Although loss of function is almost immediate, there is no specific duration of clinical death at which the non-functioning brain clearly dies. The most vulnerable cells in the brain, CA1 neurons of the hippocampus, are fatally injured by as little as 10 minutes without oxygen. However, the injured cells do not actually die until hours after resuscitation. This delayed death can be prevented "in vitro" by a simple drug treatment even after 20 minutes without oxygen. In other areas of the brain, viable human neurons have been recovered and grown in culture hours after clinical death. Brain failure after clinical death is now known to be due to a complex series of processes called reperfusion injury that occur "after" blood circulation has been restored, especially processes that interfere with blood circulation during the recovery period. Control of these processes is the subject of ongoing research.