ABSTRACT
Development of embryonic stem cells (ES) evolved out of work on mouse teratocarcinoma tumors that arise in the gonads of a few inbred strains and consist of a remarkable array of somatic tissue, juxtaposed together in a disorganized fashion. Origins of teratocarcinomas from germ cells in mice provided the concept of a stem cell (the embryonal carcinoma or EC cell) that can give rise to the multiple types of tissues found in the tumors.1,2 Following the generation of chimaeric mice by blastocyst injection of EC cells, investigators began to realize the potential value of cultured cell lines from the tumors or models for mammalian development.3
