Personal pathways to embryonic stem cells

Work with numerous colleagues on embryo stem cells in vitro began in our laboratories in 1963, using cleaving embryos and blastocysts of the rabbit. Growth of disaggregated blastomeres from cleavage-stage embryos was weak. Trophoblast outgrowths from whole-embryo cultures provided a platform supporting inner cells that differentiated into blood islands, muscle, nerve cells, phagocytic cells, connective tissue and undefined elements. Stem cell lines derived from whole or disaggregated cells of rabbit inner cell mass and embryonic disc proved to be long-lasting (immortal), growing over months and years, stable enzymically, karyotypically and morphologically, and fully capable of resuming growth after cryopreservation. To measure the embryological potency of embryo stem cells, disaggregated or entire inner cell masses were injected into the blastocoelic cavity of mouse blastocysts. They contributed to the formation of chimaeras, and partially colonized most or all recipient tissues except trophectoderm. Gene markers for coat colour enabled their instant detection in recipient fetuses, newborns and adults. Cell lineages, the multipotency of single cells, and transgenesis all descended from this approach. Mouse and rat stem cells were grown from disaggregated blastocysts cultured in vitro or from tissues extracted from early post-implantation mouse embryos. They fully recolonized bone marrow in lethally irradiated adult mouse recipients, and in mice carrying inherited anaemias, migrating via liver to bone marrow and spleen. Some may have been hepatocyte or splenocyte precursors. Non-irradiated recipients were weakly colonized by embryo stem cells overcoming histocompatibility barriers. No signs of inflammation or cancer were noted. First studies on human embryo stem cells began, but were ended by an ethical decision preventing supplies of human blastocysts for research.