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.

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