INTRODUCTION
The human body consists of tissues and organs that contain over 200 different cell types.1 these cells carry out all the functions required for viability and reproduction. It was argued long ago, that in some tissues, the differentiated cells have a shorter life span and that they are not able to self -renew.2 This led to the notion that such tissues are maintained by cells that have an extensive renewal capacity, and the ability to differentiate and generate daughter cells. These cells were termed as ‘stem cells”.2 Stem cells generate only the differentiated lineages of the tissues they reside in, ...
