levels (chemicals promoting neural growth and survival), enhanced motor ability, an increase in immune (T) cells, and improvement in conscious ability and overall neurological function (as per the Glasgow Coma Scale and National Institutes of Health Stroke Scale).
Thus, although it would be unethical to apply this treatment to a health individual to see whether their cognitive ability improves, a pure extrapolation of these results would seem to suggest that more neural stem cells implies greater cognitive and physical ability. At the same time, one must be cautious, because enhanced synaptic connectivity has been linked to autism. This can be summarized by the image below:
Another risk with neural stem cell transplantation is the potential for their development into tumors, due to the infinite cell division associated with stem cells. The first evidence of brain tumor development following NSC therapy was elucidated in 2009. Although reports of every case are not always published, this issue is a substantial challenge in neural stem cell therapy, which can result in neural stem cell injections being counterproductive instead of worthwhile.
So personally, I would disagree that neural stem cell transplantation is a viable means of augmenting intelligence. There has been almost no research to support that, and there is strong reason to believe that it may give rise to tumors or other neurological disorders, such as autism, instead. After all, our human brains likely evolved to a certain size by natural selection for a reason. So unless you have a deficit in neurological ability (e.g. by a neural pathology), I would not recommend considering this as an alternative to increasing intelligence. Better to go do math homework instead.