Martin used a different approach that avoided in vivo alteration. She reasoned that an ES cell line might be obtained by culturing cells isolated from blastocysts in a medium that had previously been conditioned by an established teratocarcinoma stem-cell line (such a medium might contain a factor that stimulates ES-cell proliferation and/or suppress their differentiation). Using this approach, she established a cell line directly from normal pre-implantation mouse embryos and confirmed its pluripotency by showing that individual cells of this line could differentiate to form a wide variety of cell types in vitro and in vivo.  Digital facsimile from PubMedCentral at this link.