Jello wrote:

Xia Zhang of the University of Saskatchewan in Saskatoon, Canada, and colleagues decided to see what effects a synthetic cannabinoid called HU210 had on rats' brains.

They tested a synthetic cannabinoid that is NOT present in marijuana.

In another study, Barr Jacobs, a neuroscientist at Princeton University, gave mice the natural cannabinoid found in marijuana, THC (D9-tetrahydrocannabinol)). But he says he detected no neurogenesis, no matter what dose he gave or the length of time he gave it for. He will present his results at the Society for Neuroscience meeting in Washington DC in November.

Tests with the real deal show no such signs of 'cell growth in the brain.'

Emphasis on might here.

there are also studies which suggest a link between mental health problems and marijuana
having been a heavy marijuana user in my early 20s and having suffered over the past 12 years bouts of mental health difficulties ranging from psychotic episodes to suicide attempts i would urge caution.
I don't know catagorically that there is a causal link ( i did have mental health problems prior to my drug use) however just as some people are susceptable to becoming alcoholics i believe my drug use triggered problems. I also believe that for the majority marijuana, like alcohol, is not a problem and can be used harmlessly and should be legalised subject to health warnings.

RE: New Scientist Breaking News - Marijuana might cause new cell growth in the brain