here we have reviewed the conventional definitions and fundamental 
 characteristics of the two basic types of stem cells , embryonic stem cells ( escs ) and somatic stem cells ( sscs ) . by taking into account the often - overlooked asymmetric cell kinetics of sscs , we consider the evidence that should sscs retain these growth kinetics in vitro , a natural kinetic barrier to ssc propagation exists .
recent discoveries showing that the tumor suppressor gene 
 p53 can act as a regulator of asymmetric cell kinetics provide a 
 target pathway for in vitro ssc propagation strategies .
 abstracthere we have reviewed the conventional definitions and fundamental 
 characteristics of the two basic types of stem cells , embryonic stem cells ( escs ) and somatic stem cells ( sscs ) . by taking into account the often - overlooked asymmetric cell kinetics of sscs , we consider the evidence that should sscs retain these growth kinetics in vitro , a natural kinetic barrier to ssc propagation exists . 
 recent discoveries showing that the tumor suppressor gene 
 p53 can act as a regulator of asymmetric cell kinetics provide a 
 target pathway for in vitro ssc propagation strategies . 
Abstract